Study On Disturbance To The Soil From Earth Pressure Balance Shield Tunneling In Weak Watery Fine Sand Strata

Earth pressure balance shield(EPS) is facing a series of technical problems, including subsidence control, soil liquefaction and quick sand, when applied in the fine sand layer with poor plasticity, high water content and large permeability coefficient. Additionally, due to scarcity of experience, accidents happened frequently. Therefore, it is of great importance to analyze the problems of disturbance on surrounding soil and other problems in construction resulted from EPS in fine sand layer.Based on Line 1 of Suzhou metro, experiments were conducted to study on the surface settlement, horizontal displacement of soil, soil pressure and pore water pressure by setting up corresponding equipments in test section and full-face fine sand section. And the underlying laws, influential range, and influence degree for soil disturbance during different stages were studied by theoretical analysis and numerical simulation.(1) In the full face fine sand strata, the disturbance to the soil is strong during construction, and the surface settlement which is much larger than the normal formation(about 80%larger) is quite hard to control, especially in the situation of stopped shield.(2) The Peck formula was applied to predict the surface settlement of Suzhou. It is suggested that the value of soil loss per length equals 0.45% times of the excavation area, and the width coefficient of transverse settling tank equals 0.44 times of buried depth of tunnel.(3) The concept of stress disturbance degree was put forward, and the relevant relation between it and the distance of one point to the edge of tunnel was proposed.(4) Longitudinal affected area is from 15m in front of the incision to 25m behind it, and horizontal affected area is about 2 times of diameter of shield on each side of the tunnel, which is less than in the clay strata.(5) With FEA of MIDAS GTS, the disturbance on soil from the shield was simulated.Based on the above achievement, efficient method of simulating the disturbance on soil from shield with FEA techniques was suggested.(6) The problems of constructing with EPS in Suzhou were summarized according to the data from test section and full face fine sand strata work. Meanwhile, some valuable advices on the constructing with shield were put forward in the end.