Research On The Variation Law And Control Technology Of Surface Settlement Of Large-section Shallow Buried Urban Tunnel

To alleviate urban traffic congestion,the construction of large-section urban tunnels in China is gradually increasing.Due to the shallow burial depth,the design span of urban tunnels is getting larger,and surrounding buildings are more densely populated.Tunnel construction can cause surface subsidence,which can damage roads and surrounding building structures,and even cause serious engineering accidents,resulting in casualties and economic losses.Therefore,it is crucial to study the laws and control measures of surface subsidence caused by excavation of large-section shallow-buried urban tunnels.This paper takes the Beishan Tunnel in Wanzhou District as the engineering background and studies the laws and control measures of surface subsidence caused by excavation of large-section shallow-buried urban tunnels through on-site monitoring data,prediction models,and numerical simulation methods.The main research results are as follows:(1)Through the analysis of surface subsidence monitoring data of each monitoring section,it is shown that tunnel surface subsidence mainly includes four development stages: minor subsidence,rapid increase of subsidence,slow subsidence,and stable subsidence.The proportion of subsidence deformation in each stage of the right tunnel is 4.02% to 15.03%,62.79% to 70.63%,and14.35% to 29.51%,and the proportion of subsidence deformation in each stage of the left tunnel is 24.90% to 40.25%,53.37% to 67.31%,and 1.52% to 7.80%.In particular,the subsidence rate in the rapid increase stage V often experiences a sudden change.(2)By analyzing the surface subsidence monitoring data of each monitoring section in the direction of tunnel face excavation and the mileage of excavation,it is concluded that the longitudinal influence range of tunnel face advancement on unexcavated monitoring sections is 26-32 m and gradually decreases with the increase of tunnel burial depth and surface filling depth.Based on linear regression fitting analysis of Peck’s formula with different monitoring data,the range of the surface subsidence value correction coefficient α in Peck’s formula is 0.2915 to 1.307,and the range of the subsidence trough width correction coefficient β is 1.073 to 0.810.By using the upper and lower limits of the correction parameters in Peck’s formula,the maximum range of ground subsidence is solved to be 61.7-276.3mm,and the transverse influence range of tunnel excavation is 36-40 m.(3)Based on the post-monitoring data of the rapid increase stage V subsidence,the applicability analysis of the gray GM(1.1)model and the gray Markov model is carried out.The results show that the prediction results of the subsidence in the last three stages using the Markov model are more consistent with the actual monitoring data,which provides a reference for short-term prediction and emergency subsidence treatment.(4)Using the MIDAS/GTS NX finite element software,the influence of excavation footage,step spacing,and sidewall guide pit step numbers on rock strain,stress,and Z-direction displacement is analyzed,and control measures are proposed to optimize excavation methods.The results show that reducing the excavation footage and step spacing and increasing the number of sidewall guide pit steps can reduce ground subsidence.