Mud pumping problem in tunnels on erosive soil deposits

High-speed train travel round the world has given rise to newer geotechnical problems that require ingenious solutions. In tunnels laid on erosive soil deposits, pore water pressure build up due to traffic loading and ensuing seepage flow loosens soil particles beneath the tunnel invert, and mud slurry is pumped out from invert weep holes with a passing train load. This causes loss of tunnel support, giving rise to uneven deformation of the concrete slab track that requires costly and difficult maintenance. From tests performed on a new model experimental set-up, it is observed that there is a limiting distance such that no mud pumping occurs if the distance of an open slit (simulating weep holes) from the point of application of cyclic loading is greater than this limiting value. Relocating the invert weep holes at a suitable elevation along tunnel walls with their intake firmly secured by a geotextile overlain by a thin sand layer, acting together as a filter, appears to be the most effective way of countering the mud pumping problem. Two full-scale model tests, one in a yard containing a tunnel invert made of Portland cement concrete and the other one in a yard containing a tunnel invert made of slag concrete, are carried out to investigate the proposed solution in the field.