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permanent earthquake displacements), and is termed baseline shift. The baseline shift has been
removed from all of the presented FLAC displacements.
6.3.3 Modeling of the Water
The water in front of slopes was modeled indirectly by including the resulting water pressures
along the boundaries. This allows a simple modeling of the water, but does not account for the
dynamic interaction of the slope and water. The water within the soil was modeled directly and
was allowed to flow during the static solutions. During the dynamic solutions, excess pore
pressures were allowed to generate, but the dissipation of these pore pressures was not modeled
(as previously addressed).
6.3.4 Boundary Conditions
The boundary conditions for the static solutions consisted of the bottom boundary being fixed in
both the horizontal and vertical directions, while the sides of the model were treated as rollers
(by fixing only the horizontal direction). During the dynamic analysis the bottom boundary was
freed in the horizontal direction to allow application of the horizontal acceleration, and the
sidewalls were treated as an infinite medium (free field), having the same properties as the
adjacent model perimeter zones. The lateral dimensions of the model were determined as
optimum from the results of prior validation case studies as five to seven times the total
embankment height in the approach fill and three times the total embankment height in front of
the embankment slope.
6.4 VALIDATION OF NUMERICAL MODEL
The use of FLAC for the seismic modeling of earth structures with and without soil improvement
required that it be validated and calibrated. Given the rather widespread use of this tool in
geotechnical engineering practice, numerous studies have been performed to assess the strengths
and limitations of the model. These investigations have focused on the accuracy of the computed
seismically-induced deformations as compared to the permanent deformations measured in well
documented case studies. Pertinent investigations are shown below.
Earth structures and liquefaction hazards (Inel et al.1993; Roth et al. 1993; Beaty and
Byrne 1998).
Gravity retaining walls in unimproved and improved soils (Dickenson and Yang 1998).
Anchored sheet pile walls in unimproved and improved soils (McCullough and
Dickenson 1998).
Pile supported wharf structures (Yang 1999).
Behavior of spread footings founded on competent soils underlain by liquefiable soil
(Naesgaard et al. 1998).
Direct comparison with dynamic centrifuge models of earth slopes (Roth et al. 1986,
1993).
The strengths and limitations of the numerical model have been well documented in these recent
publications. The nuances of the constitutive model and pore pressure generation scheme as
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