ABSTRACT:

A major problem engineers face in the analysis of excavation by the classical methods is the difficulty of predicting displacements around the support structure. To solve this problem, finite element method (FEM) is increasingly employed. However, so many engineers in the field have little knowledge of FEM that hampered the method to spread in practice. In this paper authors attempt to provide a useful tool for field engineer, who know little of FEM, to design the structure by both classical pressure diagram and FEM methods at same time. At first, calculate depth of pile and anchor force by each of classical methods, including free earth support, fixed earth support, a modified and an improved fixed earth support, and then compute the displacements by FEM method for each situation respectively. For FEM method, an elasto-plastic theory and a Mohr-Coulomb constitutive law are adopted for soils and the mesh is automatically formed. After comparison, a safer and more economical design can be selected. To support this method, a computer package on Windows 95 base has been developed by the authors. Results of a practical example show that the improvement of fixed earth support method is better for the design.

INTRODUCTION

With the development of economy, many high-rises have been built in China in the last two decades. Correspondingly, many deep excavations have been constructed. Among them, a lot of failures and problems have been encountered. In one city, failures were as many as a third of the total number of excavation in foundation engineering. Yeqing Tang (1999) investigated 168 of failures of deep excavation, and concluded that 40% of them were caused by design. From this point, we can hardly say that the theories and methods for designing deep excavation are satisfactory though many methods available at present.

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