The strength of rock blocks containing structural flaws and defects, such as veins and healed fractures (mesodefects) is particularly important for sparsely jointed rock masses under high stress. The conventional UCS test is often unsuitable for providing an accurate measure of the intact strength for rock containing mesodefect and it is difficult to conduct tests on suitably large specimens to account for mesodefects. The Leeb Hardness (LH) test is proposed to provide a quantifiable estimate of intact strength for mesodefected rock. The LH test is a lightweight, compact rebound test that has been correlated to rock strength with a large database (∼400 test records) of various rock types over a wide strength and hardness range. The effects of conducting LH tests in the proximity of mesodefects has been examined and the LH test has been used in this study to estimate the true intact rock strength of defected rock cores.
For over 20 years, practitioners have recognized that estimates of rock mass strength should account for flaws and defects within the intact block portion of the rock mass. As noted by Hoek and Brown (2019) in their discussion of intact rock strength estimation for the Hoek-Brown rock mass shear strength criterion: "…in many rock masses, defects such as veins, micro-fractures and weathered or altered components can reduce the intact rock strength in unconfined compressive strength (UCS) tests. This is particularly important to address for sparsely jointed rock masses containing defects under high stress. Ideally, tests should be carried out on specimens large enough to include representative sections containing these defects…". Because the collection and testing of large samples is often impractical, other methods have been developed to account for the effects of defects in rock mass classification and rock mass strength estimates:
• The Modified Rock Mass Rating (MRMR) system (Jakubec & Laubscher, 2000; D. H. Laubscher, 1990; D. Laubscher & Jakubec, 2001) accounts for these defects in applications for underground mine design and block caving fragmentation.
• Martin et al. (2012) used the SRM method (Mas Ivars et al., 2011) with the software code PFC2D to evaluate the scale effect implications of defects on rock block strength. This study revealed a tendency for an asymptotic lower limit of 80% of the standard laboratory unconfined compressive strength (σc) with increasing specimen size.
• Stavrou & Murphy (2018) also conducted a numerical modelling-based study of microdefects and macrodefects related to specimen scale. They used UDEC Voronoi tessellated micromechanical modelling methods to simulate unconfined compressive strength (UCS), triaxial and Brazilian tests on various specimens. The simulation results were used to develop a modified classification system based on the GSI called the micro GSI (μGSI). Day et al. (2019) have modified the geological strength index (GSI) to account for defects. Their method utilizes a harmonic weighted calculation to account for both interblock defects and intrablock defects to determine a Composite GSI (CGSI). The CGSI is then used to estimate rock mass strength via the Hoek-Brown rock mass strength criterion.
