ABSTRACT:

While planning a tunnel through a weak rock mass, the first step is to infer whether the tunnel would experience squeezing or non-squeezing ground condition? This decision, not only controls the selection of the excavation method but also the design of support system. An user friendly empirical approach has been developed to predict the ground condition. Critical values of tunnel depth, rock mass number and tunnel size beyond which squeezing ground condition will occur can be determined using the proposed correlation. This knowledge can be used either to avoid squeezing by realigning the tunnel through shallower depths or through a better rock mass or by replacing the tunnel by two or three smaller tunnels on techno-economic considerations. Should these options not be possible, the advance knowledge can be used to design the excavation and support system for tackling the squeezing ground condition.

INTRODUCTION

It is experienced that tunneling through squeezing ground condition is relatively slow and problematic because the rock mass around the opening fails and looses its inherent strength under the influence of cover pressure. The situation may mobilise high support pressures and tunnel closures requiring the use of a flexible support system. Tunneling under the non-squeezing ground condition, on the other hand, is comparatively easy because the inherent strength of the rock mass is not lost.

Therefore, an advance knowledge of the ground condition, squeezing or nonsqueezing, plays an important role in designing the support system and obtaining a good advance rate. Non-squeezing ground condition is common in tunneling projects. The Squeezing ground conditions on the other hand, are common in the lower Himalayas in India where the rock masses are weak, highly jointed, faulted, folded and tectonically disturbed and the overburden is high. The combination of the weak rock and the high stress is responsible for squeezing.

The scope of the present paper is limited to the prediction of the elastic self-supporting and elastic with supports, herein after called the non-squeezing, and the squeezing ground conditions.

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