Full-scale effects of passive earth pressure on the lateral resistance of pile caps

Pile caps are constructed to provide a connection between a structure and multiple single piles. Pile caps are often subjected to vertical and lateral loads, and overturning moments. Resistance to these loadings are provided by pile-soil-pile interaction, base and side friction along the concrete soil interface, the rotational restraint provided by the pile to pile cap connection, and passive earth resistance. The passive earth resistance may be the largest contribution to the lateral resistance of the pile cap foundation. However, due to limited full-scale test results, methods of predicting the passive earth pressure contribution have not been verified, and therefore, the passive earth resistance is often neglected in design.; This document presents the results of seven cyclic lateral load tests performed on a full-scale 4 x 3 pile group driven in a saturated clay, silt and Silty Sand profile. The 324mm O. D. steel pipe piles were attached to a concrete pile cap 5.18 m by 3.05 m in plan and 1.12 m in height. Lateral resistance was provided by pile-soil-pile interaction and passive earth resistance. The tests were performed to investigate the effects of passive earth resistance on the cyclic lateral resistance of pile cap using four different soil types.; The ultimate passive resistance obtained from this study is compared with existing earth pressure theories. The log spiral method provided the best approximation of the measured passive resistance. The displacement required to mobilize the passive force is compared with previous model and full-scale tests and ranged from 3.0% to 5.2% of the cap height. A hyperbolic model provides a strong agreement with the measured passive resistance as a function of pile cap deflection. However, this model overestimates the passive resistance for cyclic loading conditions due to the formation of a gap between the pile cap and backfill soil and reduced passive load versus deflection stiffness. The hyperbolic relationship is combined with a procedure to model the degradation in reloading stiffness for the passive resistance versus deflection as well as the effect of soil gapping.