Structural behavior of triple-layer composite lining of a water conveyance tunnel: Insight from full-scale loading tests

When constructing water conveyance shield tunnels under high internal pressure, composite linings are preferred over single-layer segmental linings due to the superior water tightness and load-bearing capacity. A triple-layer composite lining, consisting of an outer segmental lining, internal steel tube, and self-compacting concrete (SCC) filling, has recently been applied in a large-scale water conveyance tunnel project in China. However, its structural behavior under external overburden and internal water pressures remains poorly understood. This study investigates the mechanical behavior of the triple-layer composite lining through full-scale loading tests using a novel platform that simulates external and internal pressures. Results show that the composite lining remains highly elastic under combined loads with an internal pressure of 0.4 MPa. When the internal pressure increases to 0.6 MPa, cracks first appear in the SCC layer near segment joints, propagating uniformly and leading to stress redistribution. Studs on the steel tube-SCC interface strengthen bonding, reducing debonding at this interface while slightly increasing debonding at the SCC-segment interface. Despite localized SCC damage, the lining maintains excellent serviceability under cyclic pressure fluctuations. This study offers valuable insights for the design and construction of water conveyance shield tunnels with triple-layer composite linings, particularly in high-pressure environments.