Abstract

In response to high demands for minerals and in the light of rapid advancement in instrumentation and testing facilities, mines, tunnels, and infrastructures are getting deeper at an annual increasing rate. For instance, Australia’s deepest Gwalia gold mine near Leonora is extending beyond 2 km depth in the hunt for more ore. In Europe, the Gotthard sub-alpine base tunnels exceeding 10 m wide are excavated at depths greater than 2 km. At depth, explosion-like fractures occur which are known as stress spalling, slabbing, or rock bursting due to high-pressure environment, high temperature, and low porosity. Although rockburst and spalling failures have both been observed in the past 50 years and investigated by numerous researchers, this phenomenon is yet mysterious and the physics behind is still not fully understood. Recent researches have suggested that the spalling strength can be related to the crack initiation point. Various methods have therefore been proposed to identify the crack initiation at laboratory scales based on stress-strain response. Limited models are also available based on applied stresses, but only under triaxial conditions. This study aims to compare the volumetric strain method (as a strain-based technique) with the improved Griffith and Hoek-Brown criterion (GHB) (as a stress-based method) to determine the crack initiation stress threshold under triaxial loading conditions.

Introduction

The uniaxial compressive strength (UCS) test is a popular procedure used to measure the peak compressive strength and elastic constants of cylindrical rock samples. As shown in figure 1, the UCS stress-strain curve typically shows four distinct stages at various stress levels: crack closure, crack initiation (CI), unstable crack growth, and the peak strength stage [1, 2]. At the initial stage of loading, pre-existing voids and flaws dominate the mechanical behaviour of rocks [3]. After the closure of pre-existing cracks, stress-induced tensile/shear fractures start to initiate at about 30%-60% of the uniaxial compressive strength in a so-called crack initiation stage. This stage is of particular practical importance since recent studies suggest that the CI stress threshold can be safely used as a predictor of in-situ spalling and slabbing strength [1]. Moving up on the stress-strain curve after the CI level, crack tends to grow in an unstable fashion, and the crack coalesces eventually leads to strain localization and macro-scale shear failure and axial splitting formations that cause permanent deformation at the crack damage (CD) stage and beyond. After the closure of pre-existing crack until the CI stress and well below the CD, which is also known as long-term strength as defined by [4], rock responds in an elastic fashion during which the elastic modulus and Poisson’s ratio can be determined (see figure 1).

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