Development of a rock-type s matrix for damage initiation stress threshold determination using the Combined Compression and Shear Test System (C-CAST) for design of inclined pillars

Abstract

One of the basic rock mass properties used in rock engineering and rock mechanics is the unconfined uniaxial compressive strength ((σ_c) or UCS). The Hoek-Brown failure criterion remains one of the most widely used failure criteria in underground excavation stability assessment. The constants m and s are associated with the Hoek-Brown failure criterion that represents friction and cohesion respectively. The generalized Hoek-Brown failure criterion assumes that the frictional and cohesive strength components of the rock mass are mobilized simultaneously as for the Mohr-Coulomb failure criterion that was developed for soils. The criterion also assumes that rock masses fail in shear. The work of Martin (1993) showed that for massive rock masses the constant m (i.e. friction) plays no role in the rock masses with Geological Strength Index (GSI) greater than or equal to 75, and that the damage initiation is primarily governed by cohesion loss. For this case, the s value is set to 0.11, implying that all rockmasses have the same cohesion. Suorineni and Kaiser (2002) and Suorineni et al. (2009) argued that since s represents rock cohesion it should depend on the rock type. Since then, there has been a campaign to determine s values for different rock types with the objective of establishing a rock type-s value matrix like the rock-type mi by Hoek (2007). The original values of s are determined from conventional uniaxial compression tests. Suorineni et al. (2011, 2014) showed that for inclined pillar design in dipping orebodies the loading conditions differ from the standard uniaxial compression tests and so is the damage initiation stress in such pillars. Based on Suorineni (2014) a special testing procedure called combined compression and shear test (C-CAST) was developed to determine more appropriate unconfined compression strength for the design of inclined pillars in dipping orebodies. This study aimed at estimating rock type-dependent s parameters according to the damage initiation criterion (σ_ci) through the C-CAST testing system. Four different rock types including andesite, diorite, gabbro, and dolerite were tested under various angles of inclination to determine the s values at various angles of inclination. The AE technique was used for the determination of damage initiation stress thresholds. Petrographic methods for evaluating the role of mineralogy, alteration and texture on rock strength and therefore s values were conducted. The study reveals that the damage initiation takes place at about 30-60% UCS, indicating that brittle failure of rocks is dominated by micro-fracturing at low stresses. Such behavior conforms to Griffith crack wing theory, which states that failure occurs due to the propagation of microcracks at stress levels considerably below the maximum strength point in brittle rock mass. A decrease in strength of rocks, as well as s values, was recorded under increased inclination angles. This means that the role of shear stress on damage formation and strength is very crucial. Dolerite and Diorite possess relatively high strength because of the interlocking crystal structures compared with those of Andesite and Gabbro...