ABSTRACT:

Stability of tunnel faces in sandy ground is influenced not only by the mechanical properties of the ground but also by the groundwater conditions. Sand tank experiments prove that the morphological features of face collapse are influenced by the properties of ground and the groundwater conditions. Experimental simulations by mechanical-groundwater flow coupling analysis have been tried based on the finite-difference method to verify that the analysis method is suitable. Parametric numerical analyses are also carried out to study the influence of the mechanical properties on the face stability of tunnels in the water-saturated sandy grounds. The analysis results indicate that the face stability decreases remarkably when the relative density of ground is less than 80 percent.

INTRODUCTION

It is an important problem in constructing a tunnel whether an unconsolidated sandy ground with groundwater exists around the tunnel, because we empirically know that tunnelling is difficult if the sandy ground has a high groundwater level. It is generally said that the stability of tunnel faces in sandy ground is influenced not only by mechanical properties of the ground but also by groundwater conditions. However, the relationship between the properties of the ground and conditions of the groundwater involved in the stability of face has not yet been clarified. The stability of tunnels in sandy grounds has been examined and the critical hydraulic gradients of sand samples of Pliocene and Pleistocene by horizontal permeability have been tested. These tests suggested that the relative density, fine-grained fraction contents and hydraulic gradients influence the deformation of the sandy ground around the tunnel faces (Kiya et al.,1991). The previous experimental studies investigated the stability of tunnels in the loose sandy ground (e.g. Adachi et al., 1985; Hisatake, 1995). These experiments only considered dry conditions to examine the mechanical conditions effecting the collapse of tunnel.

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