Abstract: For the tunnel accident caused by water leakage, soil erosion and sand and water loss in water-rich sand stratum, in order to quantify the risks of tunnels, a new shield tunnel seepage simulation test device is developed considering the interaction between elastic sealing gasket, soil and water. The states of soil loss of Fujian standard sand and Tianjin fine sand under different gap widths and water pressures are studied through experiments. The leakage mechanism due to invasion of sand is revealed and proved by the test results. It is shown that the traditional test methods will overestimate the waterproof property of elastic sealing gaskets without considering the influence of soil. The formula for estimating the critical gap width of soil erosion is derived, and verified by the test results. The critical gap width of soil erosion is proportional to the vertical stress and soil thickness, and is inversely proportional to the soil porosity and hydraulic pressure around gap. When soil erosion happens, the mass loss is proportional to the gap width and hydraulic pressure, and is inversely proportional to the effective stress in the soil around gaps. The soil erosion makes the gradation curve smoother. For Fujian standard sand and Tianjin fine sand, a practicable assessment method for the state of soil loss is put forward considering pressure and segment gap.