Stability Research Of The Soil-rock Composite Stratum With Defects Induced By Subway Tunnelling

The construction of subway tunnels has the characteristics of high risk,high concealment,and multi-factor influence.The accidents of strata instability caused by this frequently occur,they not only caused great economic losses,but also had a bad social impacts.As an important factor inducing strata instability,strata defects threaten the safety of subway tunnels construction.However,the influence of the structural stability is more complicated for the tunnel construction in the soil-rock composite stratum.Based on this,this article relies on the soil-rock composite stratum frequently encountered in the construction of Qingdao subway tunnels,combines with the National Natural Science Foundation project(51978356): Catastrophe mechanism research of the soil-rock composite stratum with defects induced by subway tunnelling,study the instability laws of soil-rock composite stratum with defects under the disturbance of subway tunnelling by theoretical analysis,model test,numerical simulation and so on.The main research results are as follows:(1)By collecting and sorting out 62 cases of stratum instability that occurred in the construction stage of subway tunnels,a corresponding analysis model was established based on SPSS statistical analysis software,and the main causes of stratum instability were excavated during the construction stage of subway tunnels.The results showed that the main reasons of stratum instability under the disturbance of the tunnel construction are the surrounding rock condition,the strata defect,the influence of groundwater and the atmospheric precipitation.(2)For subway tunnel engineering with defective soil-rock composite stratum,take the distance between the strata defect and the tunnel and the ratio of rock thickness to span as variables,and the test process under various working conditions is monitored and analyzed through model tests,summarize the demage process of the stratum in each working condition after the tunnel excavation,the instability mode and law of soil-rock composite stratum with defects under the disturbance of subway tunnelling are obtained.(3)The instability mode of the composite stratum when the strata defect is within the influence range of the tunnel collapse arch: small area drop on the top and both sides of the tunnels → form cracks between the tunnel and the strata defect → the fracture surface develops to the soil-rock interface → the tunnel and the strata defect produce penetration failure → the collapsed arch of the strata defect and the tunnel are superimposed to form a stable collapsed arch.(4)The PFC2 D granular discrete element method calculation software is used to simulate the instability process of soil-rock composite stratum with defects under the disturbance of subway tunnelling,and then the conclusions of the model test are compared,verified and supplemented by calculation software,the influence of different distance between the strata defect and the tunnel and the ratio of rock thickness to span for the instability law of composite stratum is analyzed,extract the most unfavorable distance when the strata stability is the worst,and analyze the law of surface settlement under different working conditions.(5)The maximum surface settlement is at the center of the tunnel.The maximum surface settlement when there are strata defects is larger than when there are no strata defects.The trend of the surface settlement duration curve is basically the same,and the settlement rate has experienced faster-steady-slower.The larger the distance between the strata defect and the tunnel,the longer it takes for the strata defect to connect with the tunnel,and the slower the surface settlement rate after the tunnel is excavated.(6)When the distance between the strata defect and the tunnel are less than the most unfavorable distance,as the distance increases,the height of the final collapsed arch becomes larger,and the stability of the composite stratum becomes worse.When the distance is greater than the most unfavorable distance,as the distance increases,the height of the final collapsed arch is basically unchanged;With the increase of the the ratio of rock thickness to span,which is the increase in the thickness of the rock layer,the height of the final collapsed arch decreases,and the stability of the composite stratum becomes stronger.The strength of the stability of the composite stratum reflects the magnitude of the mutual influence between the strata defect and the tunnel,and the essence is whether the two collapsed arches are superimposed.