Lateral resistance of full-scale pile cap with gravel backfill

A static lateral load test was performed on a full-scale 3 X 3 pile group driven in saturated low-plasticity silts and clays. The steel pipe piles were attached to a concrete pile cap which created a "fixed-head" end constraint. A gravel backfill was compacted in place on the backside of the cap, Lateral resistance was therefore provided by pile-soil-pile interaction, as well as base friction and passive pressure on the cap. In this case, passive resistance contributed about 40% of the total resistance. The log-spiral method provided the best agreement with measured resistance. Estimates of passive pressure computed using the Rankine method significantly underestimated the resistance while the Coulomb method overestimated resistance. The cap movement required to fully mobilize passive resistance in the gravel backfill was about 6% of the cap height. This is somewhat larger than reported in other studies likely due to the underlying clay layer. The p-multipliers developed for the free-head pile group provided reasonable estimates of the pile-soil-pile resistance for the fixed-head pile group once gaps adjacent to the pile were considered.