Coupled stress-seepage numerical design of pressure tunnels

Abstract

Abstract The design and construction of pressure tunnels are among the most complicated of tunnels. Therefore, special attention is required in the design and construction of these tunnels to prevent failure. This research introduce PLAXIS 2D fmite element program as a tool for numerical modeling of plain concrete pressure tunnels. The numerical model cannot portray the nature, but should simulate the materials and the loading cases. The rock behaviour was approximated using elasto-plastic Mohr - Coulombs model. The shotcrete and fmal lining defmed as permeable and elastic. Analyses were based on plain strain condition. Different phenomena in terms of loading cases during construction stage and operational loading of internal water pressure were simulated. The entire loading steps in the design of pressure tunnels modeled are: initial state of stresses, 2D simulation of 3D excavation, shotcrete installation, final lining construction, simulation of temperature/shrinkageeffect of concrete lining, shotcrete decay, grouting process with prestressing effect and finally, the modeling of operational loading of internal water pressure and external water pressure in empty tunnel. Special concern is taken on the modeling of the contact between the shotcrete and the fmal lining where during shrinkage and temperature change by first filling with cold water a gap can open. This gap is filled (closed) during prestressing. Plain concrete lining of pressure tunnels is not absolutely tight lining and water seeps out of the tunnel. The seeped water is lost energy, but can also cause stability problems in the surrounding rock mass. Additionally, if the rock mass around the tunnel is tight (originally or tighten by grouting) the seeped water stays in the vicinity of the tunnel and increases the external water pressure. Such increased external water pressure decreases the gradients between internal and external pressure and reduce the seepage and losses. Modeling of this phenomenon is performed by coupled stress-seepage calculation performed by the same model. The coupling of stress - seepagewas carried out by superimposing results of consolidation and water flow analyses. Practical example was taken from the pressure tunnel of the HPP Ermenek Turkey. The calculation results are compared with results of existing tunnel and with analytical solutions. The entire simulation results for both: construction stages and operational loading by internal and external water pressure showed that PLAXIS 2D fmite element program can be used to observe and lor predict phenomena in pressure tunnels. Keywords: Pressure tunnel, plain concrete lining, permeable tunnel lining, stress-seepage analysis, PLAXIS 2D,jinite element simulation.