Lateral or Torsional Failure Modes in Vertically Loaded Defective Pile Groups

This paper focuses on investigating potential bending or torsional failure modes in axially loaded pile groups caused by defective piles and on investigating the effect of short piles on the response of vertically loaded bored pile groups socketed into rocks. A hypothetical group of six bored piles, 2.0 m in diameter and 25.5 m in length, was analyzed and six cases with different numbers and configurations of short piles in the pile group were considered using a three-dimensional pile foundation analysis program FB-Pier. It is found that the number and configuration of short piles have a significant effect on behavior of the pile group. The presence of short piles forces "intact" piles to carry larger loads. An important aspect of the group response is that the presence of short piles can result in induced lateral deflections of the pile group and additional bending moments in piles even under "concentric" axial loads at the pile cap. The lateral deflections and bending moments can cause lateral failures of the pile group in terms of both ultimate limits and serviceability limits. For the axially loaded cases analyzed, the torques developed in the piles are much smaller than the bending moments; a torsional failure mechanism is therefore not likely to occur.