ETL 1110-2-544
31 Jul 95
c. Parameters. The parameters required for
would not contribute to settlements, if the anticipated
most of the soil stress-strain models that are used in
constructions sequence was simple, and if the
finite element analyses can be determined from the
properties of the embankment materials could be
results of conventional soil laboratory tests, such as
estimated using available data for similar soils, a
triaxial tests or direct shear tests used in combination
geotechnical engineer experienced in performing finite
with consolidation tests. The results of in situ tests
element analyses might be able to develop the mesh,
(SPT, CPT, or PMT) can be used, together with data
select the soil parameter values, perform the analysis,
for similar soils, to estimate stress-strain parameters
and summarize the results in 1 or 2 days. Computer
when results of tests on undisturbed tests are not
costs would be negligible, because this type of analysis
available.
could be performed readily on a 486 computer.
b. Dynamic analyses. If the conditions analyzed
or the objectives of the analysis are more complex,
1-5. Skills Required for Geotechnical Finite
considerably more time may be required to perform
Element Analyses
finite element analyses. Among the most time-
By far the most important skill required for geotech-
consuming analyses are dynamic analyses of
nical finite element analyses is a firm understanding of
earthquake response, analyses of consolidation that
the geotechnical engineering aspects of the problem
model elasto-plastic soil stress-strain behavior, and
being analyzed, most particularly the physical behavior
three-dimensional (3-D) analyses. An example of a
of soils and rocks. It is also necessary to understand
very complex series of analyses are the 3-D analyses
the principles of mechanics and numerical analyses that
performed to estimate the movements, earth pressures,
form the basis for the finite element method. An
and pore pressures in New Melones Dam, California.
engineer with a solid background in geotechnical
These analyses (which had a research component
engineering and mechanics can become effective in
because they were unprecedented at the time) required
using the finite element method within a few weeks or
a total of about 4,000 hours of effort. About one-
months. The time required to become skilled in using
fourth of this time was needed to evaluate the stress-
the method is shortened considerably by the
strain characteristics needed for analyses, which used
opportunity to work with and learn from an engineer
both hyperbolic and Cam Clay properties.
who has already achieved mastery of the method.
When weeks or months to learn to apply the method
c. Costs. Costs of finite element analyses have
are not available or affordable, the alternative is to
decreased in the past few years because the cost of
engage an engineer who is already an expert with this
computers has decreased so dramatically. New
method. The examples summarized in subsequent
computer programs are now available that use
sections of this report include suggestions for further
graphical preprocessors and postprocessors to reduce
study and for possible sources of expert assistance that
the amount of time required to prepare input, to
may be of use in either case.
interpret, and to plot output.
1-6. Required Effort and Cost
1-7. Finite Element Codes Used on Corps'
Projects
a. Minimum time and effort. The amounts of
effort and time required for finite element analyses
a. Corps of Engineers' experience. The Corps
vary over a wide range, depending on the purpose of
of Engineers has extensive experience in the use of
the analysis and the complexity of the problem
the finite element method for the analysis of geotech-
analyzed. An example of the minimum time and effort
nical projects. A list of the most commonly used
would be a simple analysis performed to estimate
codes is provided in Chapter 5. Chapter 5 also
possible settlements within an embankment during
includes an extensive bibliography that can be used to
construction. If the geometry of the embankment was
obtain further information on the finite element
simple, if the foundation was rock or firm soil that
method.
A-3
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