ETL 1110-2-544
31 Jul 95
(1) Sampling and laboratory testing. For
with respect to the external surface geometry and the
foundation soils, relatively undisturbed samples
distribution of materials in the embankment and
should be obtained. For embankment or backfill
underlying foundation. Additionally, the mesh should
materials, laboratory compacted specimens can be
reflect the configuration of any excavations or filling
prepared. In either case, the specimens should be
operations performed as part of construction. Most
tested in the laboratory in an appropriate manner to
embankment construction problems are either 2-D
obtain the necessary parameter values for the
plane-strain or 3-D type analyses. Levees or
material model that will be used. Typical
embankment dams constructed across broad alluvial
laboratory tests for obtaining these values are 1-D
valleys are good candidates for 2-D plane strain
consolidation tests, isotropic consolidation tests,
analysis, whereas embankment dams constructed
triaxial compression tests, and direct simple shear
within narrow canyons are good candidates for a 3-D
tests.
finite element analysis. The mesh should also extend
beyond the area of interest until a known boundary
(2) Field testing. Some in situ tests, e.g., the
condition is encountered (e.g., bedrock can often be
borehole pressuremeter tests can be performed to
represented as a fixed boundary condition) or for a
obtain material property values.
sufficient distance that conditions at the boundary do
not significantly influence the calculated stresses and
(3) Correlations with index property values.
deformations in the area of interest.
Stress-strain material property values for several
soils have been published together with index
d. Construction sequence. It is important to
property values for the same soils, e.g., Duncan et
model the construction sequence in embankment
al. (1980). These published values, together with
problems for two reasons:
judgment and experience, can be used to estimate
appropriate stress-strain material property values
(1) Soil response is nonlinear.
based on index property test results for the soils of
interest.
(2) Geometry can change during construction,
e.g., fill placement.
(4) Calibration studies. In many cases, designers
have experience with local soils and are skilled at
Because the nonlinear stress-strain behavior of soils
calculating 1-D consolidation settlements using
depends on the confining pressure, it is almost always
conventional procedures. It is good practice in such
necessary to first calculate the initial in situ stresses in
cases to develop a 1-D column of finite elements that
the foundation materials. Perhaps the only exception
models the soil profile at the site of interest. The 1-D
occurs when a rock foundation is being modeled as
column can be loaded and the resulting settlements
linear elastic. In addition, it is necessary to model the
compared to those calculated using conventional
following types of construction operations in steps:
procedures. The material property values for the finite
excavation, fill placement, placement and removal of
elment analyses can be adjusted until a match is
structural components, and application of loads and
obtained. Similarly, if an independent estimate of the
pressures. The construction steps should be modeled
lateral load response, i.e., the Poisson effect, can be
in the actual order in which they are to be carried out.
made, the material property values can be adjusted
until the 1-D column results match the independent
e. Calibration of the entire model. As can be
estimate. Ideally, one set of material property values
seen from the foregoing, there are several factors that
would be found that provides a match to both the
must be carefully considered to develop a good finite
compressiblity and the lateral load response over the
element model of an embankment construction
range of applied loads in the problem to be analyzed.
problem. It is important to successful application of
The selection of a method to obtain material property
the method over the years to calibrate the entire
values depends, of course, on the type of information
process against instrumented case histories.
available. These methods are most effective when
Fortunately, several such comparisons have been
used in combination.
published. Several of these are listed among the
references in Chapter 4.
c. Finite element mesh. The finite element mesh
should reflect the geometry of the embankment, both
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