Study On Tunnel Excavation Stability And Its Influencing Factors In Backfill Area Of Soil- Rock Mixture In Mountains City

Chongqing,as a typical mountainous city,is often constructed by opening hills and filling valleys.In the process of urbanization,a deep backfill area of soil-rock mixture is often formed.Influenced by backfilling history and quality,the backfill soil of soil-rock mixture is loose and in the stage of underconsolidation and continuous settlement,which belongs to bad engineering geological conditions.When urban rail transit tunnel passes through deep and long backfill area,it will face many engineering problems,such as unclear physical and mechanical properties of stratum,inexperienced tunnel design parameters,and difficult construction technology.It is of great practical significance to study the excavation stability of tunnel in backfill area with earth-rock mixture and its influencing factors.Based on the typical underground excavation tunnel project of Chongqing Rail Transit Line 5 and Line 10 crossing deep backfill area,the stress and deformation characteristics of surrounding rock during the excavation of tunnel in backfill area with soil-rock mixture are analyzed by means of theoretical analysis,field monitoring and numerical simulation,and the influence law of different influencing factors on the stability of tunnel excavation is studied.The main research work and achievements are as follows:(1)According to the geographic location,engineering geological conditions,hydrogeological conditions and the distribution of surrounding structures and pipelines of tunnels in backfill area,and based on the excavation and support methods adopted by tunnels passing through backfill area,the research object,simulation range,parameters of rock and soil body,support structure and construction sequence of numerical simulation are determined.(2)According to the actual situation of the project,the section of the backfill section of the tunnel is selected to observe the ground settlement,measure the horizontal convergence and the vault settlement,and monitor the internal force of the secondary lining.The monitoring data are fitted by the software of MATLAB,and the reserved deformation and the final deformation range,as well as the stress state of the secondary lining,are obtained,which are used to control the stability of the tunnel in the mixed area of soil and rock.Provide reference.(3)Based on the actual situation and monitoring results of the project,the multifactor geotechnical engineering modeling software GEO_SRM2D is applied to build the model,and the numerical simulation software Flac3 D is imported.According to the actual construction sequence and the layout of the supporting structure,the excavation of the backfill tunnel is simulated numerically,and the stress distribution and deformation characteristics of the surrounding rock in each excavation step are obtained.The results show that the numerical simulation results are about 5% larger than the monitoring values by comparing with the field monitoring values,and the change rules of the two values are basically the same,which verifies the rationality of the numerical calculation model.(4)Comparing the homogeneous soil model with the multi-factor stochastic soilrock mixture model,the calculated value of the soil-rock mixture model is about 5% larger than the monitored value,while the calculated value of the homogeneous soil model is about 18% larger than the monitored value.The results show that the soil-rock mixture model can simulate excavation deformation more accurately than the homogeneous soil model.(5)Using GEO_SRM2D and Flac3 D,the backfill tunnel models with different tunnel burial depth,rock content,excavation methods and support modes are established,and the effects of various factors on the stability of tunnel surrounding rock are analyzed.The stress and displacement of surrounding rock are analyzed.The results show that the stress and displacement will increase with the increase of tunnel burial depth;the stress will increase and the displacement will decrease with the increase of rock content;the stress of surrounding rock will increase and the deformation will decrease slightly after the setting of side wall mortar bolt;compared with the tunnel excavated by the method of retaining core soil in circular excavation,the tunnel excavated by the method of upper and lower steps will be excavated.The stress of surrounding rock is larger and the displacement is smaller.