ETL 1110-2-544
31 Jul 95
Such questions can often be addressed by performing
Chapter 2
parameter studies. Parameter studies on material
Static Soil Structure Interaction
property values are relatively easy to perform once an
Problems
SSI model has been set up. If reasonable variations of
material property values result in acceptable values of
calculated stresses and deformations, further field a-
nd/or laboratory work may not be necessary. On the
Soil structure interaction (SSI) problems are those
other hand, if reasonable variations of material prop-
where earth pressures depend on structure movements
erty values result in unacceptable stresses or deforma-
or deflections and structure movements or deflections
tions, it may be necessary to modify the proposed con-
depend on earth pressures. To analyze these prob-
struction in some way or to expend further effort to
lems, the foundation soil, the structure, and the back-
characterize subsurface conditions. In the latter case,
fill or retained soil must be considered. Examples of
the effort can be focused on those aspects of the prob-
SSI problems include: anchored walls to stabilize
lem that have been found by SSI analysis to be critical
landslides, cellular cofferdams, excavation bracing
to performance.
systems, long-span flexible culverts, reinforced slopes
and embankments, retaining walls, U-frame locks, and
b. Efficient application. In addition to its use-
tunnels.
fulness for predicting performance before construc-
tion, an SSI analysis using finite elements can
contribute to efficient application of the observational
2-1. Results and Use of SSI Analyses
method. The analysis results can be used to identify
both representative and critical locations of installation
a. Principal results. The principal results that
of instrumentation that will be used to monitor perfor-
can be obtained from an SSI analysis using finite ele-
mance during construction. Field measurements
ments are the stresses and displacements of the struc-
obtained from the instrumentation during early phases
ture and the soil. In most real design problems, the
of construction can be used to calibrate the finite ele-
stresses and displacements of the soil and structure
ment model. The calibrated model can then be used
can only be calculated using a numerical method
to make more reliable predictions of final
like a finite element analysis. Conventional limit
displacements and stresses and to evaluate whether
equilibrium methods, which do not predict displace-
specific contingency plans should be implemented.
ments, are adequate for design where there is a
sufficient base of experience. When there is less
experience, or when displacements are critical, SSI
2-2. Important Features of SSI Analysis
analyses may be needed.
The following paragraphs describe several of the
An SSI analysis can be used as a design tool in the
most important features and considerations for de-
following ways:
velopment of a good finite element model for an
SSI analysis.
(1) Calculated values. Stresses and deforma-
tions of the structure and/or the soil can be calcu-
lated, and the calculated values can be compared to
2-3. Material Behavior Models
allowable values. If necessary, changes in the sys-
tem configuration or the constructed component
stiffnesses can be made, and the SSI analysis can be
a. Linear elasticity. Structural components in
repeated until the calculated stresses and deformations
SSI analyses are most frequently modeled using
are acceptable.
linear elasticity. Rock units in the foundation are
also frequently modeled using linear elasticity. Soil
behavior, on the other hand, is usually more
(2) Questions that arise. The "what if" ques-
complex.
tions that arise during the design process can be
addressed in a rational manner. For example, due
to subsurface heterogeneity and limited budgets for
b. Confining pressure. As described in the
exploratory work and laboratory testing, significant
introduction of this ETL, the stress-strain behavior of
uncertainty can exist in characterizing subsurface
soil is nonlinear and inelastic. For all cases except
conditions. This uncertainty can create questions
saturated soil under undrained conditions, the stress-
concerning the reliability of performance predictions.
strain behavior of soil is dependent on confining
A-4
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