1 Introduction

Determination of the mechanical response of rock in-situ to applied loads is a central challenge of rock mechanics. This response is controlled predominantly by two scale factors: size and time. The effect of time on rock deformability and strength is a topic of considerable interest in rock mechanics (Einstein & Meyer, 1999). It is of special importance to the geological isolation of highly radioactive nuclear waste, as it is necessary to establish that the rock can isolate the waste from the biosphere for a time period of the order of a million years. This, in turn, implies an understanding of how rock deformability and strength will change over such a period. All materials will creep when subjected to the appropriate long-term loading conditions. At a nuclear waste repository in Precambrian crystalline rock at depths of 400 m to 700 m, loading conditions will be compressive and temperatures less than 100°C. Hence, an important question related to repository design is: "Will the strength decline essentially to zero after such extended periods, or does the rock have a non-zero ultimate strength or ‘threshold’ that can be considered time-independent, for the time-scale of interest?"

2 Time-dependent weakening of crystaline rock

Specimens of crystalline rock subject to creep tests — i.e., sustained constant loading that is below the instantaneous compressive strength — are found to collapse after a period of time. Fig. 1 illustrates a typical series of results obtained, in this case, on cylindrical specimens of Lac du Bonnet (Canada) granite loaded in compression at different constant loads, expressed as a proportion of the "instantaneous" strength. Potyondy (2007) refers to this proportion as the "driving stress ratio." The majority of the tests were for unconfined loading, but some confined loading results are included. It is seen in Fig. 1 that the individual tests all were carried out at driving stress ratios greater than 0.6 and for periods of less than two weeks. Clearly, time constraints do not allow laboratory tests to be carried out to obtain data for low stress ratios, so most estimates of the rock strength for extended loading times (of the order of tens to one million years) are obtained by extrapolation from the short-term data.

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