Passive earth pressures behind integral bridge abutments
A full-scale prototype integral bridge abutment was constructed at the University of Massachusetts Amherst as part of a project conducted for the Massachusetts Highway Department. Ten tests were then conducted during which the abutment was passively displaced into the backfill. The purpose of these tests was to determine the effect of foundation type, wingwall geometry, reloading, and backfill soil type on the lateral pressures generated following placement of the backfill and during the abutment movement.
Individual tests were performed by placing and compacting the fill in lifts and then incrementally displacing the abutment into the fill. During this displacement, earth pressures at the abutment centerline, ‘quarterline,’ and wingwall centerline as well as the applied load and deflection of the abutment and wingwalls were measured. After a series of tests were performed with the abutment on a spread footing, the abutment was removed and piles were driven through precast holes in the footing. Two piles were instrumented with strain gages and inclinometers. The abutment was then replaced and the same types of tests were again performed.
Results from testing indicated that during no test was the distribution or magnitude of lateral earth pressures similar to that predicted by classical theory. The use of an uncompacted sand zone directly behind the abutment face was found to reduce lateral earth pressures significantly. This zone was, however, found to compact after one cycle of reloading. Both wingwall geometry and foundation type were found to affect the lateral earth pressure magnitude and distribution.
Equations were developed which predict the lateral earth pressure coefficient for any abutment deflection or point along the abutment height. Current design charts were also modified to account for the behavior observed during these tests.