Abstract
Conventional rockmass characterization and analysis methods for geotechnical assessment in mining, civil tunnelling, and other excavations consider only the intact rock properties and the discrete fractures that are present and form blocks within rockmasses. As modern underground excavations go deeper and enter into more high stress environments with complex excavation geometries and associated stress paths, healed structures within initially intact rock blocks such as hydrothermal veins, veinlets and stockwork (termed intrablock structures) are having an increasing influence on rockmass behaviour and should be included in modern geotechnical engineering design. Field observations indicate the conventional Geological Strength Index (GSI) does not accurately estimate rockmass strength and behaviour of complex rockmasses. A modified GSI chart to include intrablock structures and a new Composite Geological Strength Index (CGSI) methodology to combine multiple suites of rockmass structure using a weighted harmonic average are presented as tools to evaluate complex rockmasses that contain multiple suites of structure for application to geomechanical numerical models. CGSI is introduced and numerically validated using implicit and explicit finite element method numerical simulations of an underground excavation and a case study of field observations in an adit at the El Teniente mine in Chile. In both cases, the CGSI approach using the modified GSI chart results in an improved estimate of rockmass behaviour in implicit equivalent continuum numerical models when compared to a conventional GSI approach.
Highlights
Modern civil and mining engineering excavations are increasingly being constructed in complex rockmasses and situated at deeper horizons that are subject to high in situ stresses
Geological Strength Index (GSI) is a more flexible rockmass characterization tool that is directly tied into the Generalized Hoek– Brown strength criterion, which is available in many modern geotechnical software packages to control the geomechanical behaviour of continuum materials
This study demonstrates the influence of intrablock structures on the mechanical behaviour of complex rockmasses in finite element method (FEM) numerical models, which agrees with observations of rockmass behaviour in deep excavations where intrablock structures can dominate the overall behaviour at the excavation scale
Summary
Modern civil and mining engineering excavations are increasingly being constructed in complex rockmasses and situated at deeper horizons that are subject to high in situ stresses. Conventional rockmasses are comprised of intact rock blocks that are bounded by macro-scale fractures. Complex rockmasses contain mesoscale structures that behave as part of the intact rock in situ and in high quality, undisturbed, diamond drill core. These meso-scale structures such as hydrothermal veins, veinlets, and stockwork, and others, are termed by the authors as intrablock structures. Intrablock structures can influence rockmass shear and tensile strength at excavation and larger scales and can control fragmentation after moderate disturbance and comminution
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