Environmental Engineering Reference
In-Depth Information
Determination of Unsaturated Soil Property Functions
Indirect Measurement
Experiments
Direct Measurement
Experiments
Classification Tests
(Grain-Size Distribution)
Database "Mining"
Present the
Unsaturated
Soil Property Functions
Soil-Water Characteristic
Curve Measurements
Estimation of Soil-Water
Characteristic Curve
Estimation of Soil-Water
Characteristic Curve
Compute the
Unsaturated
Soil Property Functions
Compute the
Unsaturated
Soil Property Functions
Compute the
Unsaturated
Soil Property Functions
Figure 1.6
Methodologies for determination of unsaturated soil property functions.
nature. The direct application of unsaturated soil theories
at the highest level involves unsaturated soil testing with
soil suction measurements (or controls). These tests are rel-
atively complex and time consuming to perform in compar-
ison to saturated soil tests.
Other approaches for the implementation of unsaturated
soils theories in practice have been suggested wherein the
SWCC is used along with saturated soil parameters to esti-
mate unsaturated soil properties (Fredlund, 1995b). These
indirect measurements and estimation procedures open the
way for a hierarchical approach to the application of unsat-
urated soil mechanics in geotechnical engineering practice.
The assumption is made that saturated soil properties are
known. The unsaturated soil property functions take the
form of a smooth extension of the saturated soil properties.
Unsaturated soil property functions may be defined with
reasonable accuracy for many soil mechanics problems by
performing an indirect laboratory test (e.g., a pressure plate
test to measure the SWCC). The data from the SWCC can
be used to calculate the required unsaturated soil property
functions. It is also possible to obtain an indication of the
SWCC from classification soil properties, correlations, or
“mining” databases containing the results of past laboratory
soil tests.
Classification tests (e.g., grain-size distribution curve) can
also be used for the estimation of a SWCC. The grain-size
distribution curve is used to estimate the SWCC that is then
used to compute the desired unsaturated soil property func-
tion (Fredlund et al., 1997a). There may be a reduction in
the accuracy of the estimated unsaturated soil property func-
tion when using this procedure. The engineer must assess
whether or not the approximated unsaturated soil property
function is satisfactory for the analysis of the problem at
hand. This estimation procedure is somewhat analogous to
using the grain-size distribution curve to estimate the satu-
rated hydraulic conductivity of a soil (Hazen, 1911).
1.3.2 Design Protocols for Unsaturated Soil Properties
Engineering design protocols can generally be placed within
one of two primary categories: (1) preliminary design and
(2) final design. Preliminary design protocols can make
ample use of estimated unsaturated soil properties whereas
final design protocols need to rely on measured values
of the SWCC or direct measurement of unsaturated soil
property functions. Further details on the estimation and
measurement of unsaturated soil properties are provided
throughout this topic.
1.4 STAGES IN MOVING TOWARD
IMPLEMENTATION
Developing a science basis for unsaturated soil mechan-
ics can be viewed in terms of a series of stages leading
toward implementation in engineering practice. The stages
leading toward implementation are listed in Fig. 1.7 (Fred-
lund, 2000). Research studies over the past six decades have
been directed at all stages leading toward an appropriate
technology for implementation.
1.4.1 State Variable Stage
The state variable stage is the most basic and fundamental
level at which a science for unsaturated soil behavior can be
initiated. Themost important state variables for an unsaturated
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