Abstract

Rocks in deep underground are subjected to different stresses inherent to natural orogeny, therefore, rock behaviour becomes complicated in response to extraction activities of deep earth resources. Rock behaviour and failure in deep underground is not well understood, leading to flaw engineering designs. In this sense, the complete stress-strain characteristics of intact rock, i.e. the pre-peak and post-peak stress-strain regimes are relevant in the understanding of the total process of rock deformation under in-situ stress. In this study post-peak characteristics of different rocks types, having unconfined compressive strengths of 7 to 123 MPa were investigated in a series of quasi-static monotonic uniaxial and triaxial compression tests. Post-peak stress-strain measurement was achieved by implementing lateral-strain rate feedback method in a closed-loop system to apply axial load to the rocks. In uniaxial and triaxial compressive testing, measurement of complete stress-strain behaviour showed i) either class II behaviour or a combination of class I-II behaviour and ii) a noticeable shifting from class II behaviour toward class I behaviour as the ratio between confining pressure to the unconfined compressive strength increases significantly. The results of the experimental study then were used to develop an index to describe the pre-peak and post-peak behaviour of the rocks, encompassing both confined and unconfined conditions. Reasonable correlation was observed between the new index and confining pressure.

1.
Introduction

Prediction of rock failure is very important for civil engineering and mining development projects. With this, engineers can investigate the effect of rock behaviour on drilling and excavation performance and rock burst occurrence in deep mining. Some researchers have attempted to develop brittleness indices to describe rock failure behaviour based on rock compressive and tensile strength (Hucka & Das 1974, Coates 1966, Altindag 2003, Yarali & Soyer 2011). In addition, several brittleness indices based on the energy of the rock under loading have been proposed (Baron et al. 1962). However, these indices are only considered pre-peak rock behaviour. Munoz et al. (2016a) found that brittleness indices that are developed based on rock strength and pre-peak rock behaviour are unable to predict rock drilling performance. As a result, they developed new brittleness indices to describe rock failure behaviour in uniaxial compressive loading based on pre-peak and post-peak properties (Munoz et al. 2016b). This approach, however, ignores the residual strength of rocks, which might be significant under confined loading conditions. In addition, brittle to ductile transition of rock post failure behaviour is overlooked in the previous work as only uniaxial loading condition were investigated. Therefore, it is required to develop relevant methods able to describe rock pre-peak and post-peak stress-strain behaviour during uniaxial and triaxial loading condition.

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