ABSTRACT:

Samples of anisotropic gneiss oriented at 0, 30, 60 and 90 degrees to the maximum loading stress were tested in a true-triaxial deformation cell. The loading path was designed to simulate the in situ stresses that rock would be exposed to during the excavation, heating and cooling of canister holes created for the proposed containment of radioactive waste. The results show that the deformation response would not be replicated with a traditional triaxial test were the intermediate and minimum stress are assumed to be equal. In two situations the loading history induced failure when the intermediate stress was reduced. Acoustic emission and velocity data are used to characterize the damage and the sensitivity of the velocity data provide a future method to link laboratory to in situ observations.

1 INTRODUCTION

Understanding damage around boreholes and tunnels in rock is significant to numerous engineering applications. Borehole breakout and spalling, where the preferential orientation of stresses can induce failure, is a classic manifestation of this damage. Ever since the early work of Kirsch over a century ago we have endeavored to understand the complex interactions of stress and geometry on failure. More recently rock mechanics experimental testing and discrete particle numerical models have been used to deconvolve the complex fracture processes that lead to crack coalescence and the various modes of failure in rock materials (Hoek & Martin 2014). One application, which requires an understanding of failure around underground openings, is the concept of deep geological disposal of radioactive waste. Several countries have proposed schemes that involve the placement of spent fuel in canisters that will be placed in large diameter boreholes in tunnel complexes that are between 300 m and 500 m below the surface. Three such countries include Canada, Sweden and Finland where extensive investigations have been undertaken in crystalline rock to create a design that will be safe for the long term management, storage and disposal of radioactive waste.

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