ABSTRACT
The Enhanced Characterization of the Repository Block Cross Drift (Cross Drift) excavated at Yucca Mountain is being studied to determine its suitability as a permanent high-level nuclear waste repository. This report presents a summary of data collected by the U.S. Bureau of Reclamation (USBR) personnel on behalf of the U.S. Geological Survey (USGS) for the Department of Energy in the Cross Drift from Sta. 00+00 to 26+64. This report includes descriptions of lithostratigraphic units, an analysis of data from full-periphery geologic maps (FPGM) and detailed line survey (DLS) data, a detailed description of the Solitario Canyon Fault zone (SCFZ), and an analysis of geotechnical and engineering characteristics. The Cross Drift is excavated entirely within the Topopah Spring Tuff formation of the Paintbrush Group. Units exposed in the crystal-poor member of the Topopah Spring Tuff, include the Topopah Spring crystal-poor upper lithophysal zone (Tptpul) (Sta. 0+00 to 10+15), the Topopah Spring crystal-poor middle nonlithophysal zone (Tptpmn) (Sta. 10+15 to 14+44), the Topopah Spring crystal-poor lower lithophysal zone (Tptpll) (Sta. 14+44 to 23+26), and the Topopah Spring crystal-poor lower nonlithophysal zone (Tptpln) (Sta. 23+26 to 25+85). The lower portion of the Topopah Spring crystal-rich lithophysal transition subzone (Tptrl1) is exposed on the west side of the Solitario Canyon fault from Sta. 26+57.5 to 26+64. Lithologically, the units exposed in the Cross Drift are similar in comparable stratigraphic intervals of the Exploratory Studies Facility (ESF), particularly in terms of welding, secondary crystallization, fracturing, and type, size, color, and abundance of pumice and lithic clasts. The most notable difference is the lack of the intensely fractured zone (IFZ) in the Cross Drift. The as-built cross section and the pre-construction cross section compare favorably. Lithostratigraphic contacts and structures on the pre-construction cross section were encountered where expected. Discrepancies occur at Sta. 22+38 where an unexpected fault was encountered, and in the hanging wall of the SCFZ where west-dipping beds were predicted but not encountered. Also, the pre-construction cross section did not predict the Tptpul/Tptrl contact exposed at Sta. 26+57 of the Cross Drift. The SCFZ has two major normal fault strands, the eastern strand (main splay) and the western strand. The eastern strand has approximately 260 m of normal offset based on stratigraphic relationships and the as-built cross section. The western strand was not penetrated by the tunnel boring machine (TBM) based on programmatic decisions to cease tunnel advancement. The as-built cross section estimates the offset on the western strand to be about 68 m. Footwall deformation in the SCFZ was greater than anticipated by the Geotechnical Baseline Report. The blocky and ravelling ground encountered in the footwall was predicted to be more likely to occur in the hanging wall. This may indicate a broad zone of deformation associated with the intersection of additional splays with the eastern strand of the SCFZ. Analysis of DLS fracture data in the Cross Drift shows strong correlation to conclusions drawn from fracture analysis in the ESF. Particularly strong correlations occur when fracture analysis of sets from the Cross Drift are compared to sets exhibited in the ESF. Analysis of the Tptpul, Tptpmn, Tptpll, and Tptpln of the Cross Drift produced Sets 1, 2, and 3. These sets correspond well to Sets 1, 2, and 3 of the ESF. A notable exception occurs in the Tptpll of the Cross Drift, where a Set 4 was encountered which does not correspond to any sets of the Tptpll of the ESF. Additionally, subjective visual analysis of contour plots of all fractures in each respective unit of the Cross Drift agrees well with the contours identified by Clustran. Geotechnical characterization of the Cross Drift focused primarily on rock-mass quality and rock-mass mechanical properties. Descriptions are based on two empirical rock mass classification systems, rock quality (Q system) and rock-mass rating (RMR). The rock-mass quality (Q) encountered in the Cross Drift is generally good, with the exception of the Tptpll in the fair category. The rock-mass rating (RMR) encountered in the Tptpll is borderline fair to good. The rankings and ratings agree with the Geotechnical Baseline Report, however, the Q values calculated are three times better than anticipated. These high ratings indicate that very little support is required in the stratigraphic units exposed in the Cross Drift (or similar units in different locations), other than occasional spot-bolting.INTRODUCTION
Yucca Mountain is being considered as a potential site for an underground, long-term, high-level nuclear-waste repository. Located 160 km northwest of Las Vegas, Nevada (Fig.1), Yucca Mountain is on the western edge of the Nevada Test Site (NTS) and the Nellis Air Force Range. As part of its ongoing investigation, the United States Bureau of Reclamation (USBR) in collaboration with the United States Geological Survey (USGS), under the direction of the Department of Energy (DOE), has undertaken the study of the geology of Yucca Mountain. This study is part of an extensive characterization program designed to evaluate the site’s suitability as a permanent nuclear-waste repository. As part of the continuing investigation, the Enhanced Characterization of the Repository Block Cross Drift (Cross Drift) was constructed. This report focuses on the Cross Drift, a 2.7 km-long and 5.0 m-diameter roughly north-east south-west-trending tunnel designed to extend underground access to the Solitario Canyon fault zone (SCFZ) and to stratigraphic units within the proposed repository block which were not encountered, or had limited exposure, in the Exploratory Studies Facility (ESF) (Fig. 2). The starter tunnel for the Cross Drift begins in the North Ramp of the ESF at station (Sta.) 19+92. The Cross Drift starter tunnel is approximately 26.4 m in length, 10.5 m high and wide, and has a horseshoe-shaped cross section. It was excavated by drill and blast methods, and functioned as a launch chamber for the tunnel-boring machine (TBM). The TBM is a conventional open-beam machine manufactured by Robbins of Seattle, Washington. The mapping was performed on a 70 m long platform which moved independently of the TBM and provided geologists with access to the tunnel walls. The Cross Drift varies in orientation and gradient so that it passes over the ESF Main Drift and intersects the Solitario Canyon fault zone at approximately 90 degrees. In plan view, the Cross Drift is roughly sigmoidal in shape with two broad curves of 305 m radius near beginning and end. The Cross Drift begins at a bearing of 254 degrees and an incline of 0.5 percent. At Sta. 01+82, the tunnel begins a 25 degree turn to the southwest (left) to a bearing of 229 degrees. The gradient increases to 1.85 percent at Sta. 03+25, then decreases to 1.5 percent at Sta. 07+73, the point where the Cross Drift passes over the ESF Main Drift. At Sta. 16+02, the gradient decreases from 1.5 to 0.9 percent and remains at that grade until Sta. 24+67, where it begins and maintains a -3 percent decline to the terminal heading at Sta. 26+81. The Cross Drift begins a 60 degree turn to the west (right) at Sta. 23+21, to a final bearing of 289 degrees. The Cross Drift is excavated entirely within the Topopah Spring Tuff. The tunnel begins in the Topopah Spring crystal-poor upper lithophysal zone (Tptpul) and proceeds down section through the entire length of the excavation, with the exception of the SCFZ. The stratigraphic contact between the Tptpul and the underlying Topopah Spring crystal-poor middle nonlithophysal zone (Tptpmn) occurs at Sta. 10+15. The Tptpmn is exposed until Sta. 14+44, where the Topopah Spring crystal-poor lower lithophysal zone (Tptpll) contact is exposed. The Tptpll/Topopah Spring crystal-poor lower nonlithophysal zone (Tptpln) contact is at Sta. 23+26. The Tptpln continues from Sta. 23+26 to the eastern strand of the SCFZ at Sta. 25+85. Tptpul exposed in the hanging wall (west side) of the SCFZ indicates approximately 260 m of normal offset. Offset on several normal faults from Sta. 26+08 to 26+52, in conjunction with the faulting and shearing from Sta. 26+52 to 26+66, as well as possible faults obscured by the trailing gear of the TBM, combine to bring the Topopah Spring crystal-rich lithophysal zone (Tptrl) into the Cross Drift at Sta. 26+57.Objectives
This report is a summary of data collected by USBR personnel on behalf of the USGS from Sta. 00+00 to 26+64 of the Cross Drift. This report presents fracture statistics and cluster analysis, descriptions of lithostratigraphic units, engineering characteristics of the rock, and a discussion of significant faults observed in the Cross Drift. One of the principle objectives of this excavation was to expose a larger portion of the Tptpll, and to expose the Tptpln and the SCFZ. The Cross Drift traverses the repository block in an approximate northeast-southwest direction, thus providing access to investigate and characterize lithologies, faults, fractures, and conditions of the repository block which were not encountered in the approximate north-south ESF Main Drift. It is important to characterize fractures, faults, and shears because these discontinuities may provide potential pathways through which water and gases can access stored waste and transport radionuclides into the surrounding environment. Analysis of this data assists in determining whether discontinuities can be grouped into domains with common characteristics that relate to rock type and/or regional tectonic history. These relationships will help other investigators formulate more representative models of fluid movement and tectonism as it relates to Yucca Mountain.Regional Geology
Yucca Mountain lies in southern Nevada, in the Great Basin, which is part of the Basin and Range structural/physiographic province. In the Yucca Mountain area, pre-Tertiary rocks consisting of a thick sequence of Proterozoic and Paleozoic sedimentary rocks underlie approximately 1000 to 3000 m of Miocene volcanic rocks (Gibson and others, 1990). The Miocene volcanic sequence exposed at Yucca Mountain includes units of the Paintbrush and Timber Mountain Groups (Sawyer and others, 1994). The Paintbrush Group consists of pyroclastic rock and lavas originating from the Claim Canyon caldera, approximately 6 km north of the study area, and are from 12.8 to 12.7 Ma old (Byers and others, 1976; Sawyer and others, 1994). The Paintbrush Group includes a homoclinal sequence consisting of four formations, the Tiva Canyon, Yucca Mountain, Pah Canyon, and Topopah Spring Tuffs. These formations consist of pyroclastic-flow and pyroclastic-fall deposits with interbedded lavas which dip 5 to 10 degrees to the east (Byers and others, 1976; Christiansen and others, 1977; Broxton and others, 1993). Two of these formations, the Topopah Spring and Tiva Canyon Tuffs, are voluminous, densely welded, compositionally zoned pyroclastic outflow sheets that grade upward from rhyolite composition to quartz latite composition (Lipman and others, 1966; Byers and others, 1976; Schuraytz and others, 1989). The tuff and ash flows of the Timber Mountain Group were erupted from Timber Mountain caldera complex and consist of the Ammonia Tanks Tuff and the Rainer Mesa Tuff (Sawyer and others, 1994). Yucca Mountain is bounded by Yucca Wash to the north, by the Solitario Canyon fault to the west, and the Bow Ridge fault to the east. Alluvium-filled structural valleys, consisting mostly of alluvial fan deposits (fluvial and colluvial sediments) and some thin eolian deposits, lie adjacent to the Bow Ridge and Solitario Canyon faults on the east and west sides respectively. The Yucca Mountain area is cut by steeply dipping, north-south-striking normal faults which separate the Tertiary volcanics into blocks one to four km wide (Scott, 1990). The potential repository block is bounded by the Solitario Canyon fault to the west and the Ghost Dance fault to the east. Both faults dip steeply toward the west, and displacement, amount of brecciation, and number of associated splays vary considerably along their trace. (Scott and Bonk, 1984; Day and others, 1998). The Solitario Canyon fault has normal down-to-the-west displacement of about 260 m in the vicinity of the potential repository block. The Ghost Dance fault is in the central part the potential repository block, and in general, is a north-striking normal fault zone, with down to the west displacement. The Sundance fault is located in the north-central portion of the potential repository block. It is a northwest-striking, east-dipping normal fault with a maximum cumulative down-to-the-northeast displacement of 6 to 11 m (Day and others, 1998).Site Characterization Techniques
Geologic site-characterization activities performed in the Cross Drift by the USBR for the USGS include the following techniques taken from a Technical Procedure (YMP-USGS-GP-32, R2) entitled Underground Geologic Mapping (U.S. Bureau of Reclamation and U.S. Geological Survey, 1997).Full-Periphery Geologic Mapping
Geologic mapping in the Cross Drift records lithostratigraphic and structural features at a scale of 1:125 (refer to Drawings OA-46-314 through OA-46-344). These drawings are developed in the full-periphery style in which the tunnel walls are “unrolled” to produce a flat map of the tunnel periphery. Structural discontinuities with trace lengths longer than 1 m and lithostratigraphic contacts were recorded on field sheets along with any other geologically significant features, details of the tunnel support, and rock sampling and instrumentation locations. The field sheets are then digitized using AutoCAD. The maps were field checked for accuracy, consistency, and completeness. The full-periphery geologic maps are located in the Technical Data Management System with their associated data tracking numbers (Appendix 1).Detailed Line Surveys
Detailed line surveys (DLS) were conducted along the left wall, normally at springline. A metric measuring tape was affixed to the wall and discontinuities having a minimum trace length of 1 m, and intersecting within 30 cm on either side of the tape were documented. The DLS data was recorded on Fujitsu 510 pen computers, utilizing a customized Microsoft Excel 97 spreadsheet. Over 1850 fractures, cooling joints, vapor-phase partings, lithologic contacts, faults, and shears were recorded by DLS in the Cross Drift. Discontinuities were 81 percent fractures, 7 percent cooling joints, 4 percent vapor-phase partings, and 8 percent faults and shears. The DLS data are located in the Technical Data Management System: their data tracking numbers are listed in Appendix 1. The following characteristics were recorded in the DLS.| Station- | A discontinuity is located on the DLS tape to the nearest 0.01 m giving each discontinuity a unique identifier. |
| Orientation- | The orientation of a geologic feature is determined using a goniometer for strike azimuth and a Brunton compass for dip values. Orientations were recorded using the right-hand rule where the direction of the dip is 90 degrees to the right (clockwise) of strike. |
| Type- | Discontinuities include lithologic contacts, fractures, cooling joints, vapor-phase partings, faults, and shears. Vapor-phase partings are discontinuities that consist of roughly linear accumulations of vapor-phase minerals and are parallel to subparallel to lithostratigraphic layering. Fractures are discontinuities that have no visible offset. Cooling joints are a class of fracture that presumably formed as a result of stresses in the cooling volcanic sheet. Shears are discontinuities having less than 0.1 m offset, or when offset is indeterminate. Faults are discontinuities with greater than 0.1 m offset. |
| Trace length- | Trace length is the length of the discontinuity on the tunnel wall. The trace length is measured, from the DLS tape to the discontinuity’s upper end, and from the tape to its lower end. These two measurements allow the discontinuity to be accurately located relative to the DLS tape and other discontinuities. |
| Height, Width- | The height and width are measured on an imaginary extension of the discontinuity plane. A horizontal line extending on strike from the highest point on the plane defines the upper boundary of the plane. A line parallel to the dip of the discontinuity extending from the point of its greatest lateral extent defines the lateral boundary of the plane. The height and widths are the maximum dimensions of that plane, width being measured parallel to strike and height being measured parallel to dip. |
| Terminations - | The number of visible ends (terminations) are counted. The type of termination is also recorded. If the discontinuity extends out of view, such as continuing under the tunnel’s steel support, it is recorded as such. The visible ends are recorded as ending in rock or ending in another discontinuity. The angle at which one discontinuity terminates into another is specified as intersecting either at less than or greater than 45 degrees. |
| Aperture - | The minimum and maximum open, unfilled space between a discontinuity’s surfaces is recorded as the aperture. |
| Roughness - | The roughness scale is based on the scale used by the USBR (U.S. Bureau of Reclamation, 1988). Roughness (R) characterizes the small-scale asperities of a fracture surface on a scale from 1 to 6. R1 designates a stepped surface with near-normal steps and ridges. R6 designates a very smooth, shiny, and polished surface. |
| Infilling type and thickness- | Mineral coatings and infillings are identified by their appearance, color, hardness, reaction to dilute hydrochloric acid, and fluorescence in ultraviolet light. |
Sampling
Samples have been collected in the Cross Drift at the request of the individual Principal Investigators. The sample locations are shown on the full-periphery geologic maps and are listed in tabular form in Appendix 2. At the time of this report not all samples listed in Appendix 2, or shown on the full-periphery geologic maps had been surveyed and/or processed through the Sample Management Facility.
LITHOSTRATIGRAPHY
The lithostratigraphy of the Cross Drift is described using the nomenclature and unit divisions of Sawyer and others (1994) and Buesch and others (1996) (Figure 3). Lithologic descriptions record compositional data, rock color and texture, features of welding, secondary crystallization, and alteration, depositional features, and stratigraphic relationships. The percentages of pumice clasts, matrix phenocrysts, lithic fragments, and lithophysae (void spaces) are visual estimates determined using charts produced by the American Geological Institute. The percentages of matrix (including porosity) are subsequently computed by subtraction of the other rock components from 100 percent. Colors are determined on dry surfaces under conditions of tunnel lighting using a standard Munsell rock-color chart (Geological Society of America, 1991). Aspect ratio of pumice clasts are a measurement comparing length to width (i.e. 4:1). Sizes of pumice, lithophysae or other features are measured on the longest axis. Unless otherwise noted, all stratigraphic stationings are given at springline on the left wall of the tunnel and are referred to by metric position (for example, Sta. 10+66 refers to 1066 m from the intersection of the Cross Drift with the ESF North Ramp). Unit contacts were identified and recorded as the tunnel walls were mapped in 1998; contact locations were submitted as Milestone SPG470M4 in September, 1998, and are shown in Table 1.
Topopah Spring Tuff Pyroclastic-Flow Deposit
The Cross Drift exposes units of the Topopah Spring Tuff, a densely welded, pyroclastic flow that grades upward from a crystal-poor, rhyolitic composition to a crystal-rich, quartz-latite composition. Units exposed in the crystal-poor member of the Topopah Spring Tuff, include the Tptpul (Sta. 0+00 to 10+15), the Tptpmn (Sta. 10+15 to 14+44), the Tptpll (Sta. 14+44 to 23+26), and the Tptpln (Sta. 23+26 to 25+85). The lower portion of the crystal-rich transition subzone (Tptrl1) is exposed on the west side of the Solitario Canyon fault from Sta. 26+57.5 to 26+64 (right wall; this is the last exposure behind the TBM cutter head). This section provides summary rock unit and contact descriptions, outlines the stratigraphic and depositional features observed in the tunnel walls, and describes general features of welding, secondary crystallization, and alteration for the zones encountered in the Cross Drift.
Rock Unit and Contact Descriptions
Tunnel lithologies are described stratigraphically from the top of the unit down, regardless of Cross Drift stationing. Crystal-Rich Member Lithophysal Zone (Tptrl) The crystal-rich lithophysal zone is exposed in the Cross Drift from Sta. 26+57.5 to 26+64 (end of cleaned exposure on January 12, 1999). The upper faulted contact of the unit is bounded by west-dipping shears that are located at Sta. 26+57.5 on the right wall of the Cross Drift. The abundance of phenocrysts and the rock color and texture observed in this interval are consistent with these rocks being identified as the lower portion of the Tptrl1. In general, the moderately to densely welded, devitrified unit is composed of 5 to 15 percent pumice, 5 to 7 percent phenocrysts, 10 to 15 percent lithophysae, and 63 to 80 percent matrix. Pumice. Pumice clasts comprise from 5 to 15 percent of the unit. A thin swarm of clasts at Sta. 26+61.3 (0.25 m above right springline) may mark a flow-unit boundary. Pumice fragments are variably elongated (length:width of 3:1 to 12:1), range in size up to 13 cm (long axis), and are composed of a mix of grayish brown to grayish red (5YR4/2 to 10R4/2) devitrified and grayish orange-pink (10R8/2) vapor-phase material. Pumice clasts contain from 7 to 10 percent crystals of feldspar and subordinate biotite. Phenocrysts. Phenocrysts of feldspar and biotite form from 5 to 7 percent of the rock. Lithic Fragments. Lithic fragments were not observed in the Cross Drift exposures. Lithophysae. Ellipsoidal lithophysae form between 10 and 15 percent of the rock in the cleaned exposure. Most are well-formed, have aspect ratios of 1:1 to 1.5:1, and sizes that vary from 4 to 12 cm. From Sta. 26+64 to the TBM cutterhead (uncleaned exposure), lithophysae appear to be less abundant (3 to 7 percent), poorly formed, and slightly smaller (less than 7 cm). Rock Matrix. The moderately to densely welded, devitrified matrix forms between 63 and 80 percent of the rock, is pale red (5R6/2) and contains less than 20 percent vapor-phase alteration in the form of wisps and streaks. Alteration around vapor-phase features locally imparts a grayish red-purple (5RP5/2) color to the matrix. Vapor-Phase Features. Vapor-phase alteration is present in the form of spots, short stringers (<15 cm length), and partings. Ellipsoidal spots of 1 to 3 cm (long axis) comprise from 5 to 7 percent of the rock in the cleaned exposure. Partings occur on vertical spacings of 0.5 to 1.2 m and are formed of anastomosing stringers. A crudely formed parting coincides with the thin swarm of pumice observed at Sta. 26+61.3. Vapor-phase features are grayish pink (5R8/1; 5R8/2) and typically enclose a central streak of white minerals. Lower Contact. The lower contact of this unit is not exposed in the Cross Drift. Crystal-Poor Member Upper Lithophysal Zone (Tptpul) The crystal-poor upper lithophysal zone is exposed in two segments of the Cross Drift. The Cross Drift begins in the upper central portion of the zone and it exposes rocks of the central and lower portions of the zone from Sta. 0+00 to 10+15. The upper portion of the upper lithophysal zone is exposed in the hanging wall of the eastern strand of the SCFZ, from Sta. 25+90 to 26+57.5. In both exposures, the unit is moderately to densely welded, devitrified, and vapor-phase altered. In general, the rock appears grayish red-purple (5RP4/2) and contains 10 to 40 percent vapor-phase spots, stringers, and partings. The central and lower parts of the zone (Sta. 0+00 to 10+15) are composed of 0 to 15 percent pumice, 1 to 3 percent phenocrysts, 0 to 5 percent lithic fragments, 10 to 60 percent lithophysae, and 40 to 90 percent matrix. The upper part of the zone (Sta. 25+90 to 26+57.5) is composed of 5 to 15 percent pumice, 2 to 5 percent phenocrysts, less than 1 percent lithic fragments, 3 to 20 percent lithophysae, and 60 to 90 percent matrix.
Tptpul Exposures from Sta. 0+00 to 10+15: Pumice. The abundance of pumice clasts is variable comprising 0 to 15 percent of the unit. Pumice clasts are typically smaller than 5 cm but range up to 20 cm. Between Sta. 0+00 and 1+50, pumice clasts are as large as 30 cm. Clasts are grayish-red (10R5/2), dark reddish brown (10R 3/4), and very light gray (N8). Pumice clasts are flattened, and aspect ratios are typically 4:1, but are up to 8:1. Phenocrysts. Phenocrysts include feldspars (plagioclase and sanidine) and minor, partly oxidized biotite, that form 1 to 3 percent of the rock. Lithic Fragments. Lithic fragments comprise 0 to 5 percent of the rock, are generally blocky and subangular to subrounded, and scattered throughout the unit, usually aligned subparallel to lithostratigraphic layering. Fragments are generally between 1 and 2 cm but range up to 8 cm and include light gray to white (N7 to N9) volcanic rock and grayish black to blackish red (N2 to 5R2/2) volcanic rock. Lithophysae. Lithophysae generally comprise 25 to 40 percent of the rock but as much as 60 percent locally in the upper third of the zone and as low as 10 percent near the contact with the underlying middle nonlithophysal zone. Lithophysae range in size from less than 1 cm to 80 cm. Generally, in the upper third of the zone, lithophysae are smaller and make up a greater percentage of the rock. In the remainder of the zone, lithophysae are larger and make up a smaller proportion of the rock. Up to Sta. 2+90, lithophysae are typically between 4 and 10 cm with a few ranging up to 40 cm and comprise 20 to 30 percent of the rock. There is a marked decrease in the number of lithophysae between Sta. 3+25 and 3+45. Lithophysae ranging from 4 to 10 cm comprise approximately 10 percent of the rock. Starting at Sta. 4+30, the maximum size of lithophysae gradually increases from 30 cm to a maximum of 80 cm near the base of the zone. A crude bimodal distribution in the size of lithophysae is apparent starting at Sta. 4+60. From Sta. 6+90 to the base of the zone, the bimodal distribution is somewhat more evident with lithophysal cavities ranging in size from 4 to 10 cm and 20 to 50 cm. At Sta. 6+20, lithophysae comprise 20 percent of the rock. From this point toward the base of the zone, lithophysae gradually decrease to approximately 15 percent at Sta. 7+55 and 10 percent at Sta. 10+00. Aspect ratios are typically 1:1 to 5:4 with a few individual cavities up to 3:1 locally. Many of the larger cavities have irregular boundaries and appear to have formed from a number of coalesced lithophysal cavities. The lithophysae have pale red purple (5RP7/2) alteration margins from 1 to 5 mm wide. Vapor phase minerals coat the interior surfaces of lithophysal cavities. Rock Matrix. The matrix/groundmass is a moderately to densely welded, devitrified, crystal- poor, ash-flow tuff. The matrix is gray red-purple (5RP5/2) to light brown (5YR6/3). Color changes occur along irregular but distinct boundaries that form near vertical zones. Color changes commonly occur near fractures or vapor-phase partings with gray red-purple adjacent to the discontinuity. Vapor-Phase Features. Vapor-phase alteration occurs as spots, short stringers, and partings of very light gray to grayish pink (N8 to 5R8/1) material. Vapor-phase material comprises 10 to 40 percent of the rock and are most abundant in the upper half of the zone. Spots predominate over other vapor-phase features throughout the zone. They are typically ellipsoidal to spherical and 1 to 5 cm in diameter. Vapor-phase stringers are generally 10 cm in length and are more numerous in the lower third of the zone. Vapor-phase partings are generally irregular and less than 50 cm long with the rare exception exceeding 3 m. They occur sporadically within limited stratigraphic intervals throughout the zone except near the contact with the underlying lower lithophysal zone. Beyond Sta. 9+35, vapor-phase partings become increasingly more common and longer, up to 1 to 2 m. Lower Contact. The contact between the crystal-poor upper lithophysal zone and underlying middle nonlithophysal zone is located at Sta. 10+15. This position marks the top of a new stratigraphic subdivision of the middle nonlithophysal zone, termed the transition subzone (Tptpmn4), proposed by Buesch and Spengler (1998). The Tptpul-Tptpmn contact is located where there is a downward increase in the abundance of well-developed vapor-phase partings and an increase in the percentage of light-brown matrix (Buesch, 1998).
Tptpul Exposures from Sta. 25+90 to 26+57.5 A fault-bounded portion of the Tptpul is exposed in the hanging wall of the SCFZ. The upper portion of the exposure is faulted against a polylithologic matrix-supported breccia, that contains clasts derived from the crystal-poor lower nonlithophysal zone of the Tiva Canyon Tuff (Tpcpln). The lower portion of the exposure is faulted against the crystal transition subzone, the Tptrl1. The interval is brecciated by movement along the SCFZ from Sta. 25+90 to 26+00. The unit is moderately to densely welded, devitrified, vapor-phase altered, and composed of 5 to 15 percent pumice, 2 to 5 percent crystals, less than 1 percent lithic clasts, 1 to 20 percent lithophysae, and 59 to 92 percent matrix. Pumice. Pumice fragments comprise from 5 to 15 percent of the unit. Clasts are axiolitically devitrified and have grayish red (5R4/2) rims that enclose a very light gray to white (N8 to N9) interior. Most clasts are 2 to 5 cm (long axis). Clasts of quartz latite pumice were not observed in this interval. Phenocrysts. Throughout most of the exposure, phenocrysts of euhedral feldspar and minor biotite form from 2 to 3 percent of the rock. Crystal content is approximately 5 percent in the upper part of the unit adjacent to the Solitario Canyon fault. Lithic Fragments. Lithic fragments were not observed in these exposures. Lithophysae. Lithophysae vary in abundance throughout the exposed interval as shown in Table 2. From Sta. 25+90 to 26+05, lithophysae are difficult to discern due to fracturing and brecciation of the rock adjacent to the SCFZ. Throughout the remainder of the unit, lithophysal cavities are predominantly spherical to ellipsoidal and vary in size between 5 and 25 cm. Lithophysae are poorly developed in two intervals (Sta. 26+20 to 26+35 and Sta. 26+46.5 to 26+57.5). In these locations, the rock appears locally shattered and voids have irregular shapes. Intervals of shattered rock are interlayered with intervals of intact rock containing well-developed cavities. Lithophysae typically are enclosed in halos of very light gray to grayish pink (N8 to 5R8/1) vapor-phase material. Rock Matrix. Devitrified matrix is a mix of pale red-purple (5RP5/2; 5RP6/2) and pale red (5R6/2). From Sta. 25+90 to 26+30 and from Sta. 26+46 to 26+57.5, pale red-purple colors predominate; pale red matrix is more abundant from Sta. 26+30 to 26+46. Vapor-phase material occurs throughout the matrix as wisps and blebs. Shard texture has been destroyed by secondary crystallization throughout the unit. Vapor-Phase Features. Vapor-phase alteration occur as spots, short stringers, and partings of very light gray to grayish pink (N8 to 5R8/1) material. In the upper part of the unit (Sta. 25+90 to 26+20), vapor-phase material comprises 25 to 40 percent of the rock. Spots, which are predominate over other features in this interval, typically have ellipsoidal to spherical shapes and diameters of 1 to 3 cm; stringers of approximately 10 cm length are present in the lower part of the interval. From Sta. 26+20 to 26+57.5, vapor-phase material comprises 10 to 30 percent of the rock. Spots typically have ellipsoidal to spherical shapes and diameters of 0.5 to 2.5 cm. Stringers of 5 to 10 cm length vary in abundance from rare to predominant. Stringers and partings are particularly prominent features from Sta. 26+20 to 26+35. Remnants of fractures with tubular features can be seen in the brecciated interval near the Solitario Canyon fault. Lower Contact. The lower stratigraphic contact of the crystal-poor upper lithophysal zone is not exposed in this interval. The unit, which is truncated by a fault at Sta. 26+57.5, is juxtaposed against the crystal transition subzone of the crystal-rich lithophysal zone of the Topopah Spring Tuff.
Middle Nonlithophysal Zone (Tptpmn) The Cross Drift exposes the middle nonlithophysal zone from Sta. 10+15 to 14+44. In general, the moderately to densely welded, devitrified and variably vapor-phase altered unit is composed of less than 5 to 10 percent pumice (locally 25 to 35 percent), 1 to 2 percent phenocrysts, 1 to 2 percent lithic fragments, 0 to 1percent lithophysae, and 85 to 93 percent matrix. Vapor-phase spots, stringers, and partings comprise from 1 to 15 percent of the rock. Smooth, high-angle fractures are typical of the zone. The interval from Sta. 11+85 to 13+07, which contains approximately 1 percent lithophysae (Photograph 1), represents the lithophysae-bearing subzone (Tptpmn2) of Buesch and others (1996). Pumice. Pumice fragments are difficult to discern, probably because their texture has been destroyed by recrystallization of the rock matrix. Throughout most of the unit, pumice fragments are smaller than 25 mm and form from less than 5 to perhaps 10 percent of the rock. An exception occurs from Sta. 14+03 to 14+37, where a swarm of large clasts comprises between 25 and 35 percent of the unit (Photograph 2). In this interval, which occurs approximately 1.5 to 2 m stratigraphically above the lower contact of the middle nonlithophysal zone, most pumice fragments occur in sizes up to10 cm, with clasts of 20 cm occasionally present. In general, clasts are composed of a mix of granophyrically devitrified and vapor-phase altered material and have colors that include grayish orange-pink (5YR7/2), light brownish gray (5YR6/1), pale red (10R6/2), and grayish red-purple (5RP4/2). Phenocrysts. Phenocrysts of predominant feldspar and subordinate biotite comprise 1 to 2 percent of the zone. Lithic Fragments. Subangular lithic fragments, typically smaller than 2 cm, form less than 2 percent of the unit. Lithic fragments include predominant very light gray (N8), aphanitic volcanic rock and minor white (N9) and pale red (5R6/2) foliated rhyolite. Lithophysae. Lithophysae are absent or form a minor portion of the middle nonlithophysal zone in most of the Cross Drift exposures. Lithophysae comprise 3 percent of the rock immediately adjacent to the contact with the overlying upper lithophysal zone at Sta. 10+15. Below the contact, the number of lithophysae decreases rapidly to less than 1 percent. Lithophysae are generally 5 to 25 cm, poorly formed with irregular boundaries and sometimes bisected by vapor-phase partings. Aspect ratios are 2:1 to 3:1. Lithophysae also are present from Sta. 11+85 to 13+07, where they form approximately 1 percent of the unit. Here, the lithophysal cavities are poorly developed and occur as irregular or gash-like voids typically smaller than 25 cm (but ranging up to 40 cm) that are enclosed in elliptical areas of shattered rock. Rock Matrix. The moderately to densely welded, devitrified rock matrix contains a variable amount of vapor-phase alteration. Recrystallization has destroyed most shard texture. From Sta. 10+15 to 11+85 the rock matrix is predominantly light brown (5YR6/3) with roughly vertical zones of grayish red-purple (5RP5/2). Throughout most of the lithophysae-bearing subzone (Sta. 11+85 to 13+07), grayish red-purple (5RP5/2) material comprises between 50 and 90 percent of the matrix, with the remainder formed of light brown (5YR6/3) material. An exception occurs near Sta. 12+75, where the matrix is predominantly light brown. In contrast, most of the lower portion of the zone (Sta. 13+07 to 14+44) has a matrix that is predominantly light brown; grayish red-purple coloration is associated only with vertical fractures and vapor-phase features in this interval. However, in the 2 m interval stratigraphically above the lower contact, the proportion of grayish red-purple material increases to 50 to 75 percent. Vapor-Phase Features. Spots, stringers, and partings of vapor-phase material occur in varying proportions throughout the middle nonlithophysal zone. Vapor-phase features are grayish pink (5R8/2) and may enclose a central streak of white (N9) minerals. From Sta. 10+15 to 11+30, vapor-phase partings are generally poorly formed and less than 50 cm long, vapor-phase spots that comprise 2 to 3 percent of the rock. From Sta. 11+30 to 11+85, the maximum length of vapor-phase partings increases gradually from 1 to 3 m. The lithophysae-bearing subzone (Sta. 11+85 to 13+07) contains 7 to 10 percent vapor-phase spots and numerous continuous partings that occur at a vertical spacing of 25 to 40 cm (Photograph 3). Spherical spots vary from 5 to 7.5 cm in size (a population of smaller features [1 to 1.5 cm] also is present). Partings are rare in the lower portion of the zone (Sta. 13+07 to14+44) and where present, occur at a vertical spacing of 50 to 100 cm. Spots, which comprise 1 to 3 percent (locally 4 to 6 percent) of the unit in this interval, are mostly smaller than 3 cm. Lower Contact. The contact between the middle nonlithophysal zone and underlying lower lithophysal zone occurs at Sta. 14+44. The contact is marked by a downward increase in lithophysae from 0 to 2 percent above to 3 to 7 percent below. A prominent vapor-phase parting occurs approximately 1.25 m stratigraphically above the contact. The percentage of vapor-phase spots increases sharply below this parting, from less than 5 percent above to between 20 and 30 percent below.
Lower Lithophysal Zone (Tptpll) The lower lithophysal zone is exposed along the Cross Drift from Sta. 14+44 to 23+26. In general, the moderately to densely welded, devitrified, and vapor-phase altered unit is composed of 3 to 7 percent pumice (locally 10 to 35 percent), 1 to 2 percent phenocrysts, 1 to 5 percent lithic fragments (locally 12 to 15 percent), 5 to 30 percent lithophysae (locally 1 to 5 percent), and 56 to 90 percent matrix. Throughout most of the unit, vapor-phase spots, stringers, and wisps comprise between 3 and 12 percent of the rock. In several intervals, however, vapor-phase alteration products form 15 to 40 percent of the rock. Pumice. Pumice clasts are difficult to discern throughout most of the zone because they have colors that are similar to the rock matrix (dust on the tunnel walls also makes identification difficult). Pumice typically forms between 3 and 7 percent of the rock. Exceptions occur at Sta. 21+50 to 22+82, where pumice content is 10 to 15 percent and near Sta. 23+00, where pumice comprises 25 to 35 percent of the unit. It is unknown if the higher content at Sta. 23+00 is a primary depositional feature or an artifact of the degree to which pumice clasts were preserved during silicification of the rock matrix at this location. From Sta. 14+44 to 14+62, pumice clasts are devitrified, mostly smaller than 2.5 cm (long axis), have aspect ratios of 4:1 to 8:1 (length:width), and are pale brown (5YR6/2). Between Sta. 14+62 and 17+23, clasts are a mix of granophyrically devitrified and vapor-phase altered material; larger pumice fragments may contain small spherulites. Pumice in this interval range from 1.5 to 21.5 cm and are moderately elongate (4:1 to 8:1), with mottled interiors of grayish red (5R4/2) and light gray (N7) enclosed by a thin (1 mm), moderate brown (5YR4/4), devitrified rim. In the interval from Sta. 17+23 to 22+82, pumice clasts are typically 1 to 4 cm, but clasts occur to 12 cm. These fragments are variably elongated (2:1 to 10:1) and composed of a mix of devitrified (occasionally spherulitic) and vapor-phase altered material. Clasts have mottled interiors of dark yellowish brown (10YR4/2) and grayish pink (5R8/2) that are enclosed by a millimeter-thick grayish brown rim (5YR3/1). Pumice fragments near the base of the unit (Sta. 22+82 to 23+26) are mostly smaller than 7.5 cm (long axis), slightly elongated (3:1 to 5:1), and either devitrified (granophyric and spherulitic) or mixed devitrified and vapor-phase altered. Devitrified clasts have mottled pale red (5R6/2) and grayish red (5R3/1) colors; vapor-phase materials are grayish pink (5R7/2; 5R8/2). Where the rock matrix is silicified (near Sta. 23+00) pumice fragments have grayish red, devitrified cores enclosed in a moderate orange-pink (5YR8/3) selvage of argillic (?) or vapor-phase (?) altered material. Phenocrysts. Phenocrysts of feldspar and subordinate fresh to partly oxidized biotite comprise from 1 to 2 percent of the rock. Lithic Fragments. Lithic fragments typically are present in abundances that vary from 2 to 5 percent. Exceptions occur at Sta. 15+20, where lithic clasts form 12 to 15 percent of the unit, at Sta. 22+18, where lithic fragments comprise 15 to 25 percent of the rock (see Photograph 4 on p. 24), and from Sta. 21+31 to 22+82, where lithic clasts form 1 to 3 percent of the rock. In general, lithic fragments are angular to subrounded and smaller than 4.5 cm; a few clasts exceed 7.5 cm. In the comparatively lithic-rich zones, lithic fragments are coarser, ranging in size upward to 20 cm. Lithics are dominated by fragments of very light gray (N8), sugary textured hypabyssal rock, with subordinate light gray (N7; N8) and pale red (10R6/2) fragments, flow-foliated rhyolite, and pale yellowish brown (10YR6/2; 10YR7/2) porphyritic lava. Lithophysae. Lithophysae vary in size, shape, and abundance throughout the zone as shown in Table 3 (see p. 27). For ease of description, the lower lithophysal zone is divided into seven intervals with generally similar lithophysae size and abundance (interval boundaries do not have stratigraphic significance). The reader is cautioned that these descriptions are very general; more specific information is provided in Table 3. Sta. 14+44 to 14+62. Throughout this interval, lithophysae abundance, size, and shape are comparatively constant. Lithophysae comprise 3 to 7 percent of the rock in this interval. They occur as ellipsoidal to lenticular cavities that are moderately to well formed. Most have diameters of approximately 20 cm and are equant to elongate (length:width of 1:1 to 2:1), but cavity diameters range from 7 to 30 cm. Lithophysae in this interval commonly are enclosed by 1 to 3 cm of vapor-phase material. Sta. 14+62 to 15+98. This interval is characterized by downward changes in lithophysae abundance, size, shape, and degree of development. Lithophysae decrease in abundance from 12 to 15 percent in the upper (stratigraphically) part of this interval to 5 to 10 percent in the lower part. Cavities change downward from predominantly well-formed lenticular and ellipsoidal voids to a mix of well-formed lenticular and ellipsoidal voids and poorly-formed irregular shapes to predominantly poorly-formed irregular shapes. From Sta. 15+16 to 15+98, large (>75 cm), irregularly shaped lithophysae are present. Many of these cavities contain prismatic fractures and have blocky interiors; they are associated typically with intense fracturing of the surrounding rock that leads to overbreak in the tunnel walls and back and with numerous discontinuous, high-angle fractures terminating at the cavity. Overbreak makes it difficult to determine cavity dimensions. The well-developed lithophysae near the top of the interval have thick (2 to 3 cm) linings of vapor-phase minerals. In contrast, the large, irregular cavities generally are devoid of vapor-phase crystals. Sta. 15+98 to 16+30. This interval is characterized by abundant, irregularly shaped lithophysae that comprise from 20 to 30 percent of the rock (Photograph 5). The abundance of cavities is difficult to estimate due to extreme shattering and overbreak. Although some cavities have blocky, prismatic interiors that contain variably thick coatings of vapor-phase minerals, most are broken and rubbly with thin or no vapor-phase coatings. Voids range up to approximately 130 cm; subordinate ellipsoidal to gash-like cavities vary from 5 to 40 cm. The interval also hosts abundant discontinuous, high-angle fractures that do not have associated vapor-phase alteration. Sta. 16+30 to 17+23. The abundance of lithophysae changes downward through this interval; however, size and shape remain comparatively constant. Lithophyase comprise from 5 to 10 percent of the rock near the top of the interval and from 3 to 5 percent near the base. In general, lithophyase are predominantly moderately to well-formed ellipsoidal, lenticular, or gash-like cavities with subordinate, poorly formed, irregular voids. Irregular cavities are larger (>50 cm) and are associated with discontinuous, high-angle fractures. Lithophysae generally have linings of vapor-phase minerals that vary from less than 1 to 3 cm thickness. Sta. 17+23 to 17+97. This interval is characterized by moderately to moderately well-developed lithophysae (Table 3) that vary in abundance from 10 to 20 percent. Most cavities have ellipsoidal, spherical, or irregular/blocky shapes that range in size from 6 to 75 cm (Photograph 6). Sta. 17+97 to 22+82. This interval contains predominantly ellipsoidal, spherical, lenticular, and gash-like lithophysae that vary in size from 5 to 50 cm. Voids are moderately to well formed throughout most of the interval; however, lithophysae are less well-developed from Sta. 19+50 to 20+65. Lithophysae decrease in abundance downward from 7 to 10 percent (Sta. 17+97 to 19+50) to 4 to 6 percent (Sta. 19+50 to 21+31) (Photograph 7) to 3 to 7 percent grading downward to 1 to 3 percent (Sta. 21+31 to 22+82). Most cavities contain comparatively thin, drusy coatings of vapor-phase minerals.
Sta. 22+82 to 23+26. Lithophysae that range from 15 to greater than 100 cm, typically form 3 to 5 percent of the rock in this interval. Smaller cavities tend to be lenticular or gash-like features, whereas larger cavities generally have ellipsoidal to spherical or irregular shapes (the latter created by prismatic fracturing). Lithophysae have drusy coatings of vapor-phase minerals. Toward the base of the interval, lithophysae become more gash-like in appearance and are poorly formed, eventually becoming irregularly fractured areas without distinct voids.
Rock Matrix. The moderately to densely welded matrix of the lower lithophysal zone is a mix of devitrified and vapor-phase altered material. Throughout most of the zone, shard texture has been destroyed by secondary crystallization. An exception occurs near the base of the zone (Sta. 23+00) where silicification locally preserves shard texture. In general, matrix in the lower part of the zone (Sta. 17+23 to 23+26) is a mottled mix of pale red (5R6/2; 5R5/2) and light brown (5YR6/3; 5YR6/4), with variable amounts of grayish red-purple (5RP5/2). The matrix is dusky yellowish brown (10YR3/2) where silicified. Wisps and blebs of vapor-phase products occur locally as grayish orange-pink (5YR7/2) to very light gray (N8) material. In the upper part of the zone (Sta. 14+62 to 17+23), the matrix varies from comparatively homogeneous pale red (10R6/2) to mottled mixtures of pale red and grayish red-purple or pale red and light brown (5YR6/4). At the top of the zone (Sta. 14+44 to 14+62), the matrix is primarily devitrified with a pale brown (5YR6/2) color. The faces of high-angle fractures that occur in this interval have a pale red (5R6/2) color.
Vapor-Phase Features. Vapor-phase alteration occurs in the form of spots, stringers, and partings, the abundance of which varies throughout the zone as shown in Table 4. Five distinctive intervals can be identified within the Tptpll based on the vapor-phase alteration features that are present. Three of these are distinguished by abundant (more than 20 percent) vapor-phase spots (Sta. 14+44 to 14+62; Sta. 17+97 to 18+80; Sta. 21+31 to 22+82) (see Photograph 8 on p. 30). A fourth interval (Sta. 17+23 to 17+97) is characterized by abundant vapor-phase partings. The base of the unit (Sta. 22+82 to 23+26) is typified by vapor-phase spots with large (>20 cm) diameters. Throughout the remainder of the zone, vapor-phase spots typically smaller than 15 cm and subordinate stringers form from 3 to 12 percent of the rock (Photograph 9). Vapor-phase features are pinkish gray (5R8/2; 10R8/2) to light or very light gray (N7; N8); spots, stringers, and partings may contain a thin (<1 mm), central streak of white minerals. Lower Contact. The contact between the lower lithophysal zone and underlying lower nonlithophysal zone (Sta. 23+26) is distinguished by downward decreases in lithophysae from 3 to 5 percent above to 1 to 3 percent below and in vapor-phase spots from 3 to 5 percent above to less than 1 percent below, and a downward increase in the number of smooth, high-angle fractures. Photograph 10 (p. 33) shows a fault near the lower contact at Sta. 22+38.Lower Nonlithophysal Zone (Tptpln) The Tptpln, exposed from Sta. 23+26 to 25+85 (excepting the interval from Sta. 25+40 to 25+62, which is covered by the mapping platform and shotcrete), comprises moderately to densely welded, devitrified pyroclastic-flow material. It is generally composed of 3 to 20 percent pumice, 1 to 2 percent phenocrysts, 3 to 7 percent lithic fragments, 0 to 5 percent lithophysae, and 66 to 93 percent matrix. Vapor-phase alteration products form a minor component of the rock in some portions of the unit. Rocks of the lower nonlithophysal zone vary from a heterogeneous mix of grayish red and grayish orange pink (5YR7/2) to comparatively homogeneous pale red, light brown, pale brown, or grayish brown (5YR6/4). In proximity to the SCFZ, the unit is brecciated and altered. In this area, the breccia matrix varies from moderate reddish brown to grayish orange pink to pale red; breccia clasts are locally bleached to very light gray adjacent to the fault plane. Pumice. The abundance of pumice clasts varies throughout the zone partly due to depositional processes and partly due to the extent to which they were preserved during cooling, compaction, and devitrification. Clast abundances range from 2 to 3 percent up to 15 to 20 percent. A swarm of clasts at Sta. 23+47.5 (right wall) contains 25 to 30 percent pumice. Pumice clasts are slightly to moderately deformed and elongated (aspect ratios of 3:1 to 5:1) throughout the zone. Most clasts range from 1 to 6 cm as measured along their long axis; however, clasts up to 10 cm long occur in the swarm at Sta. 23+47.5. Pumice clasts typically are devitrified and contain only minor vapor-phase alteration. Devitrification textures include predominant granophyric and subordinate axiolitic and spherulitic. In the lower portions of the unit (Sta. 25+13 to 25+40), clasts have a thin, moderate brown outer selvage (5YR4/4) that encloses a mottled interior of moderate brown devitrified material and light gray (N7) vapor phase material. Elsewhere, clasts occur in mottled shades of grayish red (5R4/1; 5R4/2) or pale red (5R6/2). Pumice clasts commonly contain a few percent of sanidine, plagioclase, and biotite. Phenocrysts. Phenocrysts include predominant sanidine and plagioclase and subordinate fresh to partially oxidized biotite. Lithic Fragments. Subangular to subrounded lithic clasts are found scattered throughout the rock matrix and concentrated in discontinuous zones that parallel bedding. In general, the percentage of lithic fragments tends to be higher in the lower part of the zone, 5 to 7 percent from Sta. 24+50 to 25+85 (Photograph 11) vs. 3 to 5 percent from Sta. 23+26 to 24+50, although exceptions occur. Clast abundances can exceed 10 percent in certain zones. Most clasts range from 0.5 to 10 cm in average diameter, but they may exceed 20 cm. Lithics are dominated by fragments of light gray to white (N7-N9), sugary, hypabyssal rock that contains sparse crystals of feldspar and biotite. Subordinate types include foliated, devitrified lava in shades of white and pale red (5R6/2); grayish orange pink to pale red (5YR7/1, 5R6/2, 5YR6/1) porphyritic rock containing 25 to 40 percent crystals of feldspar, partly oxidized biotite, and minor altered hornblende (?); light gray (N7), with few phenocrysts; and rare breccia in which foliated lava and porphyritic rock clasts are enclosed in a very pale orange (10YR8/1) matrix. Lithophysae. Although lithophysae occur in two short intervals of the lower nonlithophysal zone, they typically are absent or form less than 1 percent of the rock throughout most of the Cross Drift. Lithophysae are present as well-formed voids that comprise between 1 and 3 percent of the unit from Sta. 24+30 to 25+13. These cavities are lenticular to gash-like features that contain drusy coatings of vapor-phase minerals. Vapor-phase alteration of the rock matrix adjacent to the void typically does not occur. Voids range from 20 cm to greater than 100 cm as measured along their long axes. Lithophysae also are present from Sta. 23+50 to 23+73. In this interval, they are moderately well formed lenticular, gash-like, spherical, or irregular features that vary in size from 25 to 100 cm. Void content ranges from 2 to 3 percent between Sta. 23+50 and 23+67 and from 2 to 5 percent from Sta. 23+67 to 23+73. In other portions of the lower nonlithophysal zone, lithophysae are present as gash-like features up to 50 cm long that occur along vapor-phase stringers. Incipient lithophysae, characterized by spherical or lenticular areas of fracturing that lack a distinct void, occur from Sta. 23+26 to 23+50 and Sta. 24+14 to 24+30. Rock Matrix. The rock matrix within the lower nonlithophysal zone is composed primarily of devitrified material with minor vapor-phase alteration. Shard textures typically can be discerned, but the degree of preservation varies from poor to good. Devitrified matrix appears in shades of pale brown (5YR6/2), light brown (5YR6/4), moderate yellowish brown (10YR5/3), dark yellowish brown (10YR3/2), and pale red (10R5/2; 10R6/2). Vapor-phase material forms a minor component of the matrix (0 to 30 percent) and where abundant, imparts a grayish orange-pink (5YR7/2) coloration to the rock. Vapor-Phase Features. Vapor-phase material also occurs as spots and stringers of grayish pink (5R8/2; 5R8/1) to grayish orange-pink (5YR7/2) that may enclose a central band of white (N9) vapor-phase minerals. Spots and en echelon stringers of 10 to 50 cm length that occur on vertical spacings of 20 to 50 cm form 7 to 10 percent of the rock from Sta. 23+26 to 23+55, 2 to 5 percent of the rock from Sta. 23+55 to 24+25 and Sta. 25+00 to 25+13, and 1 to 2 percent of the rock from Sta. 24+25 to 25+00. Lower Contact. The lower stratigraphic contact of the lower nonlithophysal zone is not exposed in the Cross Drift because the unit is truncated by the Solitario Canyon fault, which cross-cuts the Cross Drift at Sta. 25+83. The rock of the lower nonlithophysal zone is brecciated in the footwall of the fault for a distance of more than 23 m along the tunnel wall and is altered by vapor and deuteric fluids as described in the ensuing section.
Solitario Canyon Fault Zone (SCFZ)
The eastern strand of the SCFZ occurs at Sta. 25+85. As described in more detail in the section entitled“Significant Faults”, the fault zone is characterized by a wide zone of brecciation and cataclasis (see Photograph 12 on p. 36). This section describes the rock units that occur in proximity to the fault. Hanging Wall Relations. A 10 to 20 cm thick basal interval composed of cataclasite and clay gouge overlain by calcite-bearing material rests atop the plane of the Solitario Canyon fault. This zone is overlain by a polylithologic breccia composed of clasts of the crystal-poor member of the Tiva Canyon Tuff. The breccia is truncated by a west-dipping shear plane at Sta. 25+90, above which lies brecciated rock of the crystal-poor member of the Topopah Spring Tuff. The polylithologic breccia contains several types of clasts, all of which are from the lower portions of the crystal-poor Tiva Canyon Tuff (Photograph 13). The breccia is competent, poorly sorted, and unaltered. Breccia fragments contain 2 to 3 percent phenocrysts of feldspar, are subangular to subrounded, and range in size from less than 5 to 100 cm (a larger boulder approximately 40 by 200 cm that occurs below springline on the left rib has smooth faces similar to those characteristic of the columnar subzone). In general, the breccia is clast-supported, although local areas are supported by a crushed rock matrix. Breccia clasts are primarily composed of moderate to densely welded, devitrified pyroclastic-flow material that has a medium-light to very light gray color (5YR6/1 to 5YR8/1). Pumice fragments included in these clasts are predominantly very dusky red (10R2/2; 10R3/2) and spherulitically devitrified, although moderate orange-pink (5YR7/4), argillically altered pumice fragments form a minor population. The breccia also contains fragments of pale brown (5YR5/2), densely welded, and devitrified rock with abundant medium to medium-light gray (N5; N6; 5YR6/1) spherulites and fragments of dark yellowish orange (10YR6/6), partly welded, argillically altered rock containing moderate orange-pink (10R7/4), argillically altered pumice and grayish orange-pink (5YR7/2), vapor-phase altered pumice. Neither clasts of crystal-poor lithophysal Tiva Canyon Tuff, nor any units that lie stratigraphically below the base of the Tiva Canyon Tuff, are observed in the breccia (see Figure 3). The lithologies identified in the polylithologic breccia are consistent with rocks that occur near the contact of the crystal-poor lower nonlithophysal zone and underlying vitric zone of the Tiva Canyon Tuff. Similar rock types were described near Sta. 7+77 of the ESF North Ramp (Barr and others, 1996). The crystal-poor upper lithophysal zone of the Topopah Spring Tuff is exposed in the hanging wall of the SCFZ from Sta. 25+90 to 26+57.5 (right rib; see Rock Unit and Contact Descriptions for a complete description). The rocks in this area are shattered near the shear plane at Sta. 25+90, but they become progressively less broken with distance from the fault. Although brecciation is locally extensive, the rock has not been altered or significantly disrupted by translation of the brecciated clasts. From Sta. 26+00 to 26+57.5, the rocks are comparatively intact. Two observations clarify the stratigraphic position of this interval of the upper lithophysal zone: (1) The phenocryst content decreases slightly downward, from approximately 5 percent at Sta. 25+90 to 2 to 3 percent at Sta. 26+05; (2) The interval contains zones of intact rock with well-developed, ellipsoidal lithophysal vugs that are interlayered with zones of shattered rock with poorly developed, irregularly shaped lithophysae (Photographs 14A and 14B). The rocks exposed in this interval of the Cross Drift are generally similar to those described in the ESF North Ramp from Sta. 17+97 to 19+05 (Barr and others, 1996). The North Ramp interval also contains zones of shattered rock with poorly formed, irregularly shaped lithophysae (e.g., Sta. 18+70 to 18+78, 1.5 m above right invert) that are separated by comparatively unfractured intervals that contain well-developed, ellipsoidal lithophysae (e.g., Sta. 18+53-18+56, 1.5 m above right invert). Based on these criteria, the Tptpul interval exposed in the hanging wall of the Solitario Canyon fault in the Cross Drift is lithologically equivalent to the uppermost portion of the upper lithophysal zone in the North Ramp. Footwall Relations. Brecciated and altered rocks of the crystal-poor lower nonlithophysal zone of the Topopah Spring Tuff are exposed in the footwall of the Solitario Canyon fault. Footwall exposures are poor in proximity to the fault because shotcrete and lagging were used extensively to stabilize the tunnel walls. The rock, which is brecciated in exposures from Sta. 25+62 to 25+85, is intensely fractured from Sta. 25+72 to 25+85 (these observations were made near the right invert). Oxidation and alteration of the brecciated footwall is evident only in the 2 meters nearest the fault plane. Adjacent to the fault, the rock is strongly altered and fracture surfaces are coated with moderate reddish brown (10R4/6) oxidized material. Breccia clasts are bleached to a medium-light to very light gray (N6 to N8) and are enclosed in a very pale orange (10YR8/1) matrix. Due to the intense alteration, it is difficult to discern the nature of the matrix, but it appears to contain a component of vapor-phase material. With increasing distance from the fault plane, the matrix changes to shades of pale red (10R6/1; 10R6/2) and is composed mostly of devitrified material.
Stratigraphic and Depositional Features
A prominent flow unit boundary is indicated near the base of the middle nonlithophysal zone (Sta. 14+03) by a sharp increase in the abundance and size of pumice clasts. The swarm of large pumice, which extends to Sta. 14+37, occurs approximately 1.5 to 2 m stratigraphically above the contact between the middle nonlithophysal zone and the underlying lower lithophysal zone. Pumice fragments contained within the swarm are mildly elongated (aspect ratios of 3:1 to 5:1) and mostly smaller than 10 cm (long axis), although they range upward to 20 cm. Pumice content is approximately 25 to 35 percent within the swarm. A thin pumice swarm was described in a similar stratigraphic position (i.e., 2 m above the Tptpmn-Tptpll contact) in the ESF Main Drift at Sta. 54+00, 0.25 m below springline and at Sta. 57+13, 1.5 m below springline (Albin and others, 1997). A zone of large lithic fragments at Sta. 15+20 marks another possible flow unit boundary. Dust on the tunnel walls obscures the stratigraphic relations of this interval. Zones containing abundant, large lithic fragments occur within the lower lithophysal and lower nonlithophysal zones. Examples can be observed at Sta. 22+15 (right rib) and at numerous locations in the interval from Sta. 24+14 to 25+40. Typically these zones occur as discontinuous pods or lenses. They are laterally discontinuous, suggesting that these lithic-rich pods did not form at a flow unit boundary (i.e., at the base of a pyroclastic flow). Instead, segregation features of this type are likely to have formed within a pyroclastic flow, due either to high flow velocity, surging flow, or processes of fluidization (Fisher and Schmincke, 1984).
Welding and Secondary Crystallization
The section of the Topopah Spring Tuff that is exposed in the Cross Drift is uniformly welded and pervasively devitrified. The degree and uniformity of welding of the rocks exposed in the Cross Drift is characteristic of ignimbrites that were deposited at high temperatures, welded rapidly, and cooled very slowly. The occurrence of flattened pumice clasts and low matrix- porosity values are indicative of post-depositional welding and compaction in high temperature pyroclastic flows. As a result of high emplacement temperatures and slow rate of cooling, partial or complete secondary crystallization (devitrification) of the hot and compacting glassy pyroclasts occurs in the interior of thicker cooling units (Fisher and Schmincke, 1984). Secondary crystallization includes devitrification and vapor-phase alteration. Variations in the intensity and style of vapor-phase alteration are used to define the zone boundaries within the crystal-poor member of the Topopah Spring. In general, the upper lithophysal zone is more intensely vapor-phase altered and contains more abundant, smaller, and more well developed lithophysae than the lower lithophysal zone. Subparallel vapor-phase partings are prominent in portions of the upper lithophysal, middle nonlithophysal, lower lithophysal zones.
Comparison of the Cross Drift and the ESF
The stratigraphic section traversed by both the ESF and the Cross Drift include the lower three quarters of the Tptpul, the entire Tptpmn, and the uppermost portion of the Tptpll. The lithologic character of the units exposed in the ESF and the Cross Drift is similar in terms of the welding, devitrification, and vapor-phase alteration. The pumice and lithic fragment content is similar in type, size, and color in the various stratigraphic intervals noted in both locations. The type, size, abundance, and character of vapor-phase features such as, vapor-phase alteration, vapor-phase partings, stringers and spots, and lithophysae are similar in comparable stratigraphic intervals in the ESF and the Cross Drift. The are some notable differences however. The lithophysae-bearing subzone of the middle nonlithophysal zone (Tptpmn2) does not appear in the ESF. The zone does appear in the Cross Drift but is poorly developed. There is a well-defined bimodal distribution in the size of lithophysal cavities in the upper lithophysal zone exposed in the ESF. In the Cross Drift, the bimodal distribution is discernible, but is poorly developed. The distribution of fractures, faults, and shears is very similar in the Cross Drift and the ESF in terms of frequency, character, and orientation (Albin and others, 1997 pg. 34) with one very notable exception. The intensely fractured zone (IFZ) so prominent in the Main Drift of the ESF does not occur in the Cross Drift. The fracture frequency histogram of the Cross Drift, Drawing OA-46-346, shows a modest increase in the fracture frequency between Sta. 13+20 and 14+20, averaging 2.9 fractures per meter as compared to an overall average of 2.2 fracture per meter in the middle nonlithophysal zone. This level does not approach the 4.2 fractures per meter in the intensely fractured zone in the Main Drift. The azimuths distribution, (see Drawing OA-46-346), in the zone of increased fracture frequency does not show the strong predominance of set 1 fractures that is seen in the intensely fractured zone in the Main Drift. Set 1 fractures make up approximately 45 percent of the fractures in this zone, while set 1 fractures in the IFZ in the Main Drift make up 73 percent of all fractures (Albin and others, 1997). The IFZ was not expected to be present in the Cross Drift. The distance of the nearest point from the Cross Drift to the IFZ is over 800 m. In addition, the exposure of the middle nonlithophysal zone in the Cross Drift is not on strike with the IFZ projected from the Main Drift. The exposure of the Tptpmn in the Cross Drift is adjacent to the North end of the Ghost Dance fault, which has relatively little offset. In the Main Drift, the IFZ is adjacent to the Ghost Dance fault where the offset is greater than 30 m. This could account for the lack of an IFZ in the Cross Drift. Figure 4 shows the relationship between the Cross Drift, Ghost Dance fault, and the IFZ of the ESF Main Drift. It is the opinion of the authors of this report that the small amount of offset on the Ghost Dance fault in the vicintiy of the Cross Drift has the greatest influence on the absence of the IFZ. However, another hypothesis has been suggested. An association has been proposed between the presence of an IFZ with sections of the middle nonlithophysal zone where the lithophysae-bearing subzone (Tptpmn2) has not developed (Buesch and Spengler, 1998). The lithophysae-bearing subzone (Tptpmn2), containing vapor-phase spots and partings is exposed in the Cross Drift, but is poorly developed (see Lithostratigraphy). The occurrence of mineral coatings is very similar in both the Cross Drift and the ESF. Vapor-phase mineral coatings are nearly ubiquitous in both excavations. Calcite and opal infillings in fractures and on the lower surfaces of lithophysal cavities are common within a number of intervals in the Cross Drift and the ESF. In addition, fine blades of specular hematite occur sporadically in lithophysal cavities. Table 5 shows the percentage of calcite, opal, manganese oxide, and vapor-phase mineralization as infilling according to lithology. Numbers in parenthesis are percentages from the ESF for the same lithology.
STRUCTURE
Comparative Cross Section
The Comparative Geologic Cross Section Along the Cross Drift (Drawing OA-46-345) was developed by the underground mapping team from as-built geology of the Cross Drift. The as-built cross section was compared to the pre-construction cross section assembled by the USGS (Potter and others, 1998). Generally these sections compare favorably: the contacts and structures on the pre-construction section were encountered where expected. Although there are discrepancies between contact predictions and actual locations, these can be attributed to pinching and swelling of the lithostratigraphic zones, variations in dip, and the distance these contacts were projected from drill holes. Stratigraphically, these discrepancies involve minimal changes to predicted stratigraphic thicknesses. The fault at Sta. 22+38 has no known surface expression and is not encountered in any drill holes, therefore it does not appear on the pre-construction cross section. This fault is described in detail in the section entitled, “Significant Faults”. A significant difference in the pre-construction cross section is the presence of west-dipping panels in the hanging wall of the SCFZ, at the level of the Cross Drift alignment. Although these west dipping panels are mapped on the surface (Day and others, 1998), they do not extend to the depth of the tunnel. These west dipping panels are shown to pinch out above the tunnel alignment on the as-built cross section. Exposed in the hanging wall of the SCFZ are rocks of the Tptpul and Tptrl that dip to the east at approximately 6 to 8 degrees. See Table 6 for specific differences between these two cross sections.
Faults and Shears in the Cross Drift
Faults are defined as discontinuities displaying more than 0.1 m of offset (U.S. Bureau of Reclamation and U.S. Geological Survey, 1997, p.11). Shears are defined as discontinuities displaying less than 0.1 m of offset or having an undeterminable offset. Offset is determined visually in the tunnel by offset of reference features. For faults with displacement greater than the diameter of the tunnel (5.0 m), offset is defined from correlation of stratigraphic units.
Faults and Shears with offset less than four meters
The DLS in the Cross Drift collected data on 30 faults and 107 shears. There are 9 faults and 21 shears in the Tptpul, seven faults and 14 shears in the Tptpmn, four faults and 31 shears in the Tptpll, six faults and 34 shears in the Tptpln, one fault (Solitario Canyon fault) and five shears in the Tptpul, and one fault and two shears in the Tptrl in the hanging wall of the SCFZ. In the Tptpul, there are 9 faults with offset less than four meters. Four of these have normal offset, three have reverse offset, one with right lateral offset and one with unknown offset. Four of these faults are east dipping and six are west dipping. Three have a dip greater than 80 degrees; five have a dip greater than 70 degrees; and two dip less than 70 degrees. The average offset for these features is 0.21 meters, ranging from 0.1 to 0.40 meters. Of the 7 faults in the Tptpmn, with offset less than four meters, four have an unknown sense of movement, two have normal movement, and one has reverse movement. Six faults are west dipping and one is east dipping. Four have a dip greater than 80 degrees, two between 70 and 80 degrees and one less than 70 degrees. The average offset for these features is 0.84 meters, ranging from 0.2 to 2 meters. In the Tptpll, there are three faults, two with reverse offset and one with a normal sense of movement. One is east dipping and two are west dipping. All three have dips greater than 80 degrees. The average offset for these features is 0.87 meters, ranging from 0.2 to 2 meters. In the Tptpln, all six faults are west dipping. Three have a reverse sense of movement, one is right lateral, and two have an unknown sense of movement. Four dip greater than 80 degrees, and two dip greater than 70 degrees. The average offset for these features is 0.89 meters, ranging from 0.25 to 3 meters. In the hanging wall of the SCFZ, the faulted contact between the Tptpul and the Tptrl dips to the west and has greater than 5 meters of normal offset. Figures 5, 6, and 7 show all faults and shears with an offset of less than four meters plotted on various histograms and stereonets. Figure 5 and the stereonets show that faults and shears in the Cross Drift generally match the joint sets from the general fracture population. Figure 6 shows that faults and shears are not necessarily related to fracture density, particularly in the middle nonlithophysal zone.
Significant Faults
Thirty faults were encountered during excavation of the Cross Drift and recorded on the DLS. Of these, only a few are significant enough to warrant detailed description. Following are characterizations of significant faults, the zones associated with the faults, and their effect on the tunnel excavation and support.
Drill Hole Wash fault
The Geotechnical Baseline Report (CRWMS M&O, 1998, p. 4-14) predicted that a minor splay of the Drill Hole Wash fault would be encountered in the Cross Drift near Sta. 1+30. However, no trace of this fault is present in the Cross Drift excavation. Apparently the fault (previously thought to be the Drill Hole Wash fault) observed in the ESF at Sta. 22+65 is a localized feature which terminates before reaching the Cross Drift. This implies that the Drill Hole Wash fault, mapped by Day and others (1998), is not present at depth.
Ghost Dance fault
The Geotechnical Baseline Report (CRWMS M&O, 1998, p. 4-15) stated that the Ghost Dance fault might be encountered in the Cross Drift, but the fault should have minimal offset. The geologic cross section from the Basline Report accurately predicted the fault in the vicinity of Sta. 4+80. A shear was encountered at Sta. 4+99 (left wall, at springline) which has been identified as the northern distal end of the Ghost Dance fault. This feature is the only north-trending, conspicuous discontinuity in this portion of the tunnel. The feature is termed a shear at this location because it has less than 0.1 m discernible offset. The feature is traceable across the tunnel, and is oriented 352°/90°, with an irregular dip. The wall rock on both sides of the fault is Tptpul. The feature consists primarily of a 1 to 10 cm thick zone of silty/sandy gouge with clasts up to #8 sieve size. The gouge thickens slightly in the crown to 10 cm, but is only 2 to 4 cm thick elsewhere. This gouge is firm to hard, composed of crushed rhyolitic wall rock, is light gray (N7) in color, with some very minor layering caused by minor gradational variations. The gouge is dry, uncemented, and unmineralized. No slickensides were observed along the fault. The gouge is surrounded by a zone of intensely fractured and crushed rock. On the right wall, this fractured zone is approximately 0.4 m thick on the east side of the feature, and 0.6 m thick on the west side of the feature.
Sundance fault
The Sundance fault was identified and mapped originally by Spengler and others (1994), who believed it to be a major northwest-striking feature. Subsequent work on the surface by Day and others (1998, p. 10) and the subsurface by Albin and others (1997, p. 45) determined the fault to be a relatively minor feature of limited areal and vertical continuity. The Geotechnical Baseline Report (CRWMS M&O, 1998, p. 4-15) predicted the Sundance fault to be near Sta. 10+70 to 11+00. The Sundance fault was encountered along the left wall at Sta. 11+35.40 to 11+36.70. The fault intercepts the right wall at Sta. 11+35 to 11+36.2, approximately 35 m southwest of the location predicted. The fault is distinct in the tunnel walls, probably due to dramatic color change from light brown (5YR5/6) of the footwall (Tptpmn - not vapor-phase altered) to the medium light gray (N6) of the hanging wall (Tptpmn - vapor-phase altered), (Photograph 15). The fault appears to have several meters of normal (down-to-the-west) offset, however the amount of offset is indeterminate. Orientation of the footwall plane is 131°/81° and 146°/77° for an internal shear plane. The fault zone is composed of three distinct zones along the left wall (Photograph 16). Zone 1 is adjacent to the footwall plane, and is a matrix-supported, uncemented breccia, about 90 percent sand and silt-sized particles, with 10 percent gravel-sized clasts to about 2 cm. Clasts are subrounded to angular, rotated, composed of rhyolitic shards of light brown (5YR5/6) wall rock (Tptpmn). The matrix is pale red (10R6/2), with occasional light gray to very pale orange (N8 to 10YR8/2) patches, dry, firm to hard, unmineralized, and uncemented. Above the left wall springline, zone 1 contains up to 30 percent clasts. Zone 1 is approximately 20 cm thick on the left wall, thinning to 4 cm on the right wall. Zone 2 of the Sundance fault is approximately 0.7 m thick and is a matrix-supported breccia. The matrix is light gray to moderate orange pink (N8 to 5YR8/4), composed of silt and sand-sized particles, hard, uncemented, and unmineralized. Clasts are up to 150 mm in size, but average 4 to 20 mm. Clasts are angular to subrounded rhyolitic, densely welded tuff. This zone varies in thickness and composition across the tunnel, becoming a clast-supported breccia on the right wall. On the left wall, zone 2 is about 60 percent matrix, 40 percent clasts, changing to approximately 80 percent clasts, 20 percent matrix on the right wall. Zone 3 of the Sundance fault is a clast-supported breccia with about 90 percent clasts, 10 percent matrix. Clasts vary in size from 4 to 300 mm in size, and are angular to subangular with larger clasts generally not rotated. Matrix is composed of uncemented, unmineralized, light brownish gray (5YR6/1) silt to sand-sized particles. Zone 3 varies in thickness from 0.3 m on the left wall, to zero on the right wall. Despite the very sharp and distinct plane of the fault at the footwall, distinct slickensides are not evident. Faint, low-angle slickensides can be interpreted on the left wall, and undulations in the fault plane with low-angle plunges occur at the boundary between zones 1 and 2. The footwall rock is intact, even within the 10 cm of the fault plane. The hanging wall is slightly more fractured, with an intensely fractured zone about 1 m thick. The margins of the fault zone were unaltered except for in the immediate area of the fault which exhibits some iron oxide stainings along the right wall. All portions of the Sundance fault were dry at the time of excavation. The tunnel boring machine excavated through the Sundance fault smoothly with no fallout along the zone. Some blocky fallout occurred east of the fault at Sta. 11+31 in the upper right crown. The area is supported by rock bolts and welded wire mesh, with some additional mesh installed in the immediate area of the fault.
Fault at Sta. 13+17
The fault is exposed in the tunnel as a zone of shattered rock extending from Sta. 13+16.5 to approximately 13+19.5 along the right wall. The fault is oriented 175 to 185/80 to 85. The zone is composed primarily of shattered (intensely fractured) rock with a 10 to 20 cm thick zone of light brown (10YR8/2) gouge. The gouge consists of silt to sand-sized material derived from the wall rock (Tptpmn) with occasional deformed phenocrysts present. The gouge is dry, firm to hard, uncemented, and unmineralized, and has a slight reaction with HCl. Offset along this fault zone is difficult to determine due to a lack of markers. The absense of vapor-phase partings on the west side of the fault, present on the east side, rules out normal (down-to-the-west) offset. There may be some reverse (up-to-the-west) offset, but it seems unlikely that this offset is greater than 2 m. The gouge zone, while clearly visible on the left wall at Sta. 13+17 (Photograph 17), is barely distinguishable on the right wall, detectable only by a slight color change, with a gouge thickness less than 1 cm. Several indistinct shears are also visible in the upper right crown, but offset along these features is difficult to detect.
Fault at Sta. 21+54
This fault is exposed on the left wall from Sta. 21+53 to 21+54.7, and is oriented 157/87 at the footwall. On the left wall, the fault is composed of three zones (Photograph 18). Zone 1 is a matrix-supported breccia approximately 0.6 m thick. The breccia is composed of approximately 80 percent matrix and 20 percent small clasts. The material might be considered gouge except that in some portions of the tunnel the matrix component contains a substantial percentage of coarse sand-sized material up to #4 sieve size. The matrix material is firm to hard, very pale orange (10YR8/2), dry, uncemented, and unmineralized, and exhibits no reaction with HCl. Zone 2 is a clast-supported breccia and is approximately 0.9 m thick. The breccia is composed of approximately 80 percent angular to subangular clasts from 1 to 20 cm in size (averaging about 3 to 4 cm in size). The clasts are hard, grayish orange (10YR7/4) rhyolitic tuff. The matrix is about 20 percent silt and sand-sized particles, very pale orange (10YR8/2), uncemented, hard, and unmineralized. Zone 3 is adjacent to the hanging wall and is similar in composition to zone 1, but contains a higher percentage of small clasts, up to 35 percent. This zone is 0.2 m thick, and is composed of silty/sandy matrix. Clasts in this zone are composed of the rhyolitic tuff of the wall rock, varying from 1 to 10 cm in size, averaging about 3 cm. The clasts are generally angular to subrounded, with evidence of rotation. Offset along the fault zone is difficult to determine due to a lack of markers, but there is a visible change in the character of the rock across the fault. In the footwall the rock (Tptpll) has a small percentage of large lithophysae greater than 50 cm in size, about 5 to 10 percent 20 to 30 cm lithophysae, and abundant large vapor-phase spots 5 to 10 cm in diameter with small stringers of crystalline vapor-phase mineralization. The rock also contains a trace of white (N9) lithic clasts to 10 cm (long axis). Below springline, the percentage of spots decreases dramatically. In the hanging wall of the fault, the rock has a decreased abundance of large lithophysae and an abundance of 5 to 10 cm vapor-phase spots. The spots on the footwall side of the fault decrease below springline and this decrease appears to have dropped to the invert level in the hanging wall side of the fault, suggesting approximately 1.5 to 2 m of vertical separation. Faint, low-angle slickensides are visible on the footwall plane on the right wall just above springline, suggesting a minor left-lateral strike-slip component to the fault. The slickensides have a rake of 9 degrees.
Fault at Sta. 22+36
The fault zone is marked by a distinct plane on the hanging (east) wall of the fault, oriented 343/80. One of the features unique to this fault is that the zone of fracturing is more pronounced on the footwall (west) side than on the fault, with some drag folding and radiating fractures (fanning from the axis of folding) on the upper right wall. Also on the right wall, distinct slickensides with rakes of 45 to 50 degrees occur above springline. Sense of offset is clearly up to the west (normal). There is a distinct change in the rock, though both sides of the fault are in Tptpll (Photograph 19). The east side (hanging wall) of the fault contains less than 1 percent lithophysae and has abundant spherical to oblong vapor-phase spots 5 to 20 cm in diameter. Some short, discontinuous, widely spaced (less than 40 cm) vapor-phase stringers are present. The upthrown (western) side the rock also contains sparse lithophysae, but has abundant vapor-phase partings spaced approximately 2 to 30 cm apart. These vapor-phase partings are discontinuous, less than 40 cm long, as on the hanging wall. On the upthrown side, none of the vapor-phase spots are visible, except in the upper meter near the crown. This offset indicates greater than 5 m vertical separation. Also present on the footwall side of the fault is a series of large lithophysae (up to 60 cm) aligned along a pronounced series of closely-spaced vapor-phase partings. The partings and associated lithophysae extend from near springline at Sta. 22+36 to 22+60 in the crown. The fault zone is composed of matrix-supported breccia varying from 8 to 40 cm in thickness. Fault gouge is present adjacent to the hanging wall and composed of slickensided clay and silt-sized gouge 2 to10 mm thick. The matrix of the breccia is silt and sand-sized material, pale red (10R6/2), hard, dry, uncemented, and unmineralized. The clasts are composed of rhyolitic tuff, light brownish gray to moderate yellowish brown (5YR6/1 to 10YR4/2), and vary in size from coarse sand to fine gravel size (2 to 20 mm). Most clasts are angular and rotated.
The Solitario Canyon fault zone (SCFZ)
The SCFZ is the most laterally continuous fault and displays the most offset of any structure in the immediate vicinity of Yucca Mountain. Day and others (1998, p. 6) consider the SCFZ to be one in a series of major north-south trending, block-bounding faults. The fault has been extensively investigated by trenching at the surface in Solitario Canyon (Ramelli and others, 1996). In the Cross Drift, the SCFZ was expected to be composed of two major normal fault strands; the first (eastern strand) was projected as the “main splay” with a predicted offset of about 230 m. It was projected to be encountered near Sta. 25+65. The second (western strand) was projected to be encountered at Sta. 27+40, with a predicted offset of about 165 m (CRWMS M&O, 1998). A smaller, northeast-striking splay of the SCFZ was projected near Sta. 25+55. Displacement across the eastern strand of the fault was expected to place Tptpln against the Tptpul. The western strand was expected to drop bedded tuffs of the Pah Canyon Tuff and the pre-Pah Canyon Tuff (Tpbt2) against the Tptpmn. Between these two larger strands, several smaller faults were expected to be associated with the SCFZ faulting. The as-built geologic cross-section shows that the eastern strand was encountered at Sta. 25+85 and has approximately 260 m of normal offset. During construction, a decision was made to stop the tunnel boring machine at Sta. 26+81, between the two strands. This decision was based on programmatic considerations and the desire to preserve the western strand of the SCFZ, allowing pneumatic and hydrologic testing of the fault zone prior to disturbing the zone by excavating through it. For this reason, the TBM penetrated only the eastern (main) strand of the SCFZ. The western strand was not reached in the Cross Drift. The as-built geologic cross-section shows the projected offset on the western strand to be about 68 m, much less than predicted in the pre-construction geologic cross-section, about 165 m. The SCFZ influences rock in the footwall of the fault to about Sta. 25+00 in the form of increased shear intensity. Shears and small faults increase in intensity prior to (east of) of Sta. 25+00. The footwall rock is Tptpln to the eastern strand of the SCFZ at Sta. 25+85. Spacing of faults and shears decreases, while continuity and amount of offset increases with proximity to the the eastern strand of the SCFZ. At Sta. 25+30, a small fault oriented 200/83 is intercepted by the tunnel. Although the offset along the fault is approximately 1 m or less, the rock is intensely fractured after (west of) Sta. 25+40. Fallout up to 0.5 m occurs in the upper left crown (at the top of where the TBM gripper pad is placed) at Sta. 25+32 and continues to Sta. 25+80. Shear planes oriented 070 to 080 degrees, dipping 89 to 90 degrees, and 290 to 310 degrees, dipping 80 to 90 degrees occur in the intensely fractured rock from Sta. 25+35 to 25+80. At Sta. 25+74, a shear plane occurs on the right wall below springline. The plane is undulating, but generally strikes 095 degrees (subparallel to the tunnel) and dips 68 degrees. Offset along this shear is unknown. At Sta. 25+78, a shear oriented 040/68 degrees, intersects the tunnel on the right wall. Offset along this feature is unknown, and the shear is the only northeast-striking feature approximating the predicted northeast splay of the SCFZ. The rock from Sta. 25+80 to 25+82 is a clast-supported breccia with about 85 percent clasts and 15 percent silty/sandy matrix. The rock is shattered to the point of not having recognizable structure. Clasts are Tptpln, angular, and light brown (5YR6/4) with white (N10) spots and matrix. Some clasts display rotation, but most retain a gentle easterly dip. The matrix maybe partially composed of vapor-phase mineralization. The mineralization is possibly a remnant of vapor-phase mineralization present in the Tptpln during tectonism, or maybe the vapor-phase mineralization formed in situ. The possible presence of vapor-phase material in the matrix of the footwall breccia adjacent to the eastern strand of the SCFZ could hold information regarding the timing of movement along the fault. Because vapor-phase alteration of the breccia matrix could only have occurred during cooling and secondary crystallization of the Topopah Spring Tuff, the presence of this material indicates that at least a small amount of movement occurred along the fault shortly following deposition of the pyroclastic-flow deposit. From Sta. 25+82 to 25+85 the rock is a clast-supported breccia (zone 1 in Photograph 20 ) with approximately 75 percent clasts and 25 percent matrix. Clast size at Sta. 25+82 is about 50 to 200 mm, but decreases to 1 to 30 mm in close proximity to the eastern strand of the SCFZ at Sta. 25+85. Zone 1 exhibits a distinctive reddish-brown stain in the matrix and on the surface of the clasts. Freshly broken clast surfaces show that the staining has penetrated only about 0.5 mm maximum. The clasts appear to be derived from Tptpln. The matrix is silty and sandy material, slightly cemented, hard to firm, unmineralized, varying in color from light red (5R6/6) to moderate reddish-orange (10R6/6), to grayish orange pink (10R8/2) and moderate reddish brown (10R4/6). The main plane of displacement along the eastern strand of the SCFZ is at Sta. 25+85, (left wall, springline) (see Plate 1). The fault plane is defined by an 8 to 12 cm thick zone (zone 2 on Photograph 20 ) of fault gouge composed of about 85 percent clay and about 15 percent fine to medium sand. The gouge is firm and was slightly damp at the time of excavation in October, 1998, but dry by February, 1999. The gouge also contains a trace of angular clasts to 20 mm, and is uncemented, unmineralized, and light to moderate brown in color (5YR6/4 to 5YR4/4). The fault plane as defined by this zone strikes 162 degrees and dips 62 degrees. The clayey gouge exhibits numerous polished and slickensided surfaces. Slickenside rakes average 40 degrees. Zone 3 (Photograph 20), present on the upper left wall, is composed of a brecciated wedge that terminates just above springline. The zone 3 wedge is a matrix-supported breccia, with about 55 percent matrix and 45 percent clasts ranging from 2 to 300 mm (length). The matrix is composed of silt and sand-sized particles and is very hard, and well cemented with silica(?). The clasts are composed of medium gray to light brownish gray (N5-5YR6/1), hard, mostly angular, extremely fine grained tuff, and yellowish brown (10YR6/4), hard, subangular to subrounded tuff with occasional 2 mm pink pumice fragments. Zone 4 occurs along the western (hanging wall) margin of the zone 3 wedge. Zone 4 (Photograph 20) is parallel to the eastern strand of the SCFZ at Sta. 25+85. Zone 4 is a 5 to 10 cm thick clast-supported breccia. The breccia is cemented with calcite and silica. Clasts within zone 3 are about 50 percent by volume, and are 1 to 6 cm angular clasts of medium gray (N7), hard tuff, with approximately 5 percent phenocrysts, and grayish-orange to pale yellowish brown (10YR7/4 to 10YR6/2), mostly elliptical, moderately soft, moderately welded tuff clasts. The matrix is composed of silt and sand-sized particles, and is hard, light brown (5YR6/4), well cemented, and mineralized with some crystalline calcite visible along clast margins. Zone 4 may be the eastern boundary of the thicker matrix-supported breccia described below. The main portion of the hanging wall (zone 5 in Photograph 20) of the SCFZ extends from Sta. 25+86 to 25+88.5. Zone 5 is a matrix-supported breccia varying from 50 to 70 percent matrix with 30 to 50 percent clasts. The matrix is primarily composed of silt and sand-size particles, which are light gray to light brown (N7 to 5YR6/4), firm to hard, dry, and slightly to well cemented with occasional crystalline calcite visible along clast margins. The clasts vary from coarse sand to 1.2 m in diameter. Clast color varies from light brownish gray to medium gray (5YR6/1 to N5), and the clasts are angular to subrounded and rotated to random orientations. The clasts frequently contain abundant dark gray (N3 to N4) pumice clasts from 5 to 15 mm in diameter. The presence of the pumice fragments indicates that these clasts are from the base of the Tiva Canyon Tuff, specifically from the crystal-poor, lower nonlithophysal zone (Tpcpln). The zone 5 breccia is bounded on the west by an indistinct plane that strikes 178 degrees and dips 73 degrees. Across this plane, the composition of the clasts changes. None of the clasts penetrate the plane, indicating that this is a throughgoing boundary within the fault breccias of the SCFZ. On the west (hanging wall) side of the fault plane described above, is a zone of matrix-supported breccia that extends along the left wall from Sta. 25+85.5 to 25+89.90. This zone is similar in appearance to the matrix-supported breccia described above, but varies significantly in some aspects. Approximately 50 percent of the material is sandy matrix, which is light gray (N7), hard and slightly to moderately cemented with occasional to rare crystalline calcite visible. Clasts in the breccia are 5 to 200 mm in size, angular to subrounded, and composed of light gray to light brownish gray (N7 to 5YR6/1) fragments of Tptpul. Clasts are randomly rotated. The west side of this zone is bounded by an indistinct plane that strikes 180 degrees and dips 80 degrees. This plane, although subtle in appearance, forms the boundary between the matrix-supported breccia on the east and the less disturbed, clast-supported breccia to the west. The farthest western zone along the eastern strand of the SCFZ is composed of a clast-supported breccia extending along the left wall from Sta. 25+89.9 to 25+99.15. Clasts are composed entirely of Tptpul, varying in size from 10 to 400 mm, averaging 150 mm, and are angular to subangular, with some rotation of clasts evident. The matrix is composed of sand and silt-sized materials and composes about 20 to 35 percent of the overall breccia. The matrix is soft, uncemented, and unmineralized. This zone is bounded on the west side by a thin, discontinuous, matrix-supported breccia about 10 to 20 cm thick. At Sta. 25+99.15 is the western (hanging wall) boundary of the eastern strand of the SCFZ, composed of intensely fractured Tptpul. The Geotechnical Baseline Report (CRWMS M&O, 1998) anticipated possible difficulties excavating through the SCFZ. The orientation of the Cross Drift was curved to the west to minimize the excavation distance through the fault zone. Project personnel anticipated ravelling and blocky ground in the hanging wall. However, all ravelling, blocky ground, and fallout was experienced in the footwall of the fault. The contractor applied shotcrete to the wall between Sta. 25+38 and 25+77, obscuring nearly all geologic features. The contractor did not shotcrete the crown through this section however, allowing some visibility through the steel ribs and lagging. At Sta. 25+60, the contractor installed 6-inch ring steel, almost completely obscuring the walls and crown. Lagging was installed across the crown from springline to springline. Through this section, the tunnel walls below springline were shotcreted. From Sta. 25+50 to 25+80, rock in the invert was cleaned, but all that is visible is rubblized Tptpln. Generally the rock is sheared in 2 to 4 cm fragments surrounded by silty and sandy matrix. Some remnant structure is visible in the rock, such as vapor-phase spots, but generally the rock is shattered in appearance. When the TBM entered the gouge and breccias near Sta. 25+85, the ground stood without problems, and the contractor returned to installation of rock bolts and wire mesh for the remainder of the drive to Sta. 26+81. The degree of footwall deformation was not anticipated in the Geotechnical Baseline report. Surface exposures in Solitario Canyon indicated that the footwall deformation would probably be limited, with deformation and possible blocky ground occurring on the hanging wall side of the fault zone. The extent of the footwall deformation, extending nearly 50 m east of the fault, was greater than anticipated. A possible explanation for the footwall fracturing may be the presence of a northeast-trending splay of the SCFZ, originally anticipated at Sta. 25+50. This fault was not observed in the tunnel, nor were any distinct offsets observed east of the main splay at Sta. 25+84. The extent of footwall deformation may indicate a somewhat broad zone of deformation at the intersection of the northeast splay with the eastern strand of the SCFZ. The relative lack of difficulty in excavating through the eastern strand of the SCFZ can be explained by the character of the fault breccias present in the hanging wall. Although not well cemented west of Sta. 25+90, the breccias maintain enough internal cohesion to support the opening, with little or no ravelling, and little deterioration over time. West of the eastern strand of the SCFZ (Sta. 26+00), small faults and shears occur every few meters and continues to the present heading of the tunnel at Sta. 26+81, and is consistent with preconstruction expectations (CRWMS M&O, 1998, p. 4-17).
Analysis of DLS Fractures
Fractures were first analyzed according to lithology. Four lithologic units, Tptpul, Tptpmn, Tptpll, and Tptpln occur in the Cross Drift. A fifth, Tptprl is represented by only 7 fractures, and are grouped with the Tptpul fractures. Fractures for each lithologic unit were made into data files appropriate for analysis using the Dips program (Rock Engineering Group, University of Toronto). From these data files, stereonet contour plots were constructed. Stereonet contour plots for all fractures in each lithologic unit are included on Figures 8, 9, 10, 11, and 12. The fractures have been categorized into sets which are numbered to correspond to fracture sets discovered in the ESF Main Drift where possible. Figure 8 includes the contour plot for all Tptpul fractures (374 fractures). Three sets are visually apparent from the contours. The most prominent of these is contoured on the ESE edge of the stereonet plot, and centered around a mean orientation of approximately 195/83 (this is designated Set 2, since this corresponds to Set 2 from the ESF Main Drift). Included in this set are some with reciprocal values that plot on the opposite edge of the stereonet (WNW). The second most prominent set is on the NE edge of the stereonet, and centered around an attitude of approximately 122/83 (this is designated Set 1, and it corresponds to Set 1 from the ESF Main Drift), and this grouping also has a few reciprocal values which contour on the opposite edge of the stereonet (SSW and SW). The third set consists of the low angle fractures in two contours located SSW on the interior of the stereonet with a mean orientation of approximately 302/38 (this is designated Set 3 - it corresponds to Sets 3 and 4 from the ESF Main Drift). The contour closest to the center of the plot consists to a large measure of vapor-phase partings (VPP’s), while the one further out from the center contains more cooling joints (CJ’s) and other low angle fractures. Figure 9 includes the contour plot for all Tptpmn fractures (930 fractures). Two prominent sets are visually apparent from the contours. The most significant of these is contoured on the NNE edge of the stereonet plot, and centered around a mean orientation of approximately 122/84 (Set 1). Reciprocal values included in this set are apparent on the SSW edge of the plot. The second set is contoured at the ESE edge of the plot (mean orientation approximately 195/84 - Set 2), with some reciprocal values plotting on the WNW edge. A third set consisting primarily of VPP’s makes up a small contour near the center of the stereonet SW with an approximate mean orientation of 306/09 (Set 3). Figure 10 includes the contour plot for all Tptpll fractures (300 fractures). Two significant contours, and some lesser contours are formed on this plot. An arcuate concentration is formed along the NE and ENE edge of the plot, and has four peaks. The most significant peak has a mean orientation of approximately 135/80 (Set 1). Reciprocal values plot on the opposite SW and WSW edge of the stereonet. The other significant contour is formed near the center SW of the plot with a mean orientation of approximately 340/06 (Set 3), and consists primarily of VPP’s. Less significant contours show up at the NNW, N, and ENE edges with approximate mean orientations of 073/80, 090/80, and 115/80, respectively (these three are categorized as Set 4 for this report) . Some reciprocal values plot at SSE, S, and SSW edges. Another lesser concentration is found on the ESE edge (approximate mean orientation 200/80 - Set 2) with reciprocals at the WNW edge. Figure 12 includes the contour plot for all Tptpln fractures (199 fractures). One prominent contour is formed near the NE edge with a mean orientation of approximately 134/80 (Set 1). Four other less significant contours are formed. Three are plotted near the edge at ENE, ESE, and SE with mean orientations of approximately 170/80, 192/80, and 210/80, respectively (192/80 and 210/80 are categorized as Set 2). The fourth represents mostly VPP’s and plots near the center WSW with an approximate mean orientation of 336/17 (Set 3).
Cluster Analysis
Statistical analysis of the DLS data collected in the Cross Drift focuses on the orientation of discontinuities. The computer program Clustran, a commercially available software package, was used to perform the cluster analysis. Clustran mathematically groups the orientation data into clusters. The clusters thus derived are free of the subjective judgement, or bias, that may be present in other means of analysis. Cluster analysis provides a statistical approach to resolving sets, but cannot apply a knowledge of geological concepts. Sets derived from cluster analysis must then be viewed in terms of geological relevance. Clustran initially tests the orientation data for randomness using the chi square test, Poisson analysis, and log likelihood ratio test for quality of fit. Data from the Cross Drift are found to be nonrandom. This nonrandomness is expected because the poles plotted show clusters, not a uniform scatter. Thus, the data can be analyzed for clusters, removing outlying data and selecting statistically significant clusters. The user inputs and tests a number of different clustering radii, and identifies the one that results in the least “objective function” as the “best” radius. Statistical fits are made by Clustran to the extracted clusters of data, and includes their means and confidence intervals. If sets derived by using the “best radius” as determined by the minimized “objective function” are not geologically significant, it may be necessary to choose the next higher “minimized function” and it’s corresponding radius. Clustran allows the user to write the clusters into new data files, which may be used for further analysis (Gillett, 1987). Clustran identified seven clusters using the entire data set (1802 entries with greater than 1 m trace length), at the angle which produced the lowest objective function (3 degrees). For further analysis, the fractures were analyzed separately, according to lithology. Clustran identified some of the same clusters according to lithology. However, two clusters that were identified in the overall analysis seem insignificant, with 17 and 18 data points, respectively. In the lithologic analysis, these fractures are grouped with other clusters. The overall analysis also separates the low angle fractures (vapor-phase partings, some cooling joints and others) into two groups, where they are identified as only one in the lithologic analysis. The Tptpul cluster for low angle fractures, when contoured (Fig. 8), has two peaks, which generally corresponds to the two-low angle clusters identified by Clustran in the overall analysis. The four main lithologic units in the Cross Drift occur in four separate zones, except for the Tptpul, which occurs from Sta. 0+00 to 10+15, and is found again in the SCFZ from Sta. 25+85 to 26+57.5. A fifth unit, Tptrl, occurs from Sta. 26+57.5 to 26+64, but constitutes only seven fractures. The fractures for the Tptrl are considered together with those from the Tptpul. The results of running clustran on the Tptpul fractures without the fractures from the SCFZ were essentially the same as when all of the Tptpul fractures were considered together. All types of discontinuities recorded in the DLS were included in this analysis; therefore, the sets do not distinguish between fractures, shears, faults, cooling joints, and vapor-phase partings. Fractures oriented parallel or subparallel to the tunnel alignment will tend to be poorly represented in the DLS record, creating a “blind zone”. The tunnel bearing as well as it’s reciprocal are displayed on the scatter plot of strike azimuth vs. station (Dwg. OA-46-346). The tunnel bearing and the reciprocal bearing appear to pass through a “corridor” of sparse fracturing, which illustrates this “blind zone”. Fractures from each lithologic unit were analyzed using Clustran, which separated the fractures into clusters. It appeared that some of the clusters identified by Clustran might not be significant. The data was analyzed again by inputting the angle that resulted in the next lowest “objective function”, but the clusters resulting from the second analysis were less satisfactory. Consequently, the clusters presented here are the ones from the original analysis that resulted in the lowest “objective function”. All of the clusters are presented here, and have been made into contour plots (data points from each cluster contoured separately) for comparison and contrast with the contour plots representing the subjective analysis (all fractures within each lithologic unit contoured together). The contours of clusters identified by Clustran could in most cases be identified, and correlated with contours formed on the plots derived using all fractures within the lithologic unit (subjective analysis). The contour plot for all Tptpul fractures has Cluster 1 as a minor contour, Cluster 2 as a significant contour, and Cluster 3 as a major contour with two significant peaks. Two peaks can be identified on the overall contour plot as well as on the contour plot with only Cluster 3 fractures. The contour plotting nearest the center represents predominantly vapor phase partings, and the contour midway between the center and edge of the plot corresponds to cooling joints, and other fractures (Fig. 8). The contour plot for all Tptpmn fractures has Cluster 1 as a major contour, Cluster 2 as a significant, contour, and Cluster 3 as a minor contour (Fig. 9). The contour plot for all Tptpll fractures shows Cluster 1 as a very minor contour, Cluster 2 as a very minor contour, Cluster 3 as a minor contour, and Cluster 4 as a major contour with several peaks. Multiple peaks are shown on both the overall contour plot as well as on the contour of only Cluster 4 fractures (Fig. 10). Cluster 5 shows up as a significant contour (Fig. 11). The contour plot for all Tptpln fractures does not show Cluster 1. Cluster 2 shows as three minor isolated contours corresponding to three peaks on the contour plot for only Cluster 2 fractures. Cluster 3 shows up as a major contour, and Cluster 4 as a minor contour (Fig. 12). All of the clusters identified by Clustran have been placed into sets as follows: Sets 1 and 2 correspond to Sets 1 and 2 from the ESF Main Drift, and Set 3 corresponds to Sets 3 and 4 from the ESF Main Drift. Trends can be seen on the plot of Station vs. Strike Azimuth (Dwg OA-46-346), which roughly correspond to the joint sets/clusters in the contour plots. Concentrations of fractures are visually apparent on the scatter plot of strike azimuth vs. stationing (Dwg OA-46-346). These concentrations correspond to some of the clusters identified by Clustran. A concentration is seen approximately centered around 120 degrees azimuth on the scatter plot, which can be identified with [SET 1](Tptpul Cluster 2/Tptpmn Cluster 1/Tptpll Cluster 4/Tptpln Cluster 3). This concentration is centered around 122 degrees for the Tptpul and Tptpmn, and then shifts in the Tptpll at about Sta. 21+50, to be centered around 157 degrees, and then shifts in the Tptpln to be centered around 134 degrees. On the scatter plot, a concentration is seen between azimuth 180 and 210 which can be identified with [SET 2](Tptpul Cluster 3/Tptpmn Cluster 2/Tptpll Cluster 1/Tptpln Cluster 1 and 2).
Other Fracture Characteristics: Fracture frequency for each ten meters of tunnel is shown on Dwg. OA-46-346. The non-lithophysal units are more densely fractured. Figure 13 is a comparison of fracture frequency and litophysal percentage and illustrates that the percentage of lithophysae is inversely proportional to fracture density (1 meter and longer fractures). The Tptpul shows an increase in fracture density starting at Sta. 8+55 coinciding with a decrease in the percentage of lithophysae. Between Sta. 21+50 and 24+00, the increased number of fractures may be associated with faulting in this area. Beyond approximately Sta. 24+00, faulting continues, and with it, intense fracturing, which the chart does not show, due in part to the presence of numerous anastomosing vapor-phase partings, which truncate the fractures to less than 1 meter in length. Additionally, intensely fractured zones reduce the number of fractures 1 meter and longer. Beyond the Solitario Canyon fault, the lithology changes to Tptpul. The lithophysal character of this unit also reduces the intensity of fracturing. Each fracture on the DLS was measured for trace length above and below the DLS line. Continuity data was derived by adding these two lengths. A scatter plot of fracture trace length (continuity) vs. stationing is displayed on Figure 14. It is evident from this chart that there are practically no data points with less than 1 meter of continuity because of sampling methodology. A few fractures were surveyed which were less than 1 meter in length, at the discretion of the geologist doing the line survey. The two longest, at 50.8 and 45 meters, were a fracture (at Sta. 10+60) and a vapor-phase parting (at Sta. 15+77), respectively. Mean continiuity is 3.343 m, median continuity is 2.02 m, and the standard deviation is 3.698. Minimum and maximum aperture data was recorded for each fracture on the DLS. Maximum aperture is plotted against stationing on Figure 15. The largest aperture recorded is 520 mm (a fracture at Sta. 11+15). Figure 16 shows the percentage fractures in the Cross Drift which have an aperture size from zero to one mm, one to two mm, two to three mm, etc.up to 20 mm. This histogram shows that 67.3 percent of fractures in the Cross Drift have zero aperture. Figures 17-20 shows the percentage of fractures by lithologic unit (Tptrl combined with Tptpul), which have an aperture size from zero to one mm, one to two mm, two to three mm, etc.up to 20 mm. [Figure 17,
GEOTECHNICAL CHARACTERIZATION
Introduction
The purpose of this section is to summarize the results of rock mass rating (RMR) data collected during excavation of the Cross Drift from Sta. 00+00 to 26+64. This data may be used as part of the overall assessment of the stability of current and proposed underground excavations at Yucca Mountain. Data was collected based on two empirical rock mass classification systems: the Norwegian Geotechnical Institute rock quality system (Q) (Barton and others, 1974) and the Geomechanics Rock Mass Rating system (RMR) (Bieniawski, 1989). These rock mass classification systems were developed in response to the demand for numerical design tools. The data base used for this report is a summary of observations and data collected and documented under technical procedure, “Rock Mass Classification” (U.S. Bureau of Reclamation) YMP-USGS-GP-54, R0. Kirkaldie (1987) suggests the rock material field classification procedure consists of two primary steps: the classification process and the performance assessment. The classification process includes: (1) the identification of the rock unit; and (2)the description of the rock in terms of classification elements. Classification elements describe the physical properties of the rock units that are most relevant to engineering activities. This geotechnical characterization reports rock mass properties in terms of the classification elements of the Q and RMR systems.
Rock Mass Classification Systems
The following sections describe data collection procedures utilized in the Cross Drift for the Q and RMR systems. While Rock Quality Designation (RQD) alone is not considered to be an adequate classification system, it is a parameter in both the Q and RMR systems so RQD data is reviewed in the same detail as the Q and RMR data. Ratings are assigned to a five meter length of tunnel using both rock classification systems. The use of this relatively short rating length may have the disadvantage of introducing variations in some evaluated parameters which may be expected to be stable; yet it has the advantage of capturing expected variations in more unstable parameters. For example, considering the Q system, one might assume the number of joint sets would be constant over a long reach of tunnel. Using a five-meter rating length permits evaluation of the actual occurrence of a particular joint set; therefore the rating value for the number of joint sets may vary within a ten-meter reach of tunnel. On the other hand, the five meter rating length permits a description of the changes in fracture frequency represented by RQD. Overall, the five-meter rating length emphasizes changes in rock quality from one length to the next. When longer reaches of the tunnel or various stratigraphic units are compared, differences in the trends of the five-meter ratings and differences in the average ratings are meaningful. The Tptrl is exposed in the Cross Drift from Sta. 26+57.5 to 26+81, but is only accessible for characterization puposes until Sta. 26+64. For this reason, the Tptrl was not characterized and is therefore not included in this geotechnical evaluation.
Rock Quality Designation (RQD)
The RQD index is a rating parameter of both the Q and RMR systems for drill core. It was introduced as a quantitative measure of rock quality (Deere, 1989). The total length of core pieces which are 4 inches (about 10 cm) and longer is divided by the length of the core run, which in this case is 5 m. RQD is calculated as follows:
Summary of RQD in the Cross Drift
Table 10 summarizes the RQD values encountered in each of the stratigraphic units encountered in the Cross Drift.
Analysis of RQD
The Tptpul (Sta. 0+00 to 10+15, Sta. 25+85 to 26+57) has the lowest RQD rating of 36 (poor) including lithophysae, and 49 (poor) excluding lithophysae. Its lithophysae content range from 10 to 40 percent by volume and cavities are up to 80 cm in diameter. Fractures are difficult to distinguish in the Tptpul. The best correlation between horizontal RQD and VRQD occurs in this secton. The Tptpmn (Sta. 10+15 to 14+44) has a mean horizontal RQD rating of 60 (fair) including lithophysae, and 62 (fair) excluding lithophysae. The unit is generally characterized by lithophysae of 1 to 3 percent by volume. The Tptpll (Sta. 14+44 to 23+26) has a mean horizontal RQD rating of 42 (poor) including lithopysae, and 57 (fair) excluding lithophysae. The Tptpll is generally characterized by lithophysae of 5 to 15 percent by volume. Its lithophysae are mostly 30 cm or larger and are moderately developed. The Tptpln section (Sta. 23+26 to 25+85) produced the highest horizontal RQD ratings: 62 (fair) including lithophysae and 67 (fair) excluding the lithophysae cavities. This unit is characterized by lithophysae cavities generally less than 1 percent by volume. Figure 25 shows the RQD ratings including and excluding lithophysae for the Cross Drift.
Rock Mass Rating (RMR)
The RMR system, also known as the Geomechanics Classification, is an empirical rating system based on the sum of six rock mass parameters. Bieniawski developed the system in 1973 and, with the addition of case histories, revised it to the present form (Bieniawski, 1989). The numerical rock mass rating, RMR, is calculated according to the following equation:
| RMR = C + RQD + Js + Jcd + JwR + AJO | |
| C | is a numerical value associated with the intact rock compressive strength |
| RQD | is a numerical value associated with the rock mass RQD (the rating is not equal to the RQD value) |
| Js | is a numerical value associated with the fracture spacing of a given joint set |
| Jcd | is a numerical value associated with the condition of discontinuities. |
| JwR | is a numerical value dependent on groundwater or inflow conditions. The “R” is used to distinguish this rating from the Q system joint water rating. |
| AJO | is a numerical value associated with the orientation of discontinuities |
Summary of RMR in the Cross Drift
Most of the RMR ratings fall in the fair category. Table 12 summarizes the ratings of RMR throughout the Cross Drift.Analysis of RMR
The Tptpul and the Tptpmn RMR ratings matched the predicted RMR ratings of the Geotechnical Baseline Report almost exactly (CRWMS M&O, 1998) scoring a fair rating. The Tptpll and the Tptpln ratings are from four to eight times higher than predicted, scoring a fair rating also. Lithophysae or lack thereof, had a minimal effect on the determination of RMR. Figure 26 shows the fluctuation in RMR including and excluding lithophysae for the Cross Drift.Norwegian Geotechnical Institute Rock Quality, Q - System
The Norwegian Geotechnical Institute Q rock mass classification system establishes a numerical value for the quality of the rock for engineering purposes. A Q rating is calculated as the product of six parameters according to the following equation:| Q = (RQD/Jn) x (Jr/Ja) x (JwQ/SRF) | |
| RQD | is an integer number equal to the RQD percentage. In the equation above the numerical value of 90 is used for an RQD of 90 percent. |
| Jn | is an index number based on assessment of the number of joint sets in the rating length considered. |
| Jr | is an index number representing the roughness of the joint set. |
| Ja | is an index number based on the alteration or filling of a given joint set. |
| JwQ | is an index number based on groundwater conditions. The “Q” is used to distinguish this index from the RMR system groundwater rating. |
| SRF | is an index number based on in-situ conditions which influence the stability of the excavation. |
 SRFn = 1.809Q -0.329 (1) |
||
| SRFn | = | stress reduction factor (SRF) for non-homogeneous rock |
| Q | = | (RQD/Jn) * (Jr/Ja) * (JwQ/SRF) |
| SRF | = | SRFn |
| Q | = | [(RQD/Jn) * (Jr/Ja) * (JwQ/1.809)] 1/(1-0.329) |
| SRFh = SRFh1 + SRFh2 | ||
| SRFh | = | SRF for homogeneous rock |
| SRFh1 | = | SRF for stress controlled behavior of homogeneous rock |
| SRFh2 | = | SRF for geologic-structure controlled behavior of homogeneous rock |
| SRFh1 | = | 0.244K 0.346 * (H/UCS) 1.322 |
| SRFh2 | = | 0.176 * (UCS/H) 1.413 |
| K | = | maximum-to-minimum principal field stress ratio |
| H | = | thickness of overburden above excavation (m) |
| UCS | = | unconfined compressive strength of rock (MPa) |
If SRFh1 is larger than SRFh2, then the behavior of the competent rock mass will be controlled by stress conditions, else if SRFh2 is larger, then the behavior of the competent rock will be controlled by geologic structure. Rock mass for homogeneous rock (Qh) is expressed as follows:
Qh = [(RQD/Jn) x (Jr/Ja) x (JwQ/0.244K0.746 (H/UCS)1.322 + 0.176(UCS/H)1.413)]
The value of Q for a given rock mass region is determined as the minimum of Qn and Qh.
The field stress state was determined based on hydraulic fracturing, in situ stress measurements in the Thermal Test Facility Alcove in the ESF (Sandia, 1997), which found the principal stresses to be:
 h |
= | 1.7 MPa |
| H |
= | 2.9 MPa |
| v |
= | 4.7 MPa |
The average horizontal stress and the vertical stress are typically used to calculate the field stress ratio (Hoek, 1998), therefore, Kirsten’s parameter, K, has been determined as:
A stress ratio value of K=2 will be assumed to be constant throughout future excavations. The head of rock H is equal to the thickness of the overburden above the tunnel alignment. Both the Kirsten approach and Barton’s approach to calculating SRF were used in determining the rated Q value for each 5 m reach. However, for the Cross Drift, Kirsten’s approach was used to determine the actual reported Q value.
Summary of Q Ratings
Two Q values were calculated for each 5 m section of the Cross Drift. The value reported as the Q rating for any section was calculated using the Kirsten approach. This value is determined using the lowest Jr/Ja ratio from within the 5 m interval. The second Q value, called Q Barton is presented for comparison purposes. Table 14 summarizes the mean, median, and range of Q for two thermal-mechanical units and their related stratigraphic units in the Cross Drift. Figure 27 shows the rated Q, including and excluding lithophysae, for the Cross Drift.
Analysis of Q
Actual Q ratings is three times larger than predicted in all four stratigraphic units. They all rate above 10, in the good category, with the exception of the Tptpll which scores a fair rating. The Q ratings calculated in the tunnel: Tptpul (14), Tptpmn (12.7), Tptpln (12.3), and Tptpll (7.9), match very closely with the predicted Q ratings from the Geotechnical Baseline Report (CRWMS M&O, 1998). Barton’s Q values were also calculated using a stress reduction factor (SRF) and included for comparison purposes. These values serve to prove that the determination of Q utilizing the Kirsten method is valid.
Non-rated Areas
There are some reaches within the Cross Drift that were not rated. Some areas are not suitable for rating using the current principles and procedures of the Q and RMR systems. Other areas were obstructed from view by lagging and shotcrete. The rocks of the Tptrl, exposed from Sta. 26+57.5 to 26+64, are not characterized because the trailing gear of the TBM obscures the rocks. Table 15 presents a summary of these areas.
GROUND SUPPORT
The design ground support includes rock reinforcement with rock bolts and welded wire fabric as explained in Table 16 below. Figure 28 graphically portrays how the calculated Q (Kirsten method) including lithophysae falls into Barton’s latest ground support determination chart. With the possible exception of the Tptpll, which lies close to systematic bolting zone, all the stratigraphic units land in the area which requires spot bolting or no support at all.
Summary of Rock Mass Quality
Tptpul
The Tptpul (Sta. 0+00 to 10+15 and Sta. 23+26 to 25+85), the longest reach of the Cross Drift, has the lowest RQD rating (36 poor), yet the highest Q rating (14 good). Both the tunnel- calculated Q rating and the predicted Q score this unit as the best. Its RMR value (57 fair) equals the RMR value of the Tptpll. Its lithophysae content range from 10 to 40 percent by volume. These cavities average 10 cm in size. Fractures are difficult to distinguish, with an average of only one joint set. No keyblocks are expected to form within this unit; however, there are occasionally some horizontal cooling joints. It has 11 faults, 1 fault zone, and 25 shears or shear zones.
Tptpmn
The Tptpmn (Sta. 10+15 to 14+44) has a mean horizontal RQD rating of 60 (fair), including lithophysae, and 62 (fair), excluding lithophysae. The tunnel-calculated Q rating rates this unit as the second highest in the Cross Drift. The projected Q rating from the predictive report agrees with this assessment. The RMR system rates the Tptpmn and the Tptpln as the highest, with a rating of 60, (fair). The unit is generally characterized by lithophysae less than 3 percent by volume. The Tptpmn has 430 meters of exposure in the Cross Drift and has the least amount of fault/shear activity with a total of 6 faults, 1 fault zone, and 13 shears. It has an average of three to three+ random joint sets. The horizontal joint sets, or vapor-phase partings, cause significant problems with keyblocks at Sta. 10+80 to 11+55 and Sta. 13+10 to 13+15.
Tptpll
The Tptpll (Sta.14+44 to 23+26) has a horizontal RQD rating of 42 (poor). Its tunnel-calculated Q rating is7.9 (fair), the lowest in the Cross Drift. The predicted Q rating 10.6 (good), from the Geotechnical Baseline Report agrees with this. The RMR system rates this unit at 57 (fair). The Tptpll is generally characterized by lithophysae of 5 to 15 percent by volume and range in size from 5 to 130 cm. The larger lithophysae cavities tend to be irregular or ellipsoidal features, that exhibit primatic fracturing. The unit has an average of two+ random joint sets; however no keyblock problems are apparent. The Tptpll has 4 faults and 30 shears exposed in 882 meters of rated tunnel.
Tptpln
The Tptpln (Sta. 23+26 to 25+85) has the best horizontal RQD ratings: 62 (fair), including lithophysae, and 67 (fair), excluding the lithophysae cavities. Its tunnel-calculated Q rating is 12.3 (good). The predicted Q rating, from the predictive report agrees with this rating. The RMR system rates this unit a 60 (fair). This unit is characterized by lithophysae cavities generally less than three percent by volume. It has an average of three joint sets, with no keyblock problems. The Tptpln has 6 faults and 36 shear or shear zones.
SUMMARY
U.S. Bureau of Reclamation personnel have collected detailed information about the lithology, structure, and geotechnical properties of the rock units exposed in the Cross Drift at Yucca Mountain. This effort is part of a continuing investigation to analyze Yucca Mountain as a potential site for a geologic nuclear-waste repository. The data presented in this report can be used in geologic and hydrologic site characterization studies, regional interpretations, and repository design. The Cross Drift exposes units of the Topopah Spring Tuff, a densely welded, pyroclastic flow that grades upward from a crystal-poor, rhyolitic composition to a crystal-rich, quartz-latite composition. Units exposed in the crystal-poor member of the Topopah Spring Tuff, include the Tptpul (Sta. 0+00 to 10+15), the Tptpmn (Sta. 10+15 to 14+44), the Tptpll (Sta. 14+44 to 23+26), and the Tptpln (Sta. 23+26 to 25+85). The lower portion of the crystal-rich transition subzone (Tptrl1) is exposed on the west side of the Solitario Canyon fault from Sta. 26+57.5 to 26+64 (right wall; this is the last exposure behind the TBM cutter head). One of the principle geologic objectives of the Cross Drift excavation was to expose the Tptpln, Solitario Canyon fault zone, and a larger portion of the Tptpll. The Tptpln comprises moderately to densely welded, devitrified pyroclastic-flow material. It is generally composed of 3 to 20 percent pumice, 1 to 2 percent phenocrysts, 3 to 7 percent lithic fragments, 0 to 5 percent lithophysae, and 66 to 93 percent matrix. Vapor-phase alteration products form a minor component of the rock in some portions of the unit. Rocks of the Tptpln zone vary from a heterogeneous mix of grayish red and grayish orange pink (5YR7/2) to comparatively homogeneous pale red, light brown, pale brown, or grayish brown (5YR6/4). In proximity to the SCFZ, the unit is brecciated and altered. In this area, the breccia matrix varies from moderate reddish brown to grayish orange pink to pale red; breccia clasts are locally bleached to very light gray adjacent to the fault plane. The SCFZ is the most laterally continuous and displays the most offset of any structure in the immediate vicinity of Yucca Mountain. Based on stratigraphic relationships, the offset is estimated to be about 260 m. In the Cross Drift, the SCFZ was expected to be composed of two major fault strands; the first was projected to be encountered near Sta. 25+65 (eastern strand), the second near Sta. 27+40 (western strand). A smaller, northeast-striking splay of the SCFZ was projected near Sta. 25+55. Displacement across the fault was expected to place Tptpln against the Tptpul. The western strand was expected to drop bedded tuffs of the Pah Canyon Tuff and the pre-Pah canyon Tuff (Tpbt2) against the Tptpmn. Between these two larger strands, several smaller faults were expected to be associated with the SCFZ faulting. During construction, a decision was made to stop the tunnel boring machine at Sta. 26+81, between the two strands. This decision was based on programmatic considerations and the desire to preserve the western strand of the SCFZ, allowing pneumatic and hydrologic testing of the fault zone prior to disturbing the zone by excavating through it. For this reason, the TBM penetrated only the eastern main strand of the SCFZ. The western strand was not reached in the Cross Drift. The degree of observed footwall deformation in the eastern strand was not anticipated in the Geotechnical Baseline Report. Surface exposures in Solitario Canyon indicated that the footwall deformation would probably be limited, with deformation and possible blocky ground occurring on the hanging wall side of the fault zone. The extent of the footwall deformation, extending nearly 50 m east of the fault, was greater than anticipated. A possible explanation for the footwall fracturing may be the presence of a northeast-trending splay of the SCFZ, originally anticipated at Sta. 25+50. This fault was not observed in the tunnel, nor were any distinct offsets observed east of the main splay at Sta. 25+84. The extent of footwall deformation may indicate a somewhat broad zone of deformation at the intersection of the northeast splay with the eastern strand of the SCFZ. The relative lack of difficulty in excavating through the eastern strand of the SCFZ can be explained by the character of the fault breccias present in the hanging wall. Although not well cemented west of Sta. 25+90, the breccias maintain enough internal cohesion to support the opening, with little or no ravelling, and little deterioration over time. West of the eastern strand of the SCFZ (Sta. 26+00), small faults and shears occur every few meters and continues to the present heading of the tunnel at Sta. 26+81, and is consistent with preconstruction expectations (CRWMS M&O, 1998, p. 4-17). The lower lithophysal zone is exposed along the Cross Drift from Sta. 14+44 to 23+26. In general, the moderately to densely welded, devitrified and vapor-phase altered unit is composed of 3 to 7 percent pumice (locally 10 to 35 percent), 1 to 2 percent phenocrysts, 1 to 5 percent lithic fragments (locally 12 to 15 percent), 5 to 30 percent lithophysae (locally 1 to 5 percent), and 56 to 90 percent matrix. Throughout most of the unit, vapor-phase spots, stringers, and wisps comprise between 3 and 12 percent of the rock. In several intervals, however, vapor-phase alteration products form 15 to 40 percent of the rock. The Comparative Geologic Cross Section Along Cross Drift (Drawing OA-46-345) was developed by the underground mapping team from the as-built geology of the Cross Drift. The as-built cross section was compared to the pre-construction cross section assembled by the USGS (Potter and others, 1998). Generally these sections compare favorably: the contacts and structures on the pre-construction section were encountered where expected. Although there are discrepancies between contact predictions and actual locations, these can be attributed to pinching and swelling of the lithostratigraphic zones, variations in dip, and the distance these contacts were projected from drill holes. Stratigraphically, these discrepancies involve minimal changes to predicted stratigraphic thicknesses. The fault at Sta. 22+38 has no known surface expression and is not encountered in any drill holes, therefore it does not appear on the pre-construction cross section. The stratigraphic section traversed by both the ESF and the Cross Drift include the lower 75 percent of the Tptpul, the Tptpmn, and the Tptpll. The lithologic character of the units exposed in the ESF and the Cross Drift is similar in terms of the welding, devitrification, and vapor-phase alteration. The pumice and lithic fragment content is similar in type, size, and color in the various stratigraphic intervals noted in both locations. The type, size, abundance, and character of vapor-phase features such as, vapor-phase alteration, vapor-phase partings, stringers and spots, and lithophysae are similar in comparable stratigraphic intervals in the ESF and the Cross Drift. However, there are some notable differences. The lithophysae-bearing subzone of the middle nonlithophysal zone (Tptpmn2) does not appear in the ESF. The zone does appear in the Cross Drift, but is poorly developed. There is a well-defined bimodal distribution in the size of lithophysal cavities in the upper lithophysal zone exposed in the ESF. In the Cross Drift, the bimodal distribution is discernible, but is poorly developed. Also, the intensely fractured zone (IFZ) so prominent in the Main Drift of the ESF, does not occur in the Cross Drift. Analysis of DLS fractures in the Cross Drift showed strong correlation to conclusions drawn from fracture analysis of the ESF. Particularly strong correlations were found when fracture analysis of sets from the Cross Drift were compared to sets exhibited in the ESF. Analysis of the Tptpul, Tptpmn, Tptpll, and Tptpln of the Cross Drift produced Sets 1, 2, and 3. These sets correspond well to Sets 1, 2, and 3 of the ESF. A notable exception occurs in the Tptpll of the Cross Drift, where a Set 4 was encountered which does not correspond to any sets of the Tptpll of the ESF. Additionally, subjective visual analysis of contour plots of all fractures in each respective unit of the Cross Drift agrees well with the contours identified by Clustran. Geotechnical characterization of the Cross Drift focused primarily on rock-mass quality and rock-mass mechanical properties. Descriptions are based on two empirical rock mass classification systems, rock quality (Q system) and rock-mass rating (RMR). The rock-mass quality (Q) encountered in the Cross Drift is generally good, with the exception of the Tptpll in the fair category. The rock-mass rating (RMR) encountered in the Tptpll is borderline fair to good. The rankings and ratings agree with the Geotechnical Baseline Report, however, the Q values calculated are three times better than anticipated. These high ratings indicate that very little support is required in the stratigraphic units exposed in the Cross Drift (or similar units in different locations), other than occasional spot-bolting.
REFERENCES
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