Undrained behavior of auger cast-in-place piles in multilayered soil

Auger cast-in-place piles (ACIP) are often installed through multilayered soil profiles, which make accurate predictions of the performance of the piles more complex than piles constructed in either clay or sand deposits. This study is intended to shed some light on the undrained behavior of ACIP embedded in stratified soil and to explore a methodology to predict the ultimate pile loads. The study is based on practical measurements of load-displacement relationships of 51 static loading tests of full-scale ACIP installed through multilayered soil profiles. The study revealed that the normalized load-displacement relationships of the tested piles have deterministic range with upper and lower bounds. Equations for these bounds and the mean load-displacement relationship are developed in this study. There is a deficiency in the literature concerning the calculations of ultimate loads for piles embedded in multilayered soil. Therefore, this paper presents an attempt to estimate the ultimate pile load in undrained conditions utilizing two approaches. The first approach assumed the failure pattern of the soil beneath the pile base to be punching into the sand followed by general shear failure in clay underneath. The end-bearing resistance at the pile tip was estimated by implementing Meyerhof and Hanna's [24] shallow foundation procedure. The second approach assessed the depth of the influence zone below the pile tip using isobars of pressure around and below the pile tip due to a point load, based on the theory of elasticity and characterization of a semi-infinite soil mass (Martins [3]). Soil layers, within the zone of influence, were considered to be an equivalent geomaterial with shear strength parameters computed by weighted average of shear strength parameters of the soil sub-layers. For comparison purposes, the ultimate pile load of each test was interpreted experimentally using the method proposed by Chin (1970). Reasonable agreement was obtained between the predicated and the experimental values, with an accuracy of about ±17%.