Calculating Bias-Free Volumetric Fracture Counts (VFCs) in Underground Works and Their Use in Estimating Rock Mass Strength and Deformability Parameters

Abstract

This paper initially provides a practical example on how to estimate a bias-free volumetric fracture count (VFC—fractures/m3) in a tunnel and incorporate it into a new and unified volumetric-based Geological Strength Index (V-GSI) chart. The quantified V-GSI chart and the methods shown in the practical example were used extensively as tools to assess rock mass conditions and assist in support determinations on the WestConnex M8 Motorway tunnel project in Sydney, Australia. The reliability of the strength and deformability estimates obtained using the V-GSI ratings while tunneling within the Hawkesbury Sandstone is demonstrated here by providing an example of deformation results obtained through 3-D finite element analysis in a single location in the tunnel. The modelling results are compared to measure convergence in the tunnel in this location, which demonstrated good correlation between predicted and observed deformation. This provides validation that the V-GSI chart and associated Hoek–Brown strength and deformability equations can be used with some confidence to determine potential deformation in underground works.