Interface Stresses between Soil and Large Diameter Drilled Shaft under Lateral Loading

Results are presented of a field testing program for a full-scale, large diameter cast-in-drilled-hole (CIDH) shaft/column under large displacement cyclic lateral loading. The test shaft was extensively instrumented to enable high-precision section curvature measurements, including measurements of contact pressure at the soil-shaft interface around the shaft perimeter. Among the principal objectives of the testing was to characterize the soil-shaft interaction across a wide displacement range in order to gain insight into the adequacy of existing design guidelines (which are based principally on the testing of small diameter piles) for the large diameter shafts commonly used to support highway bridges. The component stresses of resistance and the effects of nonlinear soil resistance to relative displacements between the soil and shaft are presented in detail. INTRODUCTION Pile-supported bridges provide an economical option for highway construction, particularly in seismic regions. The inelastic deformations for a pile/column under lateral loading occur below grade; therefore, the overall lateral load behavior of the system is influenced by the interaction between the shaft and the surrounding soil, commonly modeled using p-y curves. The prediction of soil-pile-structure system behavior under lateral loading is among the most complex topics in geotechnical engineering. Current models for p-y curves are calibrated primarily from lateral load testing of relatively small diameter piles. Since the soil resistance is such a critical consideration, there is a need for better understanding of its component stresses, particularly for large-diameter piles and drilled shafts. Assistant Professor, Clarkson University, Department of Civil and Environmental Engineering, Potsdam, NY 13699 [email protected] Undergraduate Research Assistant, Clarkson University, Department of Civil and Environmental Engineering, Potsdam, NY 13699 [email protected] The beam on nonlinear Winkler foundation (BNWF) model that gives rise to the use of p-y curves in engineering design is based upon the concept that soil-shaft interaction can be represented by a compressive force per unit length of shaft (McClelland and Focht 1958). As seen in Fig. 1, this force is the resultant of several stresses, including: • Normal compressive stress on the leading face of the shaft, which in general will be some percentage of the passive soil capacity. • Shear stress along the sides of the shaft. • Active normal soil stresses on the trailing face of the shaft. Current sensor technology only allows measurements of normal stress in soil, so interface shear stresses can only be indirectly inferred. This experimental study evaluated total soil reaction p from measurements of the internal bending deformation of the shaft through use of beam theory. The normal stresses at the soil-shaft interface, which were measured during field testing with nondisplacement soil pressure cells (SPC), are compared to the total soil reaction and the interface shear stresses are calculated. PREVIOUS STUDIES OF INTERFACE STRESSES Few experimental studies of soil-shaft interface stresses have been performed because of difficulties associated with the use of SPCs. Such devices are obviously impractical for driven piles, but can be used in drilled shafts. The SPCs can be installed on the shaft wall after excavation of the shaft hole and placement of the reinforcing cage, but prior to concrete placement. Such installations are only practical when the dry method of construction is used (i.e., no slurry). Among the previous studies of laterally loaded drilled shafts which have utilized soil pressure cells (Kasch et al. 1977 and Briaud et al. 1983, 1985), the work by Bierschwale et al. (1981) is of particular interest because the pressure cell data are well documented, and data for several azimuthal angles relative to the direction of loading are available. The study consisted of lateral load testing of three separate reinforced concrete drilled shafts of varying diameters of roughly 0.9 m (2.5 ft) installed at a stiff clay site and laterally loaded at ground line. The SPCs were generally installed in the line of loading, and the results were plotted as a function of depth for various levels of head load. The authors did not evaluate distributions of p for comparison to pressure profiles. A simple equilibrium check was performed by assuming that the measured pressures were uniformly applied over the projected diameter of the shafts, and then summing the moments about the point of zero lateral stress. The results indicated a lack of equilibrium, and that a substantial increase of soil pressure near the top of the