Group reduction factors for analysis of laterally loaded pile groups

The lateral response of piles embedded in soil is typically analyzed using the beam on nonlinear Winkler springs approach, in which soil-pile interaction is modeled by nonlinear p-y curves (where p is soil resistance and y is horizontal displacement). In this approach, one of the most common methods of accounting for interaction effects in pile groups is to modify the single pile p-y curves using a p-multiplier for each row of piles in the group, with higher values for leading row and lower values for trailing rows. The leading and trailing rows interchange during seismic loading; therefore, sometimes an average p-multiplier is used for all piles in the group. This average p-multiplier is called the group reduction factor. Group reduction factors have been established from experimental data from static loading tests on small pile groups, mostly 3 x 3 groups with free pile head conditions and center-to-center pile spacings of about 3 pile diameters. In this paper, continuum simulations are used to study the group reduction factors in 3 x 3 to 6 x 6 square pile groups subjected to static loading. The study includes the effects of various parameters, including pile spacing, pile head condition, and the friction angle of soil, on the group reduction factors. Calculated group reduction factors from this study compare well with available group test data, that is, typically small pile groups. However, the study shows that design guidelines such as the American Association of State Highway and Transportation Officials (AASHTO) and Federal Emergency Management Agency (FEMA) P-751 overestimate the group reduction factors, hence the lateral resistance, in larger pile groups and larger spacings, especially for fixed pile head conditions.