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Table 8.10: Peak Horizontal Acceleration Values for the Scenario Earthquakes
Elevation Where Time
PGA VALUES
History Calculated M w = 6.2 M w = 7.0 M w = 8.5 M w = 9.0
Elevation 1.5 m (5 ft) 0.17g 0.21g 0.11g 0.12g
Elevation 6.4 m (21 ft) 0.14g 0.15g 0.10g 0.10g
The factors of safety used in the development of the displacement charts were kept to a
minimum of 1.05 and a maximum of 1.5. None of the acceleration time histories had
acceleration values high enough to produce movement of the failure mass for factors of
safety greater than 1.5 (a max < a y ). A minimum factor of safety of 1.05 was used because
at values less than 1.0, the levee is not stable under static conditions.
8.9.1.2 Results from the Newmark-Type Analyses
In order to evaluate the influence of cyclic loading and the associated loss of soil strength
on the computed slope deformations, the simplified displacement procedures were
applied using soil shear strengths that were representative of the excess pore pressure
estimated at each level of shaking. These conditions were analyzed for all four
earthquakes and three river stages. Stability analyses of the levee based on static soil
strengths demonstrated that the static stability is large for summer flow and 100-year
flood conditions, however the static factor of safety for shallow landward sliding was
found to be less than unity for the crest level flood. The factors of safety associated with
these conditions resulted in yield accelerations that were large for the former two flow
conditions. The yield acceleration was not exceeded by the input ground motions
therefore no deformation is indicated by the simple Newmark-based methods. The
probability of the levee sustaining significant damage during summer or 100-year flood
stages without the generation of significant excess pore pressures during shaking is
therefore considered remote. A yield acceleration was not determined for the crest-level
flood due to the instability indicated for static conditions.
The reduction in soil strength associated with excess pore pressure generation during
cyclic loading resulted in small yield accelerations that are exceeded by the M 8 +
earthquakes (Table 8.11). The estimated displacements using simple sliding block
procedures and residual shear strengths where appropriate are given in Table 8.12.
Table 8.11: Critical Acceleration (a y ) Values for Residual Strength Conditions
WATER LEVEL
RIVERWARD FAILURE
M w = 6.2 M w = 7.0 M w = 8.5 M w = 9.0
El. 2.1 m (7 ft) 0.14g 0.14g 0.03g N/A
100-year Flood 0.28g 0.26g 0.05g 0.07g
Top of Levee 0.43g 0.42g 0.17g 0.16g
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