LATERAL PILE CAP LOAD TESTS WITH GRAVEL BACKFILL OF LIMITED WIDTH

This study investigated the increase in passive force produced by compacting a dense granular fill adjacent to a pile cap or abutment wall when the surrounding soil is in a relative loose state. Lateral load tests were performed on a pile cap with three backfills to evaluate the static and dynamic behavior. One backfill consisted of loose silty sand while the other two consisted of dense gravel zones 3 ft. (0.91 m) and 6 ft. (1.82 m) wide between the pile cap and the loose silty sand. The 3 ft. and 6 ft. wide dense gravel zones increased the lateral resistance by 75% to 150% and 150% to 225%, respectively, relative to the loose silty sand backfill. Despite being thin relative to the overall shear length, the 3 ft. and 6 ft. wide gravel zones increased lateral resistance to 59% and 83%, respectively, of the resistance that would be provided by a backfill entirely composed of dense gravel. The dynamic stiffness for the pile cap with the gravel zone decreased about 10% after 15 cycles of loading, while the damping ratio remained relatively constant with cycling. Dynamic stiffness increased by about 10% to 40% at higher deflections, while the damping ratio decreased from an initial value of about 0.30 to around 0.26 at higher deflections.