Experimental investigation on gush disaster in shield tunnel driven through upper-silt and lower-sand strata

Gush disasters are frequent and catastrophic in shield tunnel engineering. Silt and sand are two typical kinds of strata that gush accidents happen frequently. But the investigations concerning silt-sand composite strata are rare. This research focuses on upper-silt and lower-sand strata and adopts a set of self-built experiment system. The dip angle of the strata interface (α) and the slope of the shield tunnel (β) are taken as the variables. The results show that the collapse happens rapidly as soon as gush breach opens except the experiment of single silt stratum. The collapse process of single sand or silt stratum when tunnel is horizontal is accompanied by the formation, destruction, re-formation and re-destruction of soil-arch. But soil-arch does not appear in other groups of experiments for lack of arch base except when α is 0° and β is 30°. When β is 60°, the subsidence boundary enlarges to the interface of sand and silt. Seepage comes from the sand stratum to the main subsidence surface for its hyperpermeability and permeates downwards, resulting in a minor settlement finally. The ultimate subsidence value of all experiment groups is in the range of 80 ∼ 90 mm. The effect of tunnel slope lies in that the unstable area above gush breach can be divided as Gravity Instability Area and Slip Instability Area in early stage of collapse, but Slip Instability Area will integrate into Gravity Instability Area which enlarges in the collapse process. Pressing phenomenon exists at the lower part of strata interface. One reason is that the confined aquifer property of silt in the early moment of collapse, the other reason is that the major principal stress rotates and has a larger horizontal component.