In the present study, a return mapping algorithm is formulated for an elasto-plastic swell model based on the Modified Cam-Clay(MCC) model, which can show the soft/hard behavior of swelling process by applying the finite element method (FEM). Several simple numerical examples are used to assess the efficiency and convergence of the formulation of the return mapping algorithm. This study is contributed to predicting the nonlinear mechanical response of the tunnel constructed in swelling rockmass, which contains the stress redistribution of the tunnel after excavation and the soft/hard response caused by swelling process. The numerical simulation will show the swell deformation, strain path and stress history of a tunnel which ground masses has occurred the process of water-absorption, aimed at evaluating the stability of a tunnel constructed in swelling rockmass after some years.

1. Introduction

In recent years, a problem that ground expansion by swelling clay minerals destroys the roadbed of a mountainous tunnel has occurred in various regions in Japan. The heave of tunnel invert caused by swellable minerals will seriously affect the normal operation and endanger the structural safety of the tunnel, and therefore, it is very important to make appropriate predictions of the mechanical behavior of the tunnel for the support system design and maintenance.

The expansion comes with the process of water-absorption between layers of these clay minerals (Grim, R.E., 1968). In addition, it is reported that the hygroscopic expansion, which is based on a thermo-chemical phenomenon, degrades the stiffness of the ground as the swelling clay minerals interact with surrounding intact rock (Luc Massat et al. 2016). The complicated mechanical behavior of the ground involving swelling clay minerals comes from a kind of phenomena based on multi-physics, due to the hygroscopic expansion. Although it should be to simulate the complicated behavior theoretically in terms of multi-scale/multi-physics analysis which considered the thermo-mechanics with chemical reactions, no applicable model has yet to be developed in practice.

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