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

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.

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-

dation soils, relatively undisturbed samples should be

obtained. For embankment or backfill materials, lab-

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.

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