Optimization Design of Consolidation Grouting Around High-Pressure Tunnel Considering Non-Darcian Flow Effect

Concrete-lined pressure tunnels have been widely used for water conveyance in pumped storage hydropower. With increasing internal water pressure, the tunnels are at higher risk of leakage, and the flow regime in the surrounding rocks is likely to transition from laminar to non-Darcian. Grouting has been considered as a cost-effective technique for controlling leakage in pressure tunnels, and an optimization design is usually needed to save the cost. This study proposes to use the Forchheimer's law-based model for design optimization of grouting for a concrete-lined tunnel in hard rocks subjected to 8 MPa internal water pressure. The hydraulic conductivity K of the surrounding rocks is determined from packer tests, and the excavation-induced K variation is characterized by an equivalent elasto-plastic model. The non-Darcian coefficient beta is estimated from K with a calibrated power-law scaling. It is proposed that a proper design of grouting should be able to limit the region with significant non-Darcian effect within the grouted zone, in addition to other commonly-used criteria for leakage, seepage erosion and fracturing failure in the surrounding rocks. It is found that when the quality of grouting is controlled to a standard of K = 6 x 10-8 m/s, the optimal depth of grouting is 6 m in fractured rocks and 12 m in fault zones. Hydraulic tests and seepage measurements confirm that this design scheme has been well implemented at the site, and is effective in controlling pore pressure and leakage to the level predicted by the Forchheimer's law-based simulation. The quality control criterion and depth of grouting are optimized with Forcheimer's law-based flow simulation for a pressure tunnel.It is proposed that significant non-Darcian flow effect should be limited within the grouted zone for optimization design of grouting.The optimized design scheme of grouting was implemented, and its effectiveness in leakage control is validated by seepage measurements.