Report No.: CCEER-96-4
Title: Development of a Strain Wedge Model Program for Pile Group Interference and Pile Cap Contribution Effects
Authors: M. Ashour, P. Pilling, G. Norris, and H. Perez
Date: June 1996
California Department of Transportation (Caltrans)
Department of Civil Engineering/258
University of Nevada, Reno
Reno, NV 89557
Beam on elastic foundation theory provides an efficient solution for the problem of a laterally loaded pile. The accuracy of such a solution depends upon the characterization of the interaction between the pile and the surrounding soil. A particularly good representation of the soil-pile interaction yields a more realistic solution. While traditional nonlinear "p-y" characterization provides reasonable assessment for a wide range of loaded piles, it has been found that the p-y curve (or the modulus of subgrade reaction) depends on pile properties (width, shape, bending stiffness, and pile-head conditions) as well as soil properties. The strain wedge model allows the assessment of the nonlinear p-y curve response of a laterally loaded pile based on the envisioned relationship between the three-dimensional response of a flexible pile in the soil to its one-dimensional beam on elastic foundation parameters. In addition, the strain wedge model employs stress-strain-strength behavior of the soil as established from the triaxial test and the effective stress condition to evaluate the mobilized soil behavior.
The response of a pile group, consisting of vertical, flexible piles, to static lateral loading is a complex, three-dimensional soil-pile-soil interaction problem. Although many methods exist that allow the prediction of such a response, they typically maintain limitations which prevent them from being a simple design tool capable of analyzing the response of a flexible pile group to lateral loading over a broad range of deflections. In addition, no simple models exist that can also predict the contribution of an embedded pile cap to the overall working load of a pile group for a given level of deflection.
The purpose was to develop a simple three-dimensional method of analyzing the response of a group of flexible piles and the associated pile cap to lateral loading. The corresponding method proposed as a part of this research uses the strain wedge model, previously developed for isolated, flexible piles subjected to lateral loads (Norris and Abdollaholiaee, 1985; Abdollaholiace, 1985; Gowda, 1991; and Ashour. 1996), as a basis to develop a model that can accurately characterize the response of pile groups and associated pile caps to lateral loading.
The strain wedge model, as originally proposed by Norris and Abdollaholiaec (1985), relates one-dimensional beam on an elastic foundation analyses to a three-dimensional response modeled by the strain wedge method by relating the deflection of a pile versus depth, or rotation, to the relative soil strain that exists in the passive wedge which develops in front of a pile under horizontal load. The strain wedge model assumes that the deflection of a pile under lateral load is due solely to the deformation of the soil within the passive wedge, plane stress change conditions exist within the wedge, and soil strain is constant with depth in the wedge. Pile group response is accommodated in this approach by evaluating the effects of passive wedge overlap on the relative stiffness of the soil-pile-soil system, while pile cap response is characterized by evaluating the development of a passive wedge over the depth of the pile cap.