Application of Model Pile Tests to Axial Pile Design

ABSTRACT Experiments conducted with instrumented model pile segment probes at Harvey and Empire, Louisiana are summarized and the observed behavior interpreted. Correlations are developed for the time rate of strength gain and interim recommendations are given for construction of t-z curves (soil resistance versus axial pile displacement) for use in design. The methods are compared with existing pile test results. INTRODUCTION A comprehensive program of studies of the behavior of axially loaded piles in normally consolidated clay was performed between 1982 and 1986. The studies included a large number of experiments with instrumented pile segment models which were deployed below the bottoms of boreholes. Some were near the sites of large-diameter pile load tests. Continuously recorded measurements included pore pressure and total lateral pressure plus soil shear resistance as a function of axial displacement, under a variety of static and cyclic loadings. The results of selected experiments done in Louisiana at Harvey and Empire have been given in two companion papers (Refs 1 and 2). In this paper, the complete sets of results will be used to establish proposed interim design procedures for axially loaded piles in similar clays. The procedures will then be applied to two sites at which axial pile load tests have been performed in normally consolidated clays. SUMMARY OF OBSERVED SOILPROBE BEHAVIOR A fairly clear concept of the mechanisms of clay soil behavior in interaction with axially loaded piles has been obtained in part from the Harvey and Empire test results. The principal features are summarized here as a basis for the development of correlations and design procedures. In general, when a pile is pushed or driven into the soil, the soil is displaced outward by an amount which is, in terms of analysis, referred to as cavity expansion. The soil is also remolded by the flow around the point and by shear deformations along the length of the pile. Excess lateral pressures are created by the cavity expansion, primarily in the pore water and therefore in the total pressure. Because of the rapid reduction of shear stress with radial distance from the pile, the significant lateral and shear stresses are confined to a soil band very close to the pile wall. With time during and after pile installation, the pore water pressure dissipates by a process of radial consolidation and a part of the lateral pressure is transferred to lateral effective stress. Since the process involves the outward migration of pore water there is a slight movement of the soil skeleton inward toward the pile. This results in a reduction of total pressure during consolidation. A closed-end or plugged pile forces more soil to move laterally and thereby creates somewhat greater initial lateral pressures than an open-end or thin-wall pile. The band of remolded and heavily stressed soil around the thin-wall pile is much thinner, resulting in more rapid consolidation and set-up, This was clearly demonstrated by the Harvey and Empire tests.