| Based upon a recent survey of state departments of transportations (DOTs) it is clear that there is a trend towards the increased use of drilled shafts in transportation applications in the United States. Groups of drilled shafts installed vertically are often used to resist lateral loads. Drilled shafts spaced less than 8-diameters center-to-center in the direction of the applied load are commonly considered to interact with each other. There are several procedures available to account for shaft-soil-shaft interaction, each with their own reduction factor to account for the effective softening of soil properties brought about by interaction. However, the procedures currently in use and their subsequent reduction factors, are largely from model testing and limited full-scale testing, most of the latter on groups of driven piles. To determine if there is a difference in reduction factors for driven pile and drilled shaft groups, case studies of drilled shaft groups found in the literature were modeled. The first stage of modeling was to match the load-deflection curve of a single shaft in the field using the computer program LPILE. As a result of the first stage of modeling the site was characterized and the need to model the shaft as nonlinear was established. The second stage of modeling was to match the load-deflection curve of the shaft group. There are several procedures available to model the group, but only the most common design procedures were examined: the group reduction factor method; the p-multiplier method; and the coefficient of lateral subgrade reaction reduction method. Shaft modeling was performed using the computer programs LPILE and GROUP. The results of the research showed that the reduction factors for driven piles and drilled shafts tended to overlap with driven piles having a higher average. There is greater interaction amongst shafts in a group than piles in a group on average. Therefore, when designing a drilled shaft group special attention must be given to the factors used to account for group effects. |
