1 Introduction

One of the most common tests to measure time-dependent behaviour and time-to-failure is a creep test, where the load is help constant and the strains are measured with time. A less commonly conducted test on rock specimens is a relaxation test, where one of the sample strain levels is held constant and the stress change is measured with time. A recent study by Paraskevopoulou et al. (2017) identified that the relaxation behaviour appears to be equal and opposite to the creep behaviour, as illustrated in Fig. 1. In this extensive study on two different limestones, the procedure of how to conduct a relaxation test on brittle rocks was examined for cylindrical specimens was examined. A current study, presented here in, has been initiated on prismatic specimens to examine the influence of specimen shape on the relaxation behaviour of brittle rocks.

The Jurassic limestone, from northern Switzerland, and the Cobourg limestone, from Southern Ontario, Canada have been tested in this study. The Jurassic specimens are dominantly fossil rich, whereas the Cobourg specimens have irregular fossil rich layers mixed with argillaceous layers. The latter is the potential rock mass for Canada's low and intermediate level nuclear waste repository.

The influence of specimen shape on compression tests, as well as other test types, has been examined on a variety of materials and several authors (e.g. Li et al. 2011). However, many of these studies have focused on the height to width (radius) ratio of individual geometries and less on a variety of specimen geometries from the same material. A collection of studies, including this one, that have tested cylindrical and prismatic specimen geometries under uniaxial compressive conditions are shown in Fig. 2 to illustrate the influence on the mechanical properties.

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