Study on the instability mode of a tunnel face under variable seepage conditions in sandy soil shield tunnels: Centrifuge tests and numerical simulation

Shield tunnel excavation in water-rich sandy soil experiences rapid seepage variations at the tunnel face due to groundwater level fluctuations. To study the instability mode of the tunnel face in water-rich sandy soil under variable seepage conditions, taking a metro construction project as the engineering background, a set of tunnel face instability devices including a water level control system suitable for a geotechnical centrifuge were independently developed. Centrifuge model tests were conducted using the self-designed model device. The displacement field, earth pressure, and pore pressure in the instability zone were automatically measured during the experiments. Complementary numerical simulations were conducted to analyze the limit support pressure at the tunnel face under variable seepage conditions. Results show that under variable seepage conditions, the instability zone of the tunnel face in a sandy soil shield tunnel exhibits a "wedge + prism + iterative arch" type and displays an angle of approximately 45 degrees + phi/2 (where phi represents the friction angle of the soil) between the inclined plane of the wedge and the horizontal direction. The top of the final instability zone extends to the surface to form a "chimney" shape. During the instability process, the earth pressure within the instability zone is continuously decreased, the displacement of the strata leads to an increase in the seepage channels, and the pore pressure increases. The changes in the earth and pore pressures become more pronounced when approaching the tunnel face. The increase in seepage intensity accelerates the development of the instability zone and increases the changes in the earth and pore pressures. The stability of the strata in the instability zone decreases rapidly with increasing seepage intensity. The limit support pressure at is primarily governed by the initial groundwater level and the rate of seepage intensity variation.