Abstract: In recent years, a new type of composite lining has been adopted in the construction of diversion tunnels across urban areas in China. The joint of its outer segments may open under the internal water pressure transmitted by the back-filled self-compacting concrete, thereby causing a problem of external water seepage. In this paper, a three-dimensional finite element model for the seepage in a typical tunnel section is developed; the amount of seepage flow and the external water pressure of the lining under various external conditions are examined in a case study of the Luotian Reservoir to Tiegang Reservoir water conveyance tunnel project in Shenzhen. The results show that for the tunnel in the completion stage, its seepage flow decreases by about 90% if the seepage coefficient of the outer segments is decreased by one order of magnitude. Concrete backfilling reduces the seepage by 30% ~ 40%, and the grouting ring has little effect on the seepage. During operation, the lining joint is the main leakage channel, accounting for more than 90% of the total seepage; the grouting ring in this stage can reduce the seepage significantly. The peak seepage occurs in the operation stage, and it is thought to be a control condition of the tunnel seepage. The external water pressure determination should be based on the completion stage subsequent to the self-compacting concrete backfilling.

Key words: water conveyance tunnel across urban area, composite lining, external water seepage, grouting ring, seepage flow, external water pressure