Fluid flow analysis plays a major role in various Geotechnical applications. In the analysis of stability of slopes the pore water pressure distribution is of fundamental importance and its evaluation is one of the prime objectives in the early stages of any stability study. A coupled hydro-mechanical flow model is employed in this study to simulate a rock slope with number of joint sets. This paper investigates the effects of built-up pore water pressure on the stability of the rock slope numerically (UDEC - Universal Distinct Element Code). The study includes the effects of rock slope, joint orientation and fluctuation of ground water table on the change of stability.


Rock falls are generally initiated by some climate event and induced forces that cause a change in the equilibrium of the rock mass. These events may include weathering of rocks (e.g. chemical degradation), variation of pore pressure due to rainfall infiltration and erosion of surrounding material during heavy rainy seasons. The ground water state prevailing within a rock mass attracts importance and cannot be overlooked in any slope stability problem. Ground water flows and the development of excessive pore water pressures greatly reduce the stability and impose safety problems for the slopes. Generally, the permeability of the intact rock (e.g. granite) is very low, but if the rock has discontinuities such as joints, fissures or faults, it can be significantly high, because these discontinuities provides the channels necessary for the flow of water. When these channels are interconnected it forms a flow network and the well-established flow network facilitates in building up the pore water pressure. It is known that flow channelling is very sensitive to normal stress and pore pressure is an influencing factor on the stress redistribution within rock mass (Tsang, 1990).

2.1 Hydraulic behaviour of rock joints

Flow in planar rock fractures may be idealized by means of the parallel plate model (Engelder & Scholz, 1981; Brown, 1987; ITASCA, 1996). Experiments conducted by Louis (1969) showed that this law is essentially valid for laminar flow in rock joints. Presence of fracture roughness causes the flow to be non-laminar. Witherspoon et al. (1980) tested both open and closed joints. They reported that the cubic law is still valid for rough joints, provided that the actual mechanical aperture is used. Due to the effects of roughness and tortuosity of flow, the fracture conductivity in their experiments was reduced by a factor between 1.04 and 1.65. Barton et al. (1985) proposed an empirical formula that gives the hydraulic aperture (to be used in the cubic law) as a function of the mechanical aperture and the joint roughness coefficient (JRC). For tight and rough discontinuities, the cubic law deviates from accuracy because flow through such discontinuities is strongly influenced by the stress changes. At low stress levels, the aperture and flow rate vary significantly with the normal stress, whereas at high stress levels, there is no significant change in aperture and flow volume.

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