Load-deformation behaviour of rough rock fractures subjected to internal water flow

Underground rock strata are often fractured and their permeability is mainly governed by interconnected fracture networks. Flow through fractures must be studied in order to design and operate underground activities such as tunnelling and mine operations, as well as groundwater and petroleum extraction. Flow through a fracture is primarily influenced by its aperture, and because fracture apertures can be distributed widely within a rockmass, they have closures as well as wide openings depending on the location and in-situ stress conditions. Past research studies have been carried out on defining the equivalent aperture to predict fracture flows from uni-directional flow models. However, in most civil engineering applications, plane strain conditions can be assumed (e.g. tunnels, rock slopes), and in such situations two-dimensional fracture models have been suggested for stationary fracture walls. Modelling flow through deformable fractures in plane stain, two-dimensional domain would provide profound insight into rock fracture hydraulics, and these models available now have been simulated using common numerical flow solvers. In this regard, a customised numerical solver to simulate fracture hydraulics would be an important addition to this research area. In contrast to available literature, in this PhD study, an equivalent twodimensional flow model was derived from the three-dimensional Navier-Stokes theory for deformable rock fractures. The proposed model contains pressure-velocity coupled equations, and a numerical solution is subsequently introduced by modifying the SIMPLE (Semi Implicit Method for Pressure Linked Equations) algorithm. The Writer’s own computer programme (Rock Fracture Flow Solver or RFFS) was developed to solve the proposed model using MATLAB computer language. Laboratory experiments were carried out for mated and dislocated fracture specimens using the high pressure triaxial apparatus (HPTPTA) designed and built in University of Wollongong. The fracture apertures were measured by replicating them and scanning the surfaces using a 3D laser scanner. Flows through the rock fractures were simulated using the Rock Fracture Flow Solver (RFFS), and the validity of the proposed model was verified for general underground fracture flow situations

Normal Deformation and Formation of Contacts in Rough Rock Fractures and Their Influence on Fluid Flow

Rock fracture flow was initially modeled according to the parallel plate model, which does not consider the undulating nature of fracture surfaces. The conventional parallel plate model or cubic formula was later modified to obtain precise predictions by considering the joint roughness coefficient (JRC) of the fracture walls. However, the real flow characteristics through a relatively long rough rock joint can still be modeled accurately via a two-dimensional analysis, which enables the spatial irregularity of rock fracture apertures to be considered with the fracture contacts, which act as obstacles to the flow. In this study, a new two-dimensional flow model for deformable fracture walls to predict the volumetric flow changes that result from effective normal stress fluctuations is proposed. This model was solved using the finite-volume method via a new program developed by the authors. It captures the existing contacts and newly formed contacts that occur while fracture aperture deformations take place and treats them as local boundaries. The model flow-rate predictions were compared with the simulated real rock fracture flow carried out on a high-pressure two-phase triaxial apparatus (HPTPTA) designed and built at the Univ. of Wollongong (Wollongong City, Australia). The model predictions and experiment results of volumetric flow rates were in good agreement, which verifies the accuracy of incorporating a more realistic contact treatment process between the upper and lower asperities during joint closure. The numerical simulations illustrate flow paths within the rock fracture in a more realistic manner without having to consider the entire fracture surface to be permeable

Mathematical modeling and experimental verification of fluid flow through deformable rough rock joints

Rock joints exert an enormous influence on the permeability of a rock mass because they act as interconnecting networks that provide pathways for fluids to permeate and flow within the rock structure. The apertures in rock joints are irregular in nature and induce flows that cannot be described by the parallel-plate theory based on planar joints or the classical cubic flow relationships. In this study, a two-dimensional (2D) hydraulic aperture distribution was considered to develop a mathematical model for fracture flow. In this approach, the three-dimensional Navier-Stokes equation was integrated over the joint aperture and converted to an equivalent 2D flow model. The proposed model was then solved numerically by adopting a well-known algorithm for coupling the pressure and velocity and implementing it in a computer program. The selected program is capable of predicting the deformation of the joint apertures on normal loading, the resulting flow patterns, and the volumetric flow rates associated with permeability tests conducted using a high-pressure triaxial apparatus that was designed and built at the University of Wollongong. The model output for different conditions of confining stresses and hydraulic gradients was computed, and a good agreement with the experimental results was observed