Report - Oregon State Library: State Employee Information Center ...

The earthquake time histories used in the analyses were recorded during the M w 6.2 Mammoth
Lakes earthquake (Long Valley Dam station) and M w 8.5 Michoacán earthquake (Aeropuerto
station). The motions were scaled to 0.29g and 0.12g, respectively. The SHAKE91 program was
used to generate time histories at elevation -18 m (the input location for the motions in FLAC).
The damping of the earthquake motions for numerical stability utilized Rayleigh damping. The
acceptable Rayleigh damping used in the models was determined from the validation studies to
be 5% at 5 Hz. Any baseline drift in the earthquake motions has been removed from all of the
presented FLAC displacements.
The ground motions computed with FLAC for the embankment and foundation soils (elevations
1.5m and 6.4m as previously referenced) differ from those computed using the SHAKE91
program for two primary reasons. First, the FLAC model employs a linear analysis for dynamic
ground response while the SHAKE91 model is based on the equivalent linear approach. In
addition, the FLAC program models the embankment in two dimensions as opposed to the 1D
model used in SHAKE91. The end result of these important differences is that the ground
motions computed with FLAC are greater than those computed with SHAKE91. FLAC motions
at selected elevations in the model were as large as 50% to 90% greater than the values produced
with SHAKE91. Effects such as 2D embankment response highlight the need to adjust results
from 1D models with the results of case history data, 2D and 3D response analyses of similar
earth structures, and sound engineering judgment.
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 during earthquake shaking was not modeled. Water outside
the slope was modeled as boundary pore pressure; therefore hydrodynamic effects were not
modeled.
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. Figure 8.22 shows the soil layering and the model grid used in
the numerical model analyses. The soil properties used in the numerical model for the layers
shown in the figure are given in Table 8.16.
Table 8.16: Soil Properties Used in the Numerical Model (see Figure 8.22 for layer number references).
Angle of Internal
LAYER
Dry Mass Density
Cohesion (N
Soil Type
NUMBER
(kg/m 3 Friction Porosity
1 ) 60
)
(kPa) blows/30 cm
(deg)
1 SP 1381 37 0.4 0 11
2 SM 1252 33 0.5 0 11
3 CL/SM 1211 30 0.55 0 5
4 SM-ML 1288 33 0.4 0 6
5 SM 1366 33 0.5 0 6
6 SP 1381 37 0.5 0 10
7 ML 1098 0 0.6 62 3
8 SP 1489 37 0.4 0 31
178