Enhancement of constant normal stiffness direct shear testing protocols for determining geomechanical properties of fractures

Abstract

Discontinuity behaviour can have a large impact on geotechnical engineering design; therefore, it is essential to determine their geomechanical properties to predict rockmass behaviour and mitigate any potential failure that may affect personnel safety or damage property. Geotechnical numerical software programmes that discretely simulate discontinuities rely on direct shear laboratory tests to provide the properties needed as inputs to these tools. This study presents valuable direct shear laboratory test results for the mechanical properties and behaviours of fresh, unweathered, rough fractures in the Pointe du Bois granite. The testing programme considers three separate boundary conditions: constant normal stress (CNL*) and two variations of constant normal stiffness (CNS). A new machine stiffness model is proposed to estimate the machine stiffness for tunnelling applications. Measurements of shear strength and a proposed secant dilation angle are assessed and critically evaluated. It is found in this study that there is no significant difference between shear strength parameters determined from direct shear data from CNL* and CNS boundary conditions when comparing maximum strength and residual strength. In addition, it is found that the proposed secant dilation angle, critical shear displacement, and total dilation potential all have a negative relationship with increasing normal stress.