Static and Dynamic Lateral Load Tests on a Pile Cap with Partial Gravel Backfill

Lateral pile cap tests were performed on a pile cap with two backfills to evaluate its static and dynamic behavior. In one case the backfill was loose silty sand while in the other case, a 0.91-m wide zone of gravel was compacted between the pile, cap and the loose silty sand. The relatively thin dense gravel zone increased the lateral resistance by 75 to 150% relative to the loose silty sand backfill. The dense gravel zone also increase lateral resistance to 59% 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. Stiffness increased by about 10% at higher deflections, while the damping ratio decreased from an initial value 0.30 to around 0.26 at higher deflections.