Abstract: It is difficult to maintain the pressure of the soil chamber when the earth pressure balance(EPB) shield tunnels in the waterrich gravel sandy strata. Therefore, it is necessary to improve the residual soil of the shield to reduce its permeability and improve the pressure holding effect of the soil chamber. A case study is conducted on the Xiaocaiyuan stationNorth Railway station section shield tunnel project of Kunming metro line 4, and the response characteristics of waterrich gravel sandy strata tunneled by shield under soil conditioning are examined using numerical simulation and field monitoring data. The results show the following: (1) With the declining permeability of conditioned soil, the pore water pressure on excavation face gradually increases, and when the permeability coefficient of conditioned soil in soil chamber decreases to 5×10-5 cm/s, the pore water pressure on excavation face will approach to the initial pore water pressure, which indicates that the conditioned soil with relatively low permeability has a good waterproof performance. With the rising groundwater level, the pore water pressure on excavation face gradually increases, indicating that the waterproof performance of conditioned soil is more important for the EPB shield tunneling in stratum with high groundwater level. (2) The permeability of conditioned soil in the Xiaocaiyuan stationNorth Railway station section is relatively low that the excavation face is of waterresisting condition to some extent. In the process of shield tunneling, a certain excess pore water pressure will be developed near the excavation face. The affected area of excess pore water pressure is mainly 1.5 times of the tunnel diameter in front of the shield. The soil displacement of the excavation face is far less than that of the excavation face when instability occurs, indicating that the stability of the excavation face is high. (3) Due to the low permeability of conditioned soil, a relatively high excess pore water pressure will be developed around the shield during tunneling. Meanwhile, the conditioned soil with low permeability can effectively limit the free seepage of groundwater in front of the excavation face, which can improve the stability of excavation face, and reduce the construction ground settlement. (4) With the rising groundwater level, the ground settlement and excess pore water pressure tend to increase, causing the support pressure required to maintain the stability of excavation face increases rapidly, which indicates the groundwater level has notable impact on the support pressure on excavation face.

Key words: shield tunnel, waterrich gravel sandy strata, soil conditioning, ground response, numerical simulatio