ABSTRACT:

The tunnels in the rock mass are affected by the dynamic loads developed by the seismic stress waves. An approach to estimate the shear wave velocity and concept of ratio of wavelength of earthquake shear waves to tunnel span has been discussed. Various approaches for obtaining the Barton's rock mass quality Q and estimating support pressure in tunnels for dynamic conditions are available. It has been experienced in Himalayan tunnels that a tunnel through weaker rock mass suffers more damage than a good rock mass under similar dynamic conditions. Thus, an attempt has been made in the paper to suggest Qdyn (Q value for dynamic condition) for different ground conditions for tunnels. The proposed Qdyn is suggested to be used for designing the supports using the support chart of Grimstad & Barton (1993).

1 INTRODUCTION

In most part of the World, majority of the tunnels are built in rocks. While designing the tunnels, one of the factors to be considered is the safety of rock tunnels and caverns under dynamic loads generated mainly by seismic stress waves. Since rock masses generally contain fractures in different scales, the interaction between stress waves and rock materials and fractures affects not only stress wave attenuation, but also rock mass instability. As such the tunnels in the rock mass, though less than surface structures, are also affected by dynamic loads developed by the seismic stress waves. A major reason for the reduction in underground damage is the lower levels of shaking at depth in comparison with surface motions.

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