Doctor of Philosophy
School of Civil, Mining, and Environmental Engineering
Thirukumaran, Sivanathan, Shear strength of degradable rock joints, Doctor of Philosophy thesis, School of Civil, Mining, and Environmental Engineering, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4133
The typical shear behaviour of rough joints has been studied under constant normal stress or zero normal stiffness boundary conditions (CNL), but recent studies have shown that this boundary condition may not replicate more practical situations, and that constant normal stiffness (CNS) is a more appropriate boundary condition to describe the stress-strain response of field joints. Unlike CNL conditions, a limited amount of data under CNS boundary conditions is available to date. In addition to the effect of boundary conditions, the shear behaviour of a rough joint also depends on its surface properties and initial stress acting on its interface. Despite this, exactly how these parameters affect the shear behaviour of joints is not fully understood because the stress-strain response of joints is governed by non-uniform asperity damage and the resulting gouge that accumulates on their interfaces. Therefore, an attempt has been made in this study to enhance our fundamental understanding of asperity deformation through a series of CNS shear tests that were conducted on replicas of three rough tensile (natural) joints at a range of initial normal stresses that varied from 0.4 to 1.6 MPa.
Based on the laboratory investigation, a novel 3D characterisation method was proposed to quantify the role of the asperity damage and distribution of gouge material on the surfaces of rough rock joints under CNS direct shearing. A 3D laser scanner was used to digitise the joints, and characterisation was conducted based on the scanned models before and after the shear tests. This method demonstrated how zones of asperity damage are formed and gouge material is distributed onto the surfaces of sheared joints that are related to initial normal stresses and surface roughness, in terms of a colour spectrum and contour maps which show how the asperity heights deviate from the original profile. The results of this characterisation showed that the spatial distribution of asperity damage and gouge accumulation depended on the surface morphology and initial applied normal stress. The experimental results indicated that the response of rough joints under CNS boundary shearing was greatly affected by damage to the asperities, the extent of which increased with an increasing initial level of normal stress and joint roughness. The gouge material formed as a result of damage to the asperities slowed down any further damage, an effect that was more pronounced in less rough joints.
This study also proposes a new analytical model to describe the complete shear behaviour of rough joints under constant normal stiffness (CNS) boundary conditions by incorporating the effect of damage to asperities. The effects of initial normal stress levels and joint surface roughness on the shear behaviour of joints under CNS conditions were studied in depth and the analytical model was validated through experimental results. Finally, the practical applications of the model to typical analysis of jointed rock slope stability and tunnel roof stability are also presented.