On the coupled hydro-mechanical behaviour of jointed rock masses: an application to mine slope stability

Rock mass strength is the most important single parameter required for stability analyses of rock slopes. However, the precise estimation of rock mass strength is extremely difficult due to its heterogeneous nature induced by the presence of joints. Rock mechanical behaviour is greatly influenced by the geometrical properties of the joints, such as their orientation, persistence, surface roughness and degree of interconnectivity. In addition,different hydro-geological conditions can have marked influences on the jointed rock behaviour. The main objective of this thesis is to investigate the influence of various joint geometrical properties on the hydro-mechanical behaviour of jointed rock masses under undrained hydro-geological conditions. The experimental study involved the testing of jointed rock samples under two different stress conditions – unconfined and confined. First, the influence of three partially-spanning joint geometrical properties, i.e. joint location, orientation and persistence, on the strength of rock-like brittle material under unconfined stress conditions was experimentally studied. Test specimens were prepared using a cement-mortar mixture and the joints with desirable geometrical properties were artificially embedded using water-jet cutting. Amongst the observed behaviours, the variation of strength against partially-spanning joint orientation is salient, as the critical joint orientation was found to be 45°, which would have been (45+ø/2)°, in the case of a fully-persistent joint, according to the literature (ø is the friction angle). In addition, using the results of the experimental work, two constitutive relationships using regression analysis and fuzzy logic analyses were also developed to predict the strength of rock with partially-spanning joints. The experimental results were also used to validate a two-dimensional numerical model. Numerical models were developed using both finite element (using RFPA2D software) and distinct element (using UDEC software) methods. After validating the numerical models with reasonably good agreement between experimental and numerical results using both RFPA2D and UDEC, UDEC numerical simulation work was further extended to study the behaviour of rock with complex non-persistent joint arrangements. Several important results were obtained regarding the behaviour of rock with non-persistent joints from the above numerical simulations, and among them the new parameter introduced to describe the persistency of on-persistent joints is the most significant. Unconfined testing on natural bedded sandstone specimens was also conducted to investigate the effect of bedding orientation and water saturation on the mechanical response. The results of these bedded sandstone tests revealed some important characteristics of their mechanical strength and energy-releasing behaviour during deformation. In general, it was observed that water saturation can considerably lower the strength of bedded sandstone and the energy-releasing characteristics are mainly governed by the failure mechanisms of rock specimens. Experimental testing under confined stress conditions was started after modifying an existing triaxial apparatus. In its original condition testing could only be performed under dry conditions, and the apparatus was therefore customized to include some novel features, such that it could be used to conduct testing under different hydro-geological conditions. All the experiments were conducted under undrained triaxial conditions and this thesis discusses the results of over 230 undrained triaxial tests in total. Intact Hawkesbury sandstone specimens were first tested under different combinations of confining pressure and initial pore-water pressure (i.e. to simulate stress conditions at different depths) to understand the mechanical behaviour of the rock material being tested. Jointed specimen testing considered three different joint geometrical properties – joint orientation, degree of interconnectivity and surface roughness. All joints were embedded using water-jet cutting for the cases of surface roughness and orientation with rough surface profiles, while interconnected joints were created using a diamond cutter with planar surface profiles. For the joint orientation case, six different joint orientations were considered, and five different surface roughness profiles (i.e. surface profiles with five different JRC values) were considered for the surface roughness case. Two interconnected joints were created in each sample for the case of joint degree of interconnectivity and six different joint angle combinations were considered. In each case, internal pore-water pressure fluctuation and its indirect influence on the mechanical behaviour was discussed and the critical insights identified are presented in this thesis. Some in-depth characterizations related to failure mechanisms, volumetric strain response and fracturing behaviour are also presented.. In the cases of joint orientation and degree of joint interconnectivity, the hydro-mechanical behaviour of rock specimens was observed to be governed mainly by the failure mechanisms. The failure mechanisms were dependent on the confining pressure, in addition to the joint geometrical properties. Therefore, for both joint geometrical properties (i.e. joint orientation and degree of joint interconnectivity) ‘failure mechanism matrices’ were developed to predict the failure mechanism for a given joint geometry and confining pressure. In the case of joint surface roughness, the irregular joint profiles were observed to show a ductile response unlike regular saw-toothed joints, and the pore-water pressure response and shear strength of joints were observed to be governed by the amount of actual shearing area and dilating area of the joints.Awards: Winner of the Mollie Holman Doctoral Medal for Excellence, Faculty of Art, Design and Architecture, [2014]