ETL 1110-2-544
31 Jul 95
b. Known boundary condition. The mesh
excavation, fill placement, placement and removal of
should also extend beyond the area of interest until a
structural components, and application of loads and
known boundary condition is encountered (e.g., bed-
pressures. Less often it can be important to model
rock can often be represented as a fixed boundary
other sources of load, such as thermal strains in
condition) or for a sufficient distance that conditions at
structural elements and compaction-induced lateral
the boundary do not significantly influence the
earth pressures, for example.
calculated stresses and deformations in the area of
interest.
2-7. Calibration of the Entire Model
The finite element mesh for an SSI analysis can
include several different types of elements:
a. Consider factors. As can be seen from the
foregoing, there are several factors that must be care-
(1) Two-dimensional (2-D) elements. 2-D
fully considered to develop a good finite element
elements for the soil and concrete portions of plane-
model of the SSI problem. It is important to success-
strain and axisymmetric analyses.
ful application of the method to calibrate the entire
process against instrumented case histories. Fortu-
(2) 3-D brick elements. 3-D brick elements for
nately, several such comparisons have been published
the soil and concrete portions of 3-D analyses (al-
(See Reference list in Chapter 5).
though it should be pointed out that 3-D analyses of
geotechnical engineering problems are rare because of
b. View results with caution. Whenever the
the great cost and time necessary for setting up the
method is applied in an unprecedented way, the
problem and interpreting the results, as well as due to
results should be viewed with caution until confirma-
the fact that many important aspects of 3-D problems
tion by comparison with an instrumented case history
can be modeled using 2-D meshes).
can be established.
(3) Beam or shell elements. Beam or shell
elements for sheet-pile walls, cellular cofferdams, and
2-8. Case History: Retaining Wall at
other structural components.
Bonneville Navigation Locks
(4) Bar elements. Bar elements for struts and
a. Project description. A temporary tieback
tiebacks.
wall was built to retain the excavation for construction
of a new navigation lock at Bonneville Dam on the
(5) Interface elements. Interface elements to
Columbia River between Oregon and Washington.
allow for slip between dissimilar materials such as
The geologic profile at the site, slide debris and man-
between backfill soil and a concrete retaining wall.
made fill overlying rock units, is shown in Figure 2.
The landslide occurred in the Pleistocene, and previ-
ous stability analyses had shown the landslide to be
stable in its preconstruction configuration. Figure 2
2-6. Construction Sequence
also shows excavation and a railroad line relocation
a. Construction sequence. As described in the
that took place prior to constructing the tieback wall.
introduction, it is important to model the construction
An important objective of the construction was to
sequence in soil-structure interaction problems for two
limit the magnitude of movements that would take
reasons: 1) soil response is nonlinear, and 2) the
place at the railroad line during excavation for the new
geometry can change during construction, e.g., fill
navigation lock.
placement.
The 440-ft-long wall consists of a series of reinforced
b. Initial in situ stresses. Because of the non-
concrete panels. Each panel was excavated by rock
linear stress-strain behavior of soils, it is almost always
chisel and clamshell, with the excavation supported by
necessary to first calculate the initial in situ stresses in
a bentonite-water slurry. After excavation of each
the foundation materials. Perhaps the only exception
panel, reinforcement was placed and the excavation
occurs when a rock foundation is being modeled as
was backfilled with concrete. The heights of the pan-
linear elastic. In addition, it is necessary to model the
els range from 20 to 110 ft. Following completion of
following types of construction operations in steps:
the wall panels, excavation for the navigation lock
A-6
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