Abstract
This study investigated the impact of overburden height on the hydraulic fracturing of a concrete-lined pressure tunnel, excavated in intact rock, under steady-state and transient-state conditions. Moreover, the Norwegian design criterion that only suggests increasing the overburden height as a countermeasure against hydraulic fracturing was evaluated. The Mohr–Coulomb failure criterion was implemented to investigate failure in the rock elements adjacent to the lining. A pressure tunnel with an inner diameter of 3.6 m was modeled in Abaqus Finite Element Analysis (FEA), using the finite element method (FEM). It was assumed that transient pressures occur inside the tunnel due to control gate closure in a hydroelectric power plant, downstream of the tunnel, in three different closure modes: fast (14 s), normal (18 s), and slow (26 s). For steady-state conditions, the results indicated that resistance to the fracturing of the rock increased with increasing the rock friction angle, as well as the overburden height. However, the influence of the friction angle on the resistance to rock fracture was much larger than that of the overburden height. For transient-state conditions, the results showed that, in fast, normal, and slow control gate closure modes, the required overburden heights to failure were respectively 1.07, 0.8, and 0.67 times the static head of water in the tunnel under a steady-state condition. It was concluded that increasing the height of overburden should not be the absolute solution to prevent hydraulic fracturing in pressure tunnels.
Highlights
High-pressure water tunnels, usually excavated in rock, convey water from the upstream reservoir toward the turbines in hydropower plants, and turn the energy of the flowing water into electricity.These tunnels can be either steel-lined or concrete-lined; steel-lined pressure tunnels are safer, but more costly to construct
Since this solution involves the hydraulics of water flow in the tunnel and the impacts of internal pressures on the surrounding rock, the Hammer software and Abaqus Finite Element Analysis (FEA) were linked for modeling and analyzing the system
The Hammer software [12], which works based on the method of characteristics (MOC), was employed to analyze the changes in the internal pressure inside the tunnel, as a function of time
Summary
High-pressure water tunnels, usually excavated in rock, convey water from the upstream reservoir toward the turbines in hydropower plants, and turn the energy of the flowing water into electricity. These tunnels can be either steel-lined or concrete-lined; steel-lined pressure tunnels are safer, but more costly to construct. Concrete-lined pressure tunnels are faced with serious challenges in design and construction. In cases where the rock is intact and not jointed, high internal pressure can fracture the rock. This phenomenon is called hydraulic fracturing, which makes the rock mass on the tunnel unstable
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