ABSTRACT:

Fracture toughness tests were done on Kimachi sandstone and Inada granite using two types of core based specimens. In addition to the measurement at ambient conditions, fracture toughness was also determined at elevated temperatures, confining pressures and wet conditions. Elevated temperatures appear to make the rock tougher causing the fracture toughness to increase substantially. Increasing confining pressure also increases the fracture toughness as the pressure causes closure of some pre-existing microcracks and discontinuities. Water content was found to cause reduction of fracture toughness particularly in sandstone. This effect is supposed to be due to the increase in pore pressure.

INTRODUCTION

Fracture mechanics is increasingly used for the analysis of explosive and non-explosive fragmentation of rock as well as for other applications such as the prevention of failures of mine structures and the nuclear waste containment in the proposed underground repositories (Ouchterlony 1974, Whittaker et al. 1992). Fracture mechanics properties such as fracture toughness are useful for the characterization of materials. Fracture toughness is an important material property that corresponds to the critical state of the stress intensity factor required for the onset of unstable fracture in brittle materials (Ouchterlony 1980, Schmidt 1976). It is denoted by Kit and is usually determined by experimental testing followed by analysis. Failure of rock or its fragmentation is achieved if Kit is exceeded. A number of methods have been suggested to determine the fracture toughness of rock materials (Barker 1977, Ouchterlony 1986, Fowell et al. 1995, Thiercelin 1987). The short rod specimen (Barker 1977) and chevron bend specimen (Ouchterlony 1986) have been incorporated into a standard testing method for the fracture toughness measurement of rock by the International Society for Rock Mechanics, ISRM (Ouchterlony 1988). The semi-circular bend (SCB) specimen (Chong & Kumppu 1984) is complimentary to the standard methods as depicted in Figure 1. All these specimens are core based and therefore easy to prepare. Some applications of fracture toughness of rock are: (a) A parameter for classification of rock Co) An index of fragmentation processes such as tunnel boring (c) A material property in the modeling of rock fragmentation like rock cutting, hydraulic fracturing, explosive fracturing and crater blasting. Also it can be used for stability analysis of mining structures including that associated with nuclear waste repositories.

In past most rock fracture toughness tests have been performed under ambient conditions mainly due to the experimental limitations. However, it is essential that the measurements be carried out at insitu conditions such as elevated temperatures and pressures and in wet environments. In addition to determining the fracture toughness of rock at ambient conditions, this paper addresses the experimental procedures and the associated theoretical developments to deal with problems encountered in high pressure and elevated temperature tests.

Tests were done over a range of temperatures varying from ambient conditions up to 200vC. However, depending on the underground location, in-situ stresses vary over a very wide range. Fortunately it was found that the effect of increasing confining pressure on the fracture toughness dies down beyond a certain level of pressure. Therefore the conforming pressure of the tests was restricted to a maximum value of 7.5 MPa.

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