ETL 1110-2-544
31 Jul 95
pressure. These aspects of soil behavior are en-
(2) Field testing. Some in situ tests, e.g., the
countered in most geotechnical engineering projects,
borehole pressuremeter tests, can be performed to
including projects where SSI is important. Con-
obtain material property values.
sequently, it is important that the material model be
capable of tracking these aspects of soil behavior.
(3) Correlations with index property values.
Stress-strain material property values for several soils
c. Material models. Many material models,
have been published together with index property
such as the hyperbolic model of Duncan and Chang
values for the same soils, e.g., Duncan et al. (1980).
(1970) and the Cam-Clay model (Roscoe and Burland
These published values, together with judgment and
1968), do capture these characteristics of soils. The
experience, can be used to estimate appropriate stress-
hyperbolic model uses a confining pressure-
strain material property values based on index property
dependent, nonlinear elastic formulation, with an
test results for the soils of interest.
inelastic component introduced, because the value of
the unload-reload modulus is larger than the value of
(4) Calibration studies. In many cases, designers
the virgin loading modulus. The Cam-Clay model
have experience with local soils and are skilled at
uses a plasticity formulation that also yields reduced
calculating 1-D consolidation settlements using
modulus values as the soil strength becomes mobilized
conventional procedures. It is good practice in such
and increased modulus values as the confining
cases to develop a 1-D column of finite elements that
pressure increases. One of the key benefits of plas-
models the soil profile at the site of interest. The 1-D
ticity is that it can model plastic strains that occur in
column can be loaded and the resulting settlements
directions other than the direction of the applied
compared to those calculated using conventional
stress increment. This feature becomes especially
procedures. The material property values for the finite
important when a soil mass is near failure. In such a
element analyses can be adjusted until a match is
case, the application of a load increment in one
obtained. Similarly, if an independent estimate of the
direction can cause large displacements of the soil in
lateral load response, i.e., the Poisson effect, can be
another direction if large forces had been previously
made, the material property values can be adjusted
applied in that other direction. For well-designed
until the 1-D column results match the independent
structures in which failure of large masses of soil is
estimate. Ideally, one set of material property values
not imminent, modeling this aspect of failure can
would be found that provides a match to both the
become less important.
compressibility and the lateral load response over the
range of applied loads in the SSI problem to be
analyzed.
2-4. Stress-Strain Material Properties Values
b. Selection of method. The selection of a
a. Material property values. Selection of
method to obtain material property values depends, of
appropriate stress-strain material property values is
course, on the type of information available. The
often the most important step in performing SSI
above methods are most effective when used in
analyses. There are four methods to obtain material
combination.
property values:
(1) Sampling and laboratory testing. For foun-
2-5. Finite Element Mesh
dation soils, relatively undisturbed samples should be
obtained. For embankment or backfill materials, lab-
a. Finite element mesh. The finite element mesh
oratory compacted specimens can be prepared. In
for an SSI analysis should reflect the geometry of the
either case, the specimens should be tested in the lab-
structure, the stratigraphy in the foundation, and the
oratory in an appropriate manner to obtain the neces-
configuration of any excavations and/or fills that are
sary parameter values for the material model that will
part of the work. In addition, the mesh should have
be used. Typical laboratory tests for obtaining these
sufficient refinement that deformations and stress
values are one-dimensional (1-D) consolidation tests,
gradients are smoothed as one moves from element to
isotropic consolidation tests, triaxial compression tests,
element in areas of interest.
and direct, simple shear tests.
A-5
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