1 Background

Recent studies have demonstrated important links between the strength and failure mode of brittle rock materials, how they respond to varying principal stress conditions and stress paths, and resultant field-scale responses, manifest as differences in the pattern and nature of slope failures and landform and landscape evolution (Diederichs 2003; Leith et al. 2014). However, the influence of strain rate in driving differences in intact rock strength and failure mode is less well constrained (cf. Amann et al. 2011). Furthermore, how differences in stress path and strain rate influence future failures by controlling if, where and when progressive fracture can develop has not previously been considered in sufficient detail.

2 Approach

In contrast to most conventional confined compression tests that employ an increase in axial load to cause failure, we focus on the effects of both slow and rapid (near-instantaneous, quasi-dynamic) reductions in confining pressure on the compressive strength and failure mode of rock samples. We consider the former (slow loss of confining pressure) to be representative of a longer-term, gradual loss of hillslope material, driven by small-scale but continuous rockfall activity. The latter (rapid loss of confining pressure) mimics stress conditions in remnant hillslope materials during and following large slope failures that cause a sudden reduction in lateral confinement of remaining, unfailed materials.

3 Results

We present results of laboratory tests undertaken on a medium- to coarse-grained sandstone (unconfined compressive strength = 51.0 ± 10.6 MPa). We undertook a series of confined compression tests over a confining pressure range of 1 to 10 MPa. Our results indicate that differences in strength and failure mode occur where strain rates differ, despite (un-)loading along the same stress path (constant axial load, reducing confining pressure). A gradual, controlled reduction in confining pressure causes minor, but not insignificant, strengthening relative to standard axial loading tests, though we observed standard shear failure typical of confined compression (Fig. 1a). In contrast, rapid reduction in confining pressure causes an increase in compressive strength relative to baseline axial loading tests, but results in greater rock fragmentation upon failure, indicative of hybrid shear and tensile axial splitting modes (Fig. 1b,c).

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