Study on Land Subsidence Law Caused by Tunnel Passing through Upper-soft Lower-hard Strata

Objective Shield construction in upper-soft lower-hard strata is relatively challenging. Different cutting effects of shield machine on soft and hard rocks, as well as the different resistance encountered during shield propulsion, will cause great disturbance to the strata, and the resulted relatively large land subsidence will affect the safety of urban buildings（structures） and tunnel construction. Therefore, it is necessary to conduct an in-depth research on this issue. Method Taking a tunnel project in a certain city as an example, the law of land subsidence caused by shield tunnel construction in upper-soft lower-hard strata is studied. By introducing the stratum composite ratio, a calculation formula of the maximum land subsidence caused by construction with different soft and hard rocks proportions in the tunnel section is derived; using the stratum composite ratio as a variable, tunnel excavation models with strata composite ratios of 0, 25%, 50%, 75% and 100% respectively are established with finite element software Midas; the influence of strata composite ratio on land subsidence is analyzed according to the numerical calculation results, and a comparative analysis of the theoretical calculation value, numerical simulation value and field monitoring value of the maximum land subsidence is carried out. Result ＆ Conclusion Final research results can provide references for the land subsidence problem caused by tunnel construction in upper-soft lower-hard strata