Passive Force–Deflection Behavior for 0° and 30° Skewed Abutments

Accounting for seismic forces and thermal expansion in bridge design requires an accurate passive force–deflection relationship for the abutment wall. Current design codes make no allowance for skew effects on passive force; however, small-scale experimental results indicate a significant reduction in peak passive force as skew angle increases for plane-strain cases. To explore this issue further, large-scale field tests were conducted at skew angles of 0° and 30° with unconfined backfill geometry. The abutment back wall was 11 ft (3.35 m) wide by 5.5 ft (1.68 m) high, and backfill material consisted of dense, compacted sand. The peak passive force for the 30° skew was found to be 58% of the peak passive force for the 0° skew case, which was in good agreement with the available laboratory and numerical results; however, the small difference between field and available laboratory results suggests that backfill geometry may have some effect on the reduction in peak passive force with respect to skew angle. Longitudinal displacement of the back wall at the peak passive force was found to be between 3% and 5% of the back wall height for field tests of both the 0° and 30° skew, which was consistent with previously reported values for large-scale passive force–deflection tests. For both tests, the failure geometry extended approximately 4 to 5 ft (1.22 to 1.52 m) beyond the edge of the pile cap and 16 to 18 ft (4.88 to 5.49 m) from the face of the cap when measured perpendicular to the back wall.