Research On The Control Of Surface Settlement During Shield Construction In Water-rich Fine Sand Layer

Based on the construction of shield tunneling between Xuefu Street Station and Changfeng Street Station of Taiyuan Metro Line 2,this paper aims at the formation disturbance phenomenon in the process of shield tunneling.By means of theoretical analysis and calculation,ABAQUS finite element software and field measurement,the paper analyzes the stability control of the excavation face and the filling of the gap at the tail of shield.Research of aqueous silty sand layer of shield excavation process in the key construction parameters influence on surface subsidence laws and control countermeasures,mainly including shield excavation process of surface subsidence and cutter opening rate and soil tank pressure,grouting pressure,grouting ratio and elastic modulus etc key construction parameters on the influence law of ground surface settlement,The optimal control measures of key construction parameters are put forward,which provides scientific basis for shield tunnel construction of Taiyuan Metro Line2,and has guiding significance for shield tunnel construction of subway.The main conclusions are as follows:(1)According to theoretical analysis and engineering experience,the surface deformation caused by shield tunneling is divided into five stages.Combined with numerical analysis,the surface subsidence caused by the five stages accounts for 9%,18%,24%,33%and 16%of the total subsidence respectively.The ground subsidence caused by the stability control stage of excavation face and the gap stage of shield tail accounted for 27%and 57%of the total settlement,respectively.The results show that the reasonable filling of shield tail clearance and the stability control of excavation face are the two main factors affecting the ground subsidence during tunnel excavation.(2)In the theoretical calculation and the measured data obtained on the basis of the statistical analysis of soil cabin pressure and the initial value range of cutter opening ratio,then through the finite element software ABAQUS to establish a three dimensional numerical model of shield tunnel construction,in view of the different cutter opening rate and soil tank pressure to the front of formation stress and displacement characteristics is analyzed,the influence of cutter opening rate control in 50%,soil tank pressure control within the scope of 0.1 to 0.15MPa,the surface settlement of shield excavation process control effect is best.(3)The concept of grouting proportional coefficient is put forward to quantify the change of grouting amount in the process of numerical analysis.The research shows that the surface subsidence decreases gradually with the increase of grouting proportional coefficient,but when the grouting amount reaches a certain value,its effect on improving the surface subsidence is no longer obvious.The comparison between the measured grouting pressure and the simulation analysis shows that the greater the grouting pressure is,the smaller the surface subsidence will be.When the grouting pressure is greater than 0.4MPa,the effect of the grouting pressure on further slowing down the surface subsidence is no longer obvious.It is found that the elastic modulus of grout has little effect on the control of ground subsidence.Therefore,an optimization concept of low strength grout mixture ratio is proposed.In conclusion,the surface deformation caused by shield tunnelling in water-rich silt fine sand layer can be effectively controlled when the grouting proportion coefficient is 1.7,that is,the single-ring grouting volume is 4.65m~3 and the grouting pressure is 0.3?0.4MPa.(4)For the first time,the fitting evaluation method is used to objectively evaluate the dimensional relationship of the influence of grouting parameters on the surface settlement.The results show that the influence coefficients of grouting pressure,grouting proportional coefficient and grout elastic modulus are 1.525,1.05 and 0.475 respectively,that is,the grouting pressure has the greatest influence on the surface settlement,followed by the grouting quantity and the grout elastic modulus.(5)Applying the above research results to taiyuan metro line 2 university street station?changfeng street rich water and silty sand layer of earth pressure balance shield construction range,on the basis of surface subsidence control standards,through actual grouting effect evaluation,the engineering application of the shield tail void filling rate is above 90%,slurry let stand 28 d setting body strength is greater than 6MPa,surface and surrounding buildings settlement basic control within 20mm,all through the tunneling speed,thickness of grouting layer,slurry setting body strength,surface and surrounding buildings settlement,such as comprehensive evaluation,shows that the research conclusion in the engineering practice application effect is good,The selection of parameters is reasonable and meets the requirements of construction quality standards.