Behavior of Walls and Piles in Cohesive Soils Under Cyclic Loads

Abstract

The nonlinear cyclic behavior of a soil-structure system has a significant influence on the mechanical response of this system. The cyclic response of soil-structure system has been studied experimentally and analytically. However, the results of these studies are not yet reproducing the applicability of key aspects of soil-structure behavior concepts in practice. A key prerequisite is to model the cyclic response in a facilitative and realistic way. There are several constitutive models in the literature that are available for cumulative responses, but they need many soil tests for calibration and they can be used under specific numerical codes and can be only executed by specialists. To overcome these difficulties, this research develops a simplified constitutive model (a kinematic hardening constitutive model with Von Mises failure criterion) for analyzing nonlinear plastic response of a soil-structure system subjected to cyclic loading. In addition, cumulative deformations are an essential aspect of the performance of walls and piles/caissons under cyclic loading. Therefore, reasonable estimates of the cumulative plastic displacements of structures in cohesive soils are necessary, particularly for soils which the cyclic influence may be significant. For example, the cumulative wall displacements that increase over time as the system is subjected to repeated live loading from trains passing near wall, in addition to the vertical settlements under the train track. Studying the effects of cyclic loading of railroads on the soil-wall system is necessary to improve train safety when a soil-wall system is near the tracks. As a second example, while pile and caisson anchors and foundations for offshore structures, such as wind turbines and the oil/gas exploration and production facilities have been the focus of considerable attention with respect to monotonic load capacity, much less attention has been given to cumulative displacements under cyclic loading. This issue is particularly crucial for inclined loading, since cumulative displacements can lead to loss of embedment of the caisson or pile. Since stress-strain behavior of soils is inelastic even at small strains, analyses based on linear elasticity, or on elastoplastic models that assume purely elastic behavior beneath the ultimate yield surface, cannot predict the cumulative soil deformations. Hence, an analysis that takes inelastic soil behavior at low stress levels into account, such as a bounding surface plasticity model, is required to predict cumulative displacements under cyclic loading. A cyclic nonlinear elastoplastic soil spring model has been applied to predict the monotonic and cyclic nonlinear p-y curve of piles in soft clay during the cyclic loading. Predictions of pile performance based on the kinematic hardening constitutive model used in this research are shown to match the centrifuge test results better than predictions based on the widely used API soil springs. This proposed spring model can overcome the limitations of the API clay model and can be implemented with either MATLAB or as UEL (User-defined elements) subroutine in ABAQUS/Standard. Predictions based on the spring model developed in this research shows good agreement with the measurements of cumulative displacement and soil stiffness from centrifuge tests involving cyclic loading of a single pile in soft clay.