SOIL RESPONSE AND GROUP INTERACTIONS IN TORSIONALLY LOADED LARGE-DIAMETER BORED PILE GROUPS

In a pile group subjected to lateral or moment loading, the soil movements are primarily along the loading direction. In a torsionally load pile group, however, the primary soil movements are rotational. The movements of soils in the pile group can be in arbitrary directions. As a result, the soil response in a torsionally loaded pile group is different from that in a laterally loaded pile group. In this paper, three-dimensional finite-difference analyses of a torsionally loaded single pile, a torsionally loaded pile group, a laterally loaded single pile and a laterally loaded pile group are conducted to study soil responses and group interactions in torsionally loaded pile groups. The large-diameter bored piles studied are 2 m in diameter and 13.2 m in length. The pile groups are 2×2 groups with a three-diameter spacing. The soil response is represented by horizontal stresses in front of the piles and p-y curves derived from calculated bending moments along the piles. The p-y curves of the piles in the torsionally loaded pile group are lower than those of the laterally loaded single pile and the leading pile in the laterally loaded pile group. In addition, the ultimate subgrade reaction is mobilized at a much smaller local displacement (say, 20 mm) compared with the laterally loaded single pile (> 60 mm). The horizontal normal stresses caused by lateral loading and the horizontal shear stresses caused by torsion interact and reduce the lateral pile resistance in the torsionally loaded pile group.