6 VOLUMETRIC WATER CONTENT
6.1 DEFINITION
The water content in soils is usually expressed as either a dimensionless
ratio of two masses or two volumes, or is given as a ratio of a mass per
unit volume. These dimensionless ratios can be reported either as decimal
fractions or percentages, if multiplied by 100. To avoid confusion between
the two dimensionless water content ratios, their basis (i.e., mass or
volume) should always be stated. However, in cases in which no indication
is given, the figure is assumed to be based on mass because in the determination
of the soil water content, the mass-basis figure is usually obtained first
and then converted to a volume-basis figure (Gardner 1986). In RESRAD,
the input data related to the water content in soil materials are entered
on a volume basis (volumetric water content).
The water content in soils on a mass basis, w, is defined as
the ratio of the mass of the liquid phase (water), Ml,
in the given soil sample to the mass of the solid material, Ms,
according to the following expression:
The volumetric water content, , in the soil (also called the
volume wetness or volume fraction of soil water) represents the fraction
of the total volume of soil that is occupied by the water contained in
the soil. Assuming that Vl is the volume of the liquid
phase (water) in the soil sample and that Vt is the total
volume of the sample, the volumetric water content, , can then be defined
as follows:
where Vs and Vp represent, respectively,
the volumes of the solid phase and the pore space.
From the definition presented in Equations 6.1 and 6.2, the volumetric
water content, , can be expressed in terms of the mass-basis water content,
w, according to the following formula:
where b is the bulk density of the soil (see Section
2) and w is the water density.
The volumetric water content is also expressed in terms of the total
porosity, pt, and the water saturation (or saturation
ratio), Rs, according to the following expression:
where pt is the total porosity (Section 3.1 for parameter
definition), and Rs, the saturation ratio, is defined
as the ratio of the volume of water, Vl, to the volume
of the pore space, Vp. Therefore, considering the definitions
of pt and Rs, the expression for the
volumetric water content can be rewritten as follows:
The possible values for range from near zero for dry soils approaching
zero saturation, up to the value of the total porosity for fully saturated
soils. The lower limit of zero for the volumetric water content is hardly
achievable because it is difficult to completely eliminate the water from
the soil. In sandy soils, the upper limit of , which is equal to
the total porosity pt, is also hardly achievable because
of the difficulty of eliminating all the air bubbles in the soil in order
to saturate it completely. Yet, because clayey soils swell upon wetting,
the values of for these soils can exceed their total porosity.
6.2 MEASUREMENT METHODOLOGY
Direct and indirect methods can be used to determine the volumetric
water content of soils. The direct methods consist essentially of drying
and weighing a known volume of a soil sample. The indirect methods are
based on the correlation of certain physical and physicochemical properties
of the soil with its water content.
An extensive discussion on both direct and indirect methodologies for
measuring water content in soils is presented in Gardner (1986). On FUSRAP
sites, the standard method used for determining the (mass-basis) water
content in soil materials is ASTM D 2216-90, Standard Test Method
for Laboratory Determination of Water (Moisture) Content of Soil and Rock
(ASTM 1992e). This method is related to the determination of the mass-basis
water content, w, rather than to the volumetric water content, ,
as required in RESRAD. However, the volumetric water content can be determined
from Equation 6.3. when the mass-basis water content and the bulk density
of the soil material (Section 2) are known.
Generally, in a direct measurement method, the volumetric water content of a soil sample is evaluated on the basis of three measured quantities: (1) Ww, the wet weight of the soil sample; (2) Wd, the oven-dried weight of the sample; and, (3) Vt, the field volume or the
total volume of the sample. With these measured quantities available,
the volume of the liquid phase (water), Vl, in the sample
can then be calculated as
and the volumetric water content () can finally be determined from Equations
6.2 and 6.6 as
where w is the density of water.
Variations in the direct methods for determining the volumetric water
content are related to different ways of collecting the soil samples, measuring
the field volume (Vt ), and drying the samples. Possible
direct methods of collecting the soil samples and measuring Vt
have been discussed in Section 2.2 in regard to soil densities.
The definition of a dry state for the soil sample (and the establishment
of a method to achieve this state) constitutes the key problem in determining
the volumetric water content in soils. As a common practice, such as that
described in Section 2.2, the oven-dried weight of the soil sample
is measured after drying the sample at 105C until a near constant weight
is reached (Hillel 1980b). As discussed by Gardner (1986), however, this
procedure for the ovendry method is not precise enough and could create
uncertainties and inaccuracies in the measured result. Therefore, if the
determinations of water content for a particular site are considered critical,
other procedures than the ovendry method should be adopted (Gardner 1986).
The indirect methods of measuring the water content in soils rely on
certain physical and physicochemical properties of the soil and their relation
to the volumetric water content (). Usually these relationships are complicated
and require a sophisticated methodology and equipment to express them.
The indirect methods of measuring volumetric water content are applicable
for in-situ rather than laboratory determinations and involve measuring
some property of the soil that is affected by the soil water content such
as (1) electrical conductivity, (2) neutron scattering, or (3) neutron
and gamma-ray absorption (Gardner 1986).
Similar to the discussion of the determination of soil densities, the
indirect methods used for measuring volumetric water content present some
advantages over the other related laboratory techniques. The main advantages
are (1) in-situ evaluation of the water content; (2) minimum
disturbance of the soil; (3) relatively short measurement time, (4) applicability
to deeper subsoil determinations because of minimum excavation requirements;
and (5) nondestructiveness, with the possibility of continuous or
repeated measurements at the same spot. The disadvantages of such indirect
methods are that they are more sophisticated and require expensive equipment
and highly trained operators who must be able to handle the frequent calibration
procedures, the electronics, and the sampling equipment. In the case of
a system that uses radioactive elements, the operator must be particularly
trained in the radiation aspects and radiological protection procedures
of the whole operation.
6.3 RESRAD DATA INPUT REQUIREMENTS
To use RESRAD, it is necessary to define an input value for the volumetric
water content () of the soil of the cover zone and the building foundation
material (i.e., concrete). In RESRAD, the dimensionless values of the volumetric
water content are entered as decimal fractions rather than as percentages.
For generic use of the model, a set of default values for the volumetric
water content is defined internally in the code. The default values are
= 0.05 for the cover material and = 0.01 for the building foundation
material (i.e., concrete). Considering the default values for total porosity,
0.4 and 0.1, the volumetric water content values correspond to saturations
of 0.125 and 0.1 for the cover material and concrete, respectively.
For more accurate use of the RESRAD code, site-specific values of the volumetric water content should be experimentally determined according to the methods presented in Section 6.2.