SALT-CRUST THICKNESS STUDIES
Pre-1994 Auger-Hole Studies

Since 1925, at least 3 studies that involved auger-borehole drilling have been conducted to measure the thickness of the BSF salt crust: 1) Nolan, 1927, 2) Tea (Utah State Department of Highways - USDH), 1974, and 3) Brooks, 1991.

In the earliest study, Nolan (1927, p. 35) stated that the BSF salt crust "...is reported to reach a maximum thickness of 5 feet near the central portion of the deposit." However, during boring, the greatest thickness obtained by Nolan was 3.5 feet near Salduro station on the old Western Pacific Railroad.

The USDH study (Tea, 1974, unpublished field notes, pp. 8-12) recorded ranges of total salt-crust thickness measured during 1960 and 1974 along the axis of the International Track. Measurements were taken from 13 bore-hole arrays comprised of as-few-as 2, to as-many-as 13 bore holes each. The long axis of each array was perpendicular to the axis of the northeast-trending International Track, and matched to one of 13 mile-post positions (MP-1 to MP-13) as they existed in 1960 (see Figure 1).

In 1988, Brooks (1991, 16 pp.) conducted a BLM study that attempted to duplicate the USDH study method by augering 14 bore-hole arrays comprised of as-few-as 5, to as-many-as 11 bore holes each. Each array was matched to a track mile post established along the International Track as it existed in 1988 (apparently the track had been offset to the southeast from its 1960 - 1974 position). The long axis of each 1988 array was oriented so it was perpendicular to the International Track axis.

BLM Test-Pit Studies

While thickness for the same stratum varies from one test pit to another, relationship of stratum sequence has remained relatively consistent during the 1994-1997 period (Fig. 9). For example, the dense, cemented-salt stratum at the surface has been consistently underlain by an uncemented-gypsum stratum, which is underlain by a cemented, coarse, porous-salt stratum. A second uncemented-gypsum stratum typically separates the cemented, coarse, porous-salt stratum from the uncemented, coarse-salt stratum.

During pit excavation, the dense, cemented salt and the cemented, coarse, porous-salt strata are usually removed from the test pit as intact slabs. In contrast, the uncemented-gypsum strata and the uncemented, coarse-salt stratum are typically removed with a shovel as loose crystals and as loosely aggregated masses.

Figure 9 shows the stratum sequence for each of the 10 BLM test pits excavated during the 1994-1997 period (Kohler, 1994-95, and Kohler and White, 1997). Table 13 lists the 10 test-pit locations with respect to 6 reference monitoring wells.

Figure 9. Salt-crust stratum seequence obseved in ten BLM test pits excavated during 1994-1997. Based on thr salt crust exposed in each pit, 5 distinct strata were observed (the "popcorn"-salt stratum was observed only in 1994). Only pit 4b was excavated into the underllying carbonate clay.

Evidence that the stratum sequence (as shown in Table 12 and Figure 9) has remained relatively consistent is suggested by presence of a similar sequence shown in two photographs contained in a joint Utah State University, Utah State Department of Highways engineering study of the proposed I-80 route through BSF (Nielson and others, 1960, pp. 10-11, figures 1 and 4). While individual descriptions of each stratum are not included in this report, number, orientation and relative thicknesses of each stratum shown in the two photographs appear to be consistent with correlative stratum described in BLM's 1994-1997 field investigations.

Of the ten BLM test pits, only pit 4b was excavated into the clay (carbonate-mud) stratum that bounds the bottom of the salt crust. The location of test pit 4b was just NW of MP-8 (see Figure 1), and total salt-crust thickness measured in September 1995 was 2.6 feet. Reported salt-crust thickness at MP-8 in 1988 (Brooks, 1991, Figure 4, and Table 1) ranged from 3.0 to 3.2 feet.

FY98 Salt-Crust Thickness Measurements

Figure 10. Coomparison of salt-crust strata thicknesses exposed in 4 BLM test pits excavated near monitoring well BLM-93. The uncememted, coarse-salt stratum is nnot included because none of  the 4 pits exposed its contact with the underlying carbonate clay.

This is approximately a 25% increase over the previous average-thickness measurements of dense, cemented-salt strata in test pits 1a-1c (Figure 10). Variability of average stratum thickness between test pit 1d and test pits 1a-1c was also observed in the uncemented gypsum #1, cemented, coarse, porous salt, and uncemented gypsum #2 strata. Comparison of thickness variability is not included for the uncemented, coarse salt stratum because none of the 4 test pits exposed its lower contact with the underlying carbonate clay.

Average thickness of the uncemented gypsum #1 stratum in each of the 4 test pits ranges from 0.05 to 0.09 feet (0.6 to 1.1 inches) with test pit 1d exhibiting a thickness of 0.09 feet. In contrast, a more consistent stratum thickness was observed between the 4 pits for the cemented, coarse, porous salt; thickness measurements range from 0.19 to 0.24 feet (2.25 to 2.9 inches) with test pit 1d having a thickness of 0.19 feet. Average thickness of the uncemented gypsum #2 stratum in each of the 4 test pits range from 0.05 to 0.13 feet (0.6 to 1.5 inches) with test pit 1d showing the greatest thickness of 0.13 feet.

Table 14 lists the actual ranges of individual stratum thicknesses in test pit 1d, while stratum thicknesses shown in Figure 10 for test pit 1d are averages of the stratum-thickness ranges listed in Table 14. Corresponding stratum thicknesses shown in Figure 10 for test pits 1a-1c are averages of stratum thickness ranges listed in unpublished BLM field notes (Kohler, 1994-1995).

Because the strata sequence comprising the salt crust appears to be consistent from year to year, BLM believes it is technically reasonable to measure post-Laydown thickness changes by recording the thicknesses of the uppermost three strata in all future test-pit excavations.