Pile caps are used in foundation design to aid multiple single piles to act as a pile group to resist lateral forces that may cause overturning moments. The pile cap and pile group resist these forces by pile-soil-pile interaction, base and side friction along the pile cap-backfill interface, and passive earth resistance. Passive earth resistance has been neglected in design due to a limited amount of full-scale testing. This research presents the results of a combination of hydraulic actuator and eccentric-mass shaker full-scale testing of a pile cap with loose silty sand backfill to quantify the contribution of the passive earth resistance to the lateral force resistance. The test cap is 1.12 m tall and 5.18 x 3.05 m in plan view, connecting 12 steel pipe piles (324mm O.D) placed in a 4 x 3 pattern with center-to-center spacing of 4.4 and 3.3 pile-diameters in the long and short dimensions, respectively. The hydraulic actuator applied a static load to the system (backfill + pile group) while the eccentric-mass shaker introduced cyclic and dynamic loading to the system. The passive earth resistance accounted for approximately 22% of the total system resistance, with piles contributing approximately 78%. Furthermore, the results produce general correlations between cyclic and dynamic effects on degradation of the backfill provided by the testing and soil characteristics obtained, including target (static) displacement, dynamic displacement amplitude, stiffness, and damping. The dynamic displacement amplitudes during the eccentric mass shaker tests typically ranged between .4 and 2 mm for frequencies between 5 and 9.5 Hz representing behavior under reloading conditions rather than virgin loading conditions. Generally, the presence of the loose silty sand backfill nearly doubled the dynamic stiffness of the pile cap. The stiffness of the backfill and pile cap combined was typically between 100 and 200 kN/mm for frequencies between 4 and 8 Hz, while the stiffness for the backfill alone was typically a decreasing trend between 100 and 40 kN/mm for the same frequency range. The overall isolated loose silty sand damping ratio shows a general increasing trend with values from 32% to 55% for frequencies 3 and 8 Hz.

A series of lateral load tests were performed on a full-scale pile cap with three different backfill conditions, namely: with no backfill present, with densely compacted clean sand in place, and with loosely compacted clean sand in place. In addition to being displaced under a static loading, the pile cap was subjected to low frequency, small displacement loading cycles from load actuators and higher frequency, small displacement, dynamic loading cycles from an eccentric mass shaker. The passive earth pressure from the backfill was found to significantly increase the load capacity of the pile cap. At a displacement of about 46 mm, the loosely and densely compacted backfills increased the total resistance of the pile cap otherwise without backfill by 50% and 245%, respectively. The maximum passive earth pressure for the densely compacted backfill occurred at a displacement of approximately 50 mm, which corresponds to a displacement to pile cap height ratio of 0.03. Contrastingly passive earth pressure for the loosely compacted backfill occurred at a displacement of approximately 40 mm. Under low and high frequency cyclic loadings, the stiffness of the pile cap system increased with the presence of the backfill material. The loosely compacted backfill generally provided double the stiffness of the no backfill case. The densely compacted backfill generally provided double the stiffness of the loosely compacted sand, thus quadrupling the stiffness of the pile cap relative to the case with no backfill present. Under low frequency cyclic loadings, the damping ratio of the pile cap system decreased with cap displacement and with increasing stiffness of backfill material. After about 20 mm of pile cap displacement, the average damping ratio was about 18% with the looser backfill and about 24% for the denser backfill. Under higher frequency cyclic loadings, the damping ratio of the pile cap system was quite variable and appeared to vary with frequency. Damping ratios appear to peak in the vicinity of the natural frequency of the pile cap system for each backfill condition. On the whole, damping ratios tend to range between 10 and 30%, with an average of about 20% for the range of frequencies and displacement amplitudes occurring during the tests. The similar amount of damping for different ranges of frequency suggests that dynamic loadings do not appreciably increase the apparent resistance of the pile cap relative to slowly applied cyclic loadings.