ABSTRACT:

The objective of this work was to predict the effects of various sequences of excavating a NATM tunnel face and the influence of different pillar widths between multiple tunnels on the settlement using the Finite Element Method (FEM). A number of 2-D plane-strain FEM analyses, using ABAQUS software, have been performed to investigate settlement profiles for single and multiple shallow tunnels in London Clay. The Modified Cam-clay plasticity soil model with non-linear porous elasticity has been adopted in the analyses. The Hypothetical Modulus of Elasticity (HME) soft lining approach is used t o consider 3-D tunnelling problems. Three excavation sequences have been examined to find the best possible model for predicting the settlement for a single tunnel. The predictions were compared with the measurements recorded during construction of the Heathrow Express Trial tunnel in London Clay. Verification of the results for a single tunnel has allowed the proposed FEM model to be applied t o multiple tunnel cases where the tunnel separation has an important influence on the settlement and stress-strain redistribution around the individual tunnels. Three pillar widths between the tunnels were considered for analysing the spacing effect on the settlement profile predicted.

INTRODUCTION

The New Austrian Tunnelling Method (NATM) (Rabcewicz, 1964) has become very attractive for tunnel construction due to its many advantages over conventional tunnelling methods. These include lower investment in tunnelling equipment, flexibility to employ complex excavation geometries, less overall support cost, adaptability in different ground conditions. The essential characteristics of this method are the utilisation of the ground around the tunnel as a support member, use of shotcrete as a preliminary support and the application of systematic deformation measurements. However, there have been a number of well-publicised collapses, which has led to comprehensive reviews of NATM applications (HSE Report, 1996).

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