Investigation of underwater shield tunnel excavation face stability in lower dense and upper loose strata

In order to study the stability of the excavation face in the lower dense and upper loose, saturated sandy-pebble strata, physical model tests are conducted. The study concentrated on researching the progressing failure laws of the shield tunnel excavation face under various buried depths, and numerical simulations are employed to confirm the test results. Meanwhile, the theory calculation model of excavation face adapted to this stratum is put forward, and the theoretical formula of limited effective support compressive stress is presented. The results indicated that, as the baffle moves, the peak displacements of surface settlement successively occur in stages of no settlement, slow settlement and rapid settlement. The decrease of buried depth will increase the magnitude and range of surface settlement and will break the limiting state of equilibrium of the strata faster. Simultaneously, the decrease of buried depth will accelerate the seepage velocity of water in the strata without affecting the range of seepage path of water. In addition, the decreased burial depth will increase unstable range of the stratum. Under the equal burial depth, the unstable range of the lower dense and upper loose is bigger than that of homogeneous strata. Finally, the limit effective support compressive stress decreases with reduced burial depth.