Composite Geological Strength Index Approach with Application to Hydrothermal Vein Networks and Other Intrablock Structures in Complex Rockmasses

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. Keywords Rockmass characterization Á Complex rockmasses Á Healed intrablock structures Á Geological Strength Index Á Finite element method numerical models Á Composite Geological Strength Index

Where geology meets engineering in hydrothermally altered environments: considering veins in geotechnical engineering

“…For field assessment of geomechanical rockmass properties, the Composite Geological Strength Index (CGSI) equations by Day et al (2019) enable suites of rockmass structures to be considered individually, and the accompanying revised chart has been developed to account for intrablock structures in rockmass characterization using CGSI, and subsequently include these structures in continuum numerical models of excavations in shear-based stress environments.…”

“…rockmasses comprised of intact rock and fractures) into numerical modelling input properties for continuum numerical models to simulate shear-driven rockmass yield in lower stress environments. To characterize complex rockmasses at the field rockmass or excavation scale, the Composite GSI methodology and revised GSI chart that includes intrablock structures by Day et al (2019) can be used to evaluate multiple suites of rockmass structures (any combination of interblock or intrablock structures) (CGSI chart shown here in Figure 3). By adapting GSI to complex rockmasses, the system retains the benefits of determining parameters for the Hoek-Brown strength criterion that can be input directly into numerical models that simulate sheardriven rockmass yield.…”

“…From a geomechanical perspective, intrablock structures are defined as meso-scale structures within fracture-bounded blocks of otherwise intact rock. Intrablock structures influence rockmass shear and tensile strength as well as deformability (stiffness), and their mechanical behaviour depends on thickness, persistence, orientation, mineralization, and the relative mineralogy and associated mechanical properties of the surrounding wall rocks (Day et al 2019;Clark 2020). Porphyry and related skarn ore deposits, which are commonly extracted using open pit or cave mass mining methods, contain pervasive and variable stockwork veining throughout.…”

The Continuum Voronoi Block Model for simulation of fracture process in hard rocks

“…The Voronoi mosaic allowed the intact rock failure simulation between the discontinuities (i.e., rock bridges). They concluded that Voronoi blocks made it possible to simulate the rock mass fracture process. Day et al 37 . proposed a new methodology that modifies the Geological Strength Index (GSI), by incorporating the effect of intrablock structures, such as veins, in the behavior of the rock mass.…”

“…They concluded that Voronoi blocks made it possible to simulate the rock mass fracture process. • Day et al 37 proposed a new methodology that modifies the Geological Strength Index (GSI), by incorporating the effect of intrablock structures, such as veins, in the behavior of the rock mass. To validate this new methodology, they performed simulations in RS2 and used Voronoi blocks to represent the rock structure.…”