Keywords: upheave failure, excavation, finite element method, aquifer, interface element, grouting.

In recent years, deep excavations have been comprehensively adopted in many projects from the construction of buildings to stations of mass rapid transit systems in urban areas. Deep excavation is a very complicated soil-structure interaction problem. Excavation may fail due to different failure modes such as the sand boiling failure, the basal heave failure and the push in failure. This study will concentrate on the upheave failure which is usually caused by the ground water pressure in an aquifer beneath the excavation site.

As shown in Figure 265.1, when an excavation case is located at a site with a high-permeability aquifer beneath the clayey layer (so-called undrained layer) and the diaphragm wall is penetrated into the aquifer, the upheave failure may occur during excavation due to the fact that the total weight of soils, , above the aquifer in the excavation zone is smaller than the upward pore water pressure, . The factor of safety against the upheave failure () is generally expressed as:

where and are the unit weight and thickness of each layer above the aquifer, respectively; and are the unit weight of water and head at the top of the aquifer, respectively.

The analysis of the upheave failure using Equation (32) is quite simple. Generally, the friction between the diaphragm wall and soils is not considered in the analysis for conservative design. Besides, the soil upheave deformation cannot be estimated using Equation (32). As a result, the objective of this study is to establish a finite-element-method (FEM) analytic procedure which is capable of estimating the deformation behaviour of the upheave failure induced by excavation. The CRISP program [1] is used in this study. The interface element is employed in the FEM analysis to be able to properly simulate the upheave behavior. An ongoing construction of a station in Taipei Massive Rapid Transit is used to establish and examine the analytic procedure. Also, the effect of the factor of safety on the upheave failure is considered in the procedure. The soil parameters are determined based on Kung [2]. The results of the FEM analysis showed that the behaviour of the upheave failure, including the upheave failure surface, the critical excavation depth and the significant soil upheave deformation, can be properly estimated using the proposed procedure.

1

CRISP Consortium Ltd. (2002). CRISP: CRItical State soil mechanics Program in 2 dimensions, user manual, Version 4.3, London.

2

Kung, T.C. (2003). "Surface Settlement Induced by Excavation with Consideration of Small Strain Behavior of Taipei Silty Clay." PhD Dissertation, Department of Construction Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan.

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