Modeling Pile Behavior in Large Pile Groups Under Lateral Loading

Large pile groups, defined as pile groups containing a large number of closely spaced vertical piles, were examined using a three-dimensional finite-difference based numerical modeling approach. The specific case of a large pile group subject to only translational loading at the groundline was considered, assuming that a rigid pile cap, whose base is located at the groundline, was present to enforce equal horizontal displacements of all pile heads. Research efforts focused on local pile-soil interaction using p-y curves as the primary assessment tool and p-multipliers to characterize group effects. Analysis efforts were preceded by an extensive review of lateral pile-soil interaction to provide an assessment of the existing state of knowledge, and a critical review of the three-dimensional modeling approach in terms of its formulation and application to simulating laterally loaded piles and pile groups. Rationalization of a large pile group into a two-pile in-line configuration and a single pile with periodic boundaries was undertaken for the purpose of the research representing typical leading and immediately trailing piles, and internal piles, respectively. Factors considered were: (a) soil type; (b) pile type; (c) initial soil stress states; (d) pile head restraint; and (e) pile spacing. Isolated pile models have provided a benchmark for both the in-line and periodic models. A total of 30 analyses were completed. Overall, the large pile group study indicated that initial stress state, pile type and pile head restraint resulted in some differences, but these were relatively weak compared with the influence of soil behavior and movement. Marked decreases in lateral resistance for interior piles were attributed to the different stiffness and strength characteristics of the soil models, and effects resulting from the boundary conditions employed. Much lower p-multipliers compared with current small pile group recommendations are therefore recommended for large pile groups, implying a comparatively softer translational stiffness for design. While the study enabled greater insight into the mechanics of large pile group lateral stiffness, various issues such as installation effects, pile, pile head and soil conditions remain, ensuring that the task of assessing lateral group stiffness remains a challenging endeavor.