ABSTRACT

In order to investigate the hydromechanical coupling properties of rock joints, a new test apparatus including different flow systems consist of constant head, constant pressure, constant flow rates and also back pressure system is developed to perform test under wide range of hydraulic head and hydraulic gradient. Experimental test under normal and shear mode are performed and results show that, increasing normal stress cause a decrease in hydraulic conductivity. Furthermore, the results indicate that the inlet water pressure directly affects hydraulic conductivity. During shear process; increasing shear displacement causes a decrease in hydraulic conductivity about one order, and sudden increase of about three orders of magnitude. Changes of Inlet water pressure during shear do not show significant changes in hydraulic conductivity. It is clarified that there is complexity of flow in a rock joint, and the parallel plate model is difficult to apply specifically to the evaluation of the hydraulic conductivity of a rock joint. The main reason is the variable flow path in rock joint according to the flow conditions.

INTRODUCTION

Fluid flow and hydraulic conductivity in fractured rocks is a wide subject in rock mechanics and has a road relevance in many fields of engineering such as hydrogeology, civil and petroleum, geothermal engineering, and isolation of nuclear and hazardous waste disposal. In civil engineering constructions, hydraulic conductivity changes due to stress perturbation and joint normal and shear displacement and in petroleum and geothermal reservoirs, hydraulic conductivity depend on fluid pressure variation. The first step in understanding rock mass conductivity is the comprehension of single rock joint. However, the hydraulic properties of rock joint under deep and severe underground condition are not clarified.

Several hydromechanical testing apparatus were developed to evaluate rock joint permeability, under various mechanical and flow conditions. Shear-flow coupling test apparatus could be divided in two categories consist of radial flow and linear or one- dimensional flow. In radial flow testing apparatus, water is injected into the joint through hole located in the center of the joint and flows around the specimen, with free outlet conditions. Esaki et al. (1991 & 1998) developed shear-flow coupling test apparatus which adapted radial flow hydraulic conductivity test. They have performed tests on granite and sandstone, with constant water head at joint inlet and free outlet condition. They concluded that the hydraulic conductivity increased rapidly by about 1.2–1.6 orders of magnitude during the first 5 mm of shear displacement and then gradually becomes constant in the residual shear region. Furthermore, with increasing shear in residual region there are deviation between experimental results and Barton (1985) model. Boulon el al. (1995) has conducted the series of tests on replica and found that transmissivity is largely influenced by the normal relative displacement accompanying the shearing, and is quite different before and after the peak shear stresses. After peak shear displacement transmissivity is more dependent on normal relative displacement.

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