| With the accelerating process of urbanization,the urban population and vehicles are more and more intensive,and the urban traffic pressure is more and more serious.The development of urban underground space and three-dimensional traffic has become an important part of the current urban infrastructure construction.As an important part of three-dimensional transportation,subway ushers in a historic development peak.Shield method is one of the common methods of urban subway tunnel construction.Due to the dense ground buildings and complex underground pipelines in the city,it is inevitable for shield tunneling to be carried out in a close distance.The shield tunnel of Wuxi Metro Line 3 starts from an old building at a short distance and passes through it.The old building is not reinforced and can not be tested neither.The stratum encountered by shield excavation face is mainly sand layer,and micro confined water is developed.In order to control the ground settlement accurately and ensure the smooth start of the shield machine,the clay shock method is adopted to deal with the shield clearance,and the foam is used to improve the micro bearing sand layer.In this paper,the properties and diffusion mechanism of the slurry and the improvement technology of the micro pressure sand layer are studied.The main research contents and achievements are as follows:(1)The main components of the shock are bentonite,water and water glass,and three admixtures,thickener,stabilizer and dispersant,are added.The experimental results show that each component has an effect on the strength,viscosity,fluidity and material composition of gram mud,among which the water content has the greatest effect,and the water content will affect the function of other components.It is characterized by high viscosity,low fluidity,no hardening.The unconfined compressive strength of gram mud effect is low,about 0.02～0.06 MPa.(2)According to the laboratory test,the best ratio of mud control efficiency is: soil water ratio 1:4,water glass ratio 1:10,cellulose derivative 1.0%,stabilizer 0.4%,dispersant 0.3%.The practical application of Wuxi Metro Line 3 shows that the clay shock method can effectively fill the shield gap temporarily,control the ground displacement and surface settlement,and ensure the safety of the short-distance crossing construction.(3)The clay shock method is a shield synchronous grouting method.The horizontal projection of mud control effect on the surface of the shield is fan-shaped after the clay shock flows and diffuses in the gap of the shield.The boundary of the flow and diffusion is the clay shock grouted before,the soil ballast,the soil and the shield.Therefore,the clay shock fan-shaped filling and diffusion model is deduced.The example analysis shows that the clay shock is distributed in the shield section along the circumferential pressure The pressure decreases in the process of flow diffusion.Under the action of gravity,the pressure at the top of the shield is the smallest,and the pressure at the bottom of the shield is the largest.(4)The micro confined soil in the shield area of Wuxi Metro Line 3 is mainly medium coarse sand and fine sand.The best improvement method is the foam concentration of 3%and the foaming pressure of 0.1MPa The foam injection rate is 10% to 20%.The field application shows that the improved scheme can effectively ensure the stability of the speed,thrust and torque of the shield machine,and can provide reference for similar shield projects in Wuxi area. |
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