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75 Figure 7-11. Seismic coefficient charts for c- soils for 35. kh = 0.3 would be (i) 0.4 for no cohesion; (ii) 0.25 for a wall sure imparted to the retaining wall during a seismic event. This height 20 feet with 200 psf cohesion, and (iii) be 0.1 for a wall phenomenon could be a factor in explaining the good per- height at 10 feet with 200 psf cohesion. formance of retaining walls in past earthquakes. To illustration this, traditionally reduction factors on the 7.3.3 Implication to Design order of about 0.5 have been applied to the site-adjusted PGA From this example, it can be observed that a small amount to determine the seismic coefficient used in wall design. Wall of cohesion would have a significant effect in reducing the movement is a recognized justification for the reduction fac- dynamic active earth pressure for design. The reduction for tor as previously discussed. However, the wall movement con- typical design situations could be on the order of about 50 per- cept may not be correct for retaining walls supported on piles, cent to 75 percent. For many combinations of smaller kh con- particularly if battered piles are used to limit the movement of ditions (which would be very prevalent for CEUS conditions) the wall. In this case the contributions of a small amount of and also shorter wall heights, a rather small cohesion value cohesion (for example, 200 psf) could effectively reduce the would imply that the slope is stable and the soil capacity, in it- seismic coefficient of a 20-foot tall wall by a factor of 0.5, self, would have inherent shear strength to resist the inertial soil thereby achieving the same effects as would occur for a wall loading leading to the situation of zero additional earth pres- that is able to move. Figure 7-12. Seismic coefficient charts for c- soils for 40.