Research On The Foundation Stability Analysis And Treatment Measures Of New Subway Tunnel Through Coal Mine Collapse Area

How to ensure the stability of the subway tunnel foundation is a major technical problem in the subway construction must be solved firstly.In many cases,this problem can be solved by suitable location selection and applicable traditional technical methods during the stage of investigation and design of subway construction.However,there are some exceptions.Because there are some limitations,Urumqi Metro Line 1 has to pass across two collapse areas of Liudaowan coal mine(commonly known as South trough and North trough).Building subway tunnel above collapse area is unprecedented in the world and there is no successful experience for reference.Therefore,how to cross the collapse areas safely to ensure the safety of the subway tunnel during long period has been a major problem since the design of the subway tunnel placed in front of the builders.This article chooses subway crossing collapse areas as study object,the formation process of the collapse areas,residual deformation and the stability of surrounding rock around the areas are deeply investigated by using comprehensive methods.Based on the above investigation,the comprehensive plan("Pile foundation + Underground beam + Anchor pile + Cable stayed anchor")is designed and proposed with good effects.At last the new subway tunnel has been successfully passed through the coal mine collapse area.The main conclusions of this paper are as follows:(1)Based on the catastrophe theory and the generalized H-B failure criteria,the nonlinear instability model of top coal of remaining goaf in collapse area is established,to predict the stability of the remaining goaf in collapse area by using the model,the prediction results are stable,indicating late the remaining goafs in collapse area do not occur instability and collapse failure phenomenon.(2)The collapse area residual movement and deformation prediction model and dynamic prediction model are established,to verify the South trough and North trough by using the model,the maximum expected value and the maximum measured value of residual movement and deformation are close,and the relative error in the range of ±7%;the maximum residual settlement value of dynamic prediction is 303mm,and the maximum measured residual settlement value is 299mm,indicating the residual settlement is too large,do not meet the vertical design allowable settlement,and the collapse area cannot be directly used as the subway tunnel foundation,so we must carry on the reinforcement treatment.(3)The results show that under the subway load,the hanging wall rock of the collapse area is mainly destroyed,and the footwall rock of the collapse area is almost not affected,so it is necessary to reinforce the hanging wall rock.The shear failure zone based on structural mechanics on the top of the hanging wall in the collapse area of the South trough under subway load is 11.3m,that of North trough is 14.1m;the shear failure zone based on UDEC discrete element numerical simulation on the top of the hanging wall in the collapse area of the South trough under subway load is 12m,that of North trough is 15m;the shear failure zone based on similar simulation test on the top of the hanging wall in the collapse area of the South trough under subway load is 13.2m,that of North trough is 15.6m.The above results provide the basis for the reinforcement design of the side wall rock in the foundation treatment scheme.(4)Based on the above research results and considering the construction process,construction period,the feasibility of reinforcement scheme,the reinforcement effect,the reinforcement cost and other factors,this paper puts forward a new comprehensive design schemes of subway tunnel crossing the collapse area "Pile foundation + Underground beam + Anchor pile + Cable stayed anchor".After the application of the project,the settlement of the reinforced foundation meets the requirements of the code,the foundation bearing capacity and compressive strength can meet the design requirements,so the reinforced foundation can be used as the new subway tunnel foundation.In view of the main problems and technical difficulties faced by the new subway tunnel passing through the coal mine collapse area,in this paper,several innovative research results are gained as follows.(1)A comprehensive crossing scheme of "Pile foundation + Underground beam + Anchor pile + Cable stayed anchor" is proposed and designed for a newly built subway tunnel through the collapse area.(2)Based on the catastrophe theory and the generalized H-B failure criteria,the nonlinear instability model of top coal of steeply inclined extra-thick coal goaf is established.(3)The collapse area residual movement and deformation prediction model and residual settlement dynamic prediction model of steeply inclined extra-thick coal are established.At present,the main structure of the subway tunnel and the backfill of the upper part have been completed,the monitoring data show that all the control indicators meet the requirements of the national standard,indicating that the technical achievement obtained in this paper have been applied successfully,it can also play an important role in the construction of similar projects at home and abroad.