Abstract
Frequent pressure transients are identified as the cause of block failures in many unlined hydropower tunnels. The primary design objective of such tunnels is to prevent hydraulic jacking at design static pressure and mass oscillation but neglects the effect of short transients, i.e., water hammer. The issue has not been studied from the perspective of hydro-mechanical interactions due to frequent pore pressure changes in the rock mass. This article mainly focuses on the effect of pressure transients at different static heads, or different effective normal stresses across the joints and the effect of time period of pressure transient. Further, the change in such behaviour due to different mechanical properties of rock joints, such as stiffness, friction angle and dilation, is investigated. Numerical simulations of observed pore pressure response in the rock mass during a pressure transient are carried out using distinct element code 3DEC. The results show that relative joint deformation due to short pressure transients are the highest when joint normal stresses are 1.5–2.5 times higher than static water pressure in the tunnel and thus the vulnerability to weakening of such joints by hydraulic fatigue is higher. Further, results show that water hammers can travel up to 4 m into the rock mass even in stiff joint conditions and sufficiently high normal stresses. Results further indicate that the hydraulic impact due to water hammer is smaller as compared to mass oscillation. It is concluded that water hammers, wherever applicable along the waterway, can still contribute to hydraulic fatigue of rock joints in addition to the effect of mass oscillation and cannot be neglected when pressure transients occur frequently. Tunnel filling/dewatering and mass oscillations cause macroscopic joint displacements or block movements over long-term operation which is the major cause of block falls in unlined pressure tunnels.
Highlights
Hydromechanical coupling in fractured media is a major field of scientific research in rock engineering and has applications in engineering projects that involve fluid flow in rock mass, such as reservoir engineering, nuclear waste disposal, inflow during underground excavations, design of dam foundations, etc
A previously unexplored application is in the field of design of unlined pressure tunnels functioning as waterways for hydropower projects, where there is direct contact between flowing water and rock joints exposed to tunnel wall
It is seen that the boreholes which intersect the conductive joints, i.e., BH1 and BH4 strongly respond to pressure transients whereas BH2 is non-responsive, since it does not have direct hydraulic contact with the conductive joints
Summary
Hydromechanical coupling in fractured media is a major field of scientific research in rock engineering and has applications in engineering projects that involve fluid flow in rock mass, such as reservoir engineering, nuclear waste disposal, inflow during underground excavations, design of dam foundations, etc. A previously unexplored application is in the field of design of unlined pressure tunnels functioning as waterways for hydropower projects, where there is direct contact between flowing water and rock joints exposed to tunnel wall. Steel or concrete lining is carried out in places where the minor principal stress is smaller This is a well-tested design criterion with applications all over the world (Panthi 2014; Broch 2010; Marwa 2004) and especially in Norway, where more than 95% of hydropower pressure tunnels are unlined, with a history of over 100 years. An unlined water tunnel is filled at a slow pace so that there are no sudden changes in pore pressure and effective stresses in the rock mass. Mass oscillation can continue for hours after a load change event in the power plant, whereas water hammer dies down relatively faster
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