Study On Stability And Surface Subsidence Law Of Excavation Surface Of Shield Tunneling In Nanning Sand

It is of great theoretical and engineering application to study the stability of surface excavation and the law of surface subsidence of sand tunnel in Nanning area.This paper relies on Nanning Metro Line 2 tunnel project,using theoretical analysis,numerical calculation and on-site.Monitoring the combination of methods to study the main work and conclusions are:(1)According to the permeability coefficient and the grain gradation curve,the mudshield balance shield machine is selected as the shield model for the construction.Based on the analysis of the stability of the excavation surface and the improvement of the tunneling efficiency,the cutterhead structure of the mud shield machine is determined as For the panel cutter;consider the effect of cutting the soil and tool life and other factors,the cutter on the knife to determine the knife and the first knife.(2)The calculation mechanism and influencing factors of the excavation surface of the sand strata are analyzed.The calculation formula of the lateral pressure coefficient of the three-dimensional too sand basis is considered,and the calculation method of the lateral pressure coefficient based on the rotation theory of the principal stress axis is given.Based on the prism-trapezoidal wedge model,The numerical control method is used to verify the rationality of the formula.The minimum support force required for the tunnel surface excavation is given.(3)A monitoring program for surface subsidence of tunnel is established and field monitoring is completed.Based on the Peck theory,the correction coefficient of the settlement groove width is 0.22?0.31,and the correction coefficient of the formation loss rate is 4.07?4.87%.Based on the Logistic curve,the vertical settlement of the surface is divided into slow growth stage,rapid growth stage and stable stage.The sharp growth stage is within 15m before and after excavation.The three-stage method,the difference method and the linear regression method are used to analyze the prediction accuracy of the vertical settlement of the sand surface.The results show that the three-stage method is the best for predicting the vertical settlement of the surface.(4)The control measures of surface subsidence of shield tunnel are put forward.The results show that the maximum settlement value of the sandy soil strata is larger than that of the cohesive soil strata,and the stage of the vertical subsidence is smaller than that of the cohesive soil strata.The sedimentation trough curves of the sandy soil strata are "deep and narrow" "The form of the cohesive soil subsidence curve was" shallow and wide "in the form.The measured results are the same as those of FLAC,and the engineering control measures are reasonable and effective.