Numerical simulation of pore pressure in rock joints during pressure transient in an unlined hydropower tunnel

Abstract

Recent experimental result shows that pressure transients of both short and long time periods; i.e. water hammer and mass oscillation; can travel into the rock mass of an unlined hydropower tunnel through rock joints. It is observed that the rock mass can be both responsive and unresponsive to such pressure transients and mainly depends on the condition of joint surfaces. A deeper understanding of this behavior is needed in order to investigate the effects of pressure transients on the long-term stability of unlined water tunnels because additional seepage forces acting on the rock blocks due to pore pressure variations are known to cause block falls and large overbreaks. This article aims to simulate the delayed pore pressure response of the rock mass observed during pressure transients in an unlined hydropower tunnel. A 2D distinct element numerical simulation is carried out using the UDEC code. It is observed through in-situ monitoring that pressure transients travel into the rock mass mainly through the most conductive joint sets present in the rock mass. Hence, a simplified generic single joint system model is used for the simulation. Results show that complete simulation of the delayed response cannot be achieved using a 2-dimensional model, even if pore pressure magnitude could be matched by using appropriate hydraulic aperture. A fully coupled 3-dimensional hydromechanical model is recommended.

Numerical simulation of pore pressure in rock joints during pressure transient in an unlined hydropower tunnel