A Finite Element Model for Optimum Design of Plain Concrete Pressure Tunnels under High Internal Pressure

Abstract

Plain concrete lining of pressure tunnels are not absolutely tight and water can seep out of the tunnel. Seeped water is lost of energy in hydropower system, but can also cause serious stability problems in the surrounding rock mass. If the rock mass around the tunnel is tight (originally or tighten by grouting) seeped water, however, stays in the vicinity of the tunnel and increases the external (ground) water pressure. Such increased external water pressure decreases the gradients between internal and external pressure and reduce the seepage and losses. For simulation of hydraulic mechanical interaction in the process of cracking, a coupled seepage-stress method based on the 2D elasto-plastic finite element method (FEM) is proposed. The coupling has been carried out by superimposing results of consolidation and water flow analyses. The coupling principle produces the change of stress field and leads to change of permeability coefficient and the redistribution of the seepage field. The calculation results are compared with results of existing tunnel and with the analytical solutions. A design criterion based on this study can be suggested for pressure tunnel design procedure in stable rock conditions.