Investigating the Effects of Transient Flow in Concrete-Lined Pressure Tunnels, and Developing a New Analytical Formula for Pressure Wave Velocity

Abstract

Abstract One of the most common problems in concrete-lined pressure tunnels is the development of cracks in the concrete lining as a result of high internal transient pressure. Due to the rapid operations of hydraulic systems, transient flow establishes in the tunnel, and thereby, the unsteady water flow imposes high pressure on the lining. In this situation, an increase in the width and depth of cracks leads to leakage from the lining, and ultimately results in structural failure. Therefore, the effects of transient flow, as well as the pressure fluctuations inside pressure tunnels, must be considered in the design. In the present study, a two-dimensional concrete-lined pressure tunnel is modeled using Abaqus FEA (Finite Element Analysis) software, and steady-state and transient flow analyses are performed using the HAMMER software. The loads that result from the hydraulic analyses are used for the structural analysis of the lining. In this regard, first, the pressure wave velocity in the tunnel is calculated by a quasi-static method, using numerical modeling and a developed analytical formula. Then the effects of the grouted zone on the generated pressure waves are examined. Finally, the impact of the pressure wave on crack formation in the lining is studied, and the bearing capacity of the lining is evaluated. The stress analysis of the cracked elements in the concrete lining is conducted based on concrete damage plasticity behavior. It is observed that numerical modeling results indicate higher pressure wave velocity values than the analytical formula results, which can be attributed to the consideration of the rock load in the numerical model. Further, the operation schedule of the control gates influences the formation of cracks in the concrete lining. Considering non-linear behavior, the ultimate bearing capacity of the tunnel concrete lining is obtained, which can be used for the optimal design of the hydraulic systems.