A study of the coupled hydromechanical behavior of jointed rock masses around pressure tunnels

Hydrojacking of rock masses around recently commissioned pressure tunnels, can be attributed principally to the use of empirical cover criteria, developed from experience in massive unweathered hard rocks, when applied to the design of tunnels in softer and/or jointed rock.; The initiation and evolution of hydraulic jacking (hydrojacking) of fractured rock masses were studied, as the internal pressure in an unlined tunnel increased. The effects of tunnel depth {dollar}(Lsb{lcub}r{rcub}),{dollar} ground surface inclination {dollar}(beta),{dollar} geologic features (e.g., attitude and strength of rock joints) and tectonic history on the in situ stresses and hydrojacking of the mass were investigated using the Discrete Finite Element Method (DFEM), which is a numerical procedure capable of modeling the coupled hydromechanical behavior of fractured media.; The DFEM analyses showed that the total normal stresses on joints oblique to the principal stress directions were not constant during pressurization of the tunnel. The changes in total normal stress on a given joint were caused by a reduction in shear strength of joints in the medium and/or movements of rock wedges induced by water pressure increases during pressurization of the tunnel. These findings suggest that the jacking pressures obtained from hydrojacking tests on individual joints may be different from the pressure at which the joints open when the pressure tunnel is filled.; In fractured rock masses with continuous joints which daylight middle to steeply inclined valley walls {dollar}(beta>20spcirc),{dollar} failure by hydrojacking was preceded by sliding of a wedge toward the valley side and opening of steep joints. This observation was used to formulate a criterion for failure whose premise is that global hydrojacking occurs when steeply dipping joints open as movement of the wedge begins along shallow dipping joints. A cover criterion was derived from the stability of rock wedges which incorporates the attitude and strength of the joint sets and the inclination of the valley walls. The Norwegian minimum cover criterion is recovered as a special case of this new criterion.