Update on Rock-Type Specific Hoek-Brown Constant “s”

ABSTRACT: Martin (1993) established the m-zero failure criterion following extensive field and laboratory testing at the Atomic Energy of Canada Limited Underground Research laboratory. The m-zero criterion is widely used for assessing excavations in rockmasses with a Geological Strength Index ≥75. Based on the m-zero criterion stopes in massive sulfides in underground nickel mines that were expected to fail did not fail. This observation prompted further studies on the m-zero criterion at the Geomechanics Research Centre. According to the m-zero failure criterion, damage in massive rockmasses is caused by destruction of the Hoek-Brown failure criterion constant "s" with the frictional constant "m" playing no role. The value of the constant "s" to initiate damage was established as 0.11. However, this value was based on studies on the La du Bonnet granite. Suorineni and Kaiser (2002) argued that the "s" value of 0.11 should vary depending on the rock type, and hence the need for rock type-specific "s" values. Suorineni et al. (2009) presented a procedure for the determination of rock type specific "s" values. This paper presents "s" values for different rock types from Kazakhstan in a continuing effort to provide users of the criterion with a database of rock type-specific "s" values. 1. INTRODUCTION There are various failure criteria that exist in rock engineering for the evaluation of the performance of underground excavations in rock. Two frequently used failure criteria are the Mohr-Coulomb (Mohr, 1900; Coulomb) and Hoek-Brown (1980) failure criteria. This paper focuses on the latter, given its history. Given the long history of the Mohr-Coulomb failure criterion which dates to the 1700s, one would ask why there was the need for another failure criterion in the 1980s? The Mohr-Coulomb failure criterion has its foundation in soil mechanics. Hoek and Brown (1980) provide the key reasons why the Hoek-Brown failure criterion was necessary for rocks. These authors noted that to utilize the knowledge of stresses induced around underground excavations it was necessary to have available a criterion or a set of rules which will predict the response of a rockmass to a given set of induced stresses. The search for such a criterion for rockmasses had been challenging because of the complexity of the rockmass compared to a soil in that the behaviour of the rockmass transitions from an intact rock to a heavily jointed rockmass as shown in Fig. 1. All the rockmass components in Fig.1 are of concern to the underground excavation designer or rock engineer. The stability of the entire system of underground openings which make up a mine or any underground excavations network depends upon the behaviour of the entire rock mass surrounding these openings, including all the various components of the rockmass in Fig. 1. This observation implies that a failure criterion developed for soils is unlikely to be applicable to a rockmass, and hence the need for a more suitable failure criterion for rockmasses.

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