Effect of Overburden Height on Hydraulic Fracturing of Concrete-Lined Pressure Tunnels Excavated in Intact Rock: A Numerical Study

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.