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76
Mobilization of cohesion could significantly reduce seis- 3. Choose an appropriate sliding surface search scheme.
mic earth pressures to include such reductions in design prac- Circular, linear, multi-linear, or random surfaces can be
tice is not always straight forward due to uncertainties in es- examined by SLIDE and other commercial slope stabil-
tablishing the magnitude of the cohesion for compacted fills ity analysis programs.
where mixed c- conditions exist under field conditions. This 4. Apply the earth pressure as a boundary force on the face
is particularly the case for cohesionless fills, where the degree of the retained soil. The location of the force is assumed at
of saturation has a significant effect on the apparent cohesion one-third from the base (1/3 H, where H is retained soil
from capillarity. height) for static cases. For seismic cases the location can be
From a design perspective, uncertainties in the amount of reasonably assumed at mid height (0.5 H) of the retained
cohesion or apparent cohesion makes it difficult to incorporate soil. However, different application points between 1/3 H
the contributions of cohesion in many situations, particularly and 2/3 H from the base can be examined to determine the
in cases where clean backfill materials are being used, regard- maximum seismic earth pressure. The angle of applied
less of the potential benefits of partial saturation. However, force depends on assumed friction angle between wall and
where cohesive soils are being used for backfill or where native soil. A horizontal load simulates a smooth wall, whereas a
soils have a clear cohesive content, then the designer should load inclined at degrees indicates that the friction angle
give consideration to incorporating some effects of cohesion in between wall and soil is equal or greater than internal fric-
the determination of the seismic coefficient. tion angle of the soil.
5. Change the magnitude of the applied load until a minimum
7.4 GLE Approach for Determining ratio of C/D of 1.0 is obtained. The C/D ratio is equivalent
Seismic Active Pressures to the factor of safety for the analyses. The force correspon-
To overcome the limitations of the M-O method for cases ding to a C/D ratio of 1.0 is equal to total earth pressure on
involving nonhomogeneous soils and complex backslope the retaining structure.
geometry, conventional limit-equilibrium slope stability com- 6. Verify design assumptions and material properties by
puter programs may be used. The concept has been illustrated, examining the loads on individual slices in the output.
in a paper by Chugh (1995). For the purpose of both evalu-
ation of this approach and application to examples used for The program SLIDE was calibrated against M-O solutions
the recommended methodology (Appendix F), the computer by considering examples shown on Figures 7-14 and 7-15.
program SLIDE (RocScience, 2005), a program widely used The first set of figures shows the application of SLIDE for
by geotechnical consultants, was used. computing active earth pressure on a wall with horizontal
The basic principle in using such programs for earth pres- backfill. The two analyses in Figure 7-14A show the compu-
sure computations is illustrated in Figure 7-13. Steps in the tation of the active earth pressure for a homogeneous backfill
analysis are as follows: and seismic acceleration of 0.2g and 0.4g. The calculated re-
sults are identical to results from the M-O equation. The two
1. Setup the model geometry, ground water profile, and analyses in Figure 7-14B show computation of the active
design soil properties. The internal face of the wall, or the earth pressure for a case with nonhomogenous backfill. Fig-
plane where the earth pressure needs to be calculated, ures 7-15A and 7-15B show the similar analyses for a wall
should be modeled as a free boundary. with sloping backfill.
2. Choose an appropriate slope stability analysis method.
Spencer's method generally yields good results because it 7.5 Height-Dependent Seismic
satisfies the equilibrium of forces and moments. Design Coefficients
Current AASHTO LRFD Bridge Design Specifications use
peak ground acceleration in conjunction with M-O analysis
to compute seismic earth pressures for retaining walls. Ex-
cept for MSE walls where amplification factors as a function
of peak ground acceleration are used, based on studies by
Segrestin and Bastick (1988), the current approach makes no
adjustments in assigned ground acceleration for wall height.
Chapter 6 provides a fundamental approach for making these
adjustments based on scattering analyses for elastic soils. To
confirm that the recommendations in Chapter 6 apply for sit-
Figure 7-13. Adoption of slope uations where there is an impedance contrast between foun-
stability programs to compute seismic dation and fills, and the possible influence of nonlinear soil
earth pressure (Chugh, 1995). behavior, an additional set of analyses was performed. Results