Year

2005

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering - Faculty of Engineering

Abstract

This research project has considered the impact of roughness on two-phase (air + water) flow in rough discontinuous geological media. Existing analytical approaches to twophase flow tend to be highly theoretical and involve significant idealisation of the rock mass properties because of the variable distribution of rock mass discontinuities and the impact of fracture roughness. The anisotropic nature of fracture orientation, aperture topology and persistence combine to limit the coupled analysis of single and two-phase flow. The research has been driven by the need for improvements in the prediction of two-phase flows in the vicinity of underground mining development and infrastructure. The aim of the work has been to develop and verify robust simplified two-phase flow models that can be accurately applied to improve hydromechanical rock mass analysis. Isothermal two-phase flow behaviour is encountered in geotechnical engineering particularly in underground construction and extraction associated in coal mining or in developments near to coal measures geological environment. The hazards presented by the potential for outburst can lead to loss of life as well as loss of production. The financial consequences of the loss of just a day of production from a typical Australian longwall mine in 2002 would lie within the range of 7000 to 20,000 tonnes (approximately A$ 0.25 to 0.5 million). The development of a simple coupled twophase flow model would facilitate the ready quantification of geomechanical risks facing the industry. A detailed review of the literature on two-phase hydraulics and fracture roughness has identified opportunities for the development of new analytical techniques. Refined laboratory testing methods have been combined to gather high resolution roughness and aperture data. The resulting data have been used to study the nature of fracture roughness using Fourier series. Specialised two-phase triaxial testing has been conducted and the test results have been analysed using the detailed knowledge of the fracture roughness and aperture to study the role of free fracture flow. The findings of the theoretical and laboratory testing schedule have identified the opportunity to characterise fracture roughness and to relate the results of the testing to hydraulic and mechanical apertures. The study has developed a new technique to assess the frequency composition of fracture roughness that can be related to the Joint Roughness Coefficient (JRC). Laboratory testing has also validated a proposed coupled annular two-phase flow model that is based upon the conservation of mass. The model relates the phase height parameters to the fracture mechanical aperture and enables the calculation of phase flow rates. Consideration of hydromechanical analysis has lead to the development of a coupled homogenous fluid model that has highlighted the applicability of some of the study results to improve computer modelling capability using the existing UDEC code. Finite difference analysis has illustrated the development of non-parallel fracture flow and related the discretised mechanical aperture to the fracture hydraulic aperture. The homogenous fluid model has been found to enable hydromechanical analysis of rough fractures without the need for prior knowledge of the fracture JRC. These breakthroughs have revealed the potential for further application to field testing environments and improved hydromechanical modelling. Such an application of the study outcomes could increase the hydromechanical data that could be collected from borehole site investigation.

02Whole.pdf (4823 kB)

Share

COinS