Study On The Mechanism Of Ground Settelement Induced By Water And Sand Gushing Of Subway Tunnel In Rich-water Strata

At present,the construction of underground transportation in China is in a period of vigorous development.As of December 31,2021,269 subway lines with a total mileage of 8,708 km have been opened in 51 cities.However,in the process of subway construction,water and sand gushing are often encountered,leading to surface subsidence and even large-scale collapse,resulting in road and building damage,heavy economic losses,and casualties,which will have a serious impact on people’s production and life.Therefore,it is of great theoretical value and engineering significance to study the mechanism of ground settelement induced by water and sand gushing in wate-rich stratum.In this thesis,case statistics,model test,numerical simulation and theoretical research are used to study the occurrence mechanism and development process of land subsidence induced by water and sand gushing of tunnel in water-rich stratum.The cases of ground collapse at home and abroad are statisticsed,and analyzes the disaster characteristics,disaster-causing structures and hazard model of water and sand gushing in tunnel.Using large-scale physical model and CFD-DEM coupling simulation method,the disaster evolution process of land subsidence induced by water and sand gushing are discussed,the evolution law of displacement field and seepage field are revealsed,the influence of different control factors on land subsidence are analyzed,and the correlation between water and sand gushing and land subsidence is established.Based on the unified strength theory,the critical failure conditions of clay induced by water and sand gushing is deduced,and the prediction method of land subsidence is put forward.The research has achieved the following results:(1)Based on the statistical analysis of the cases of ground collapse in subway induced by water and sand gushing at home and abroad,the ground collapse is mainly caused by water and sand gushing,water and sand leakage,accounting for 78%.Moreover,50% of the accidents are located in the sand layer.The disaster-causing structures of land subsidence caused by water and sand gushing can be divided into three categories: water-bearing sand layer(rainfall induced type,precipitation failure type,water blocking structure failure type),unfavorable geology(direct exposure type,water blocking structure failure type)and artificial pipeline(water leakage accumulation type,pipeline burst type).The hazard models of soil instability can be divided into three types: overall sliding instability,overall breaking instability and seepage instability.(2)The large-scale model test system of tunnel under the action of seepage is independently developed,including toughened tank box,excavation system,intelligent pressurized water supply control system,optical fiber data acquisition system,etc.Taking Huangdao District,Qingdao as the engineering background,the whole process of land subsidence induced by water and sand gushing is reproduced.The ground deformation has a good correlation with water and sand gushing,and the surface deformation generally lags behind the water and sand gushing.The surface deformation experiences three stages: slow deformation,rapid deformation and ground collapse.The seepage pressure and soil pressure first increase and then decrease.(3)The CFD-DEM method of ground settlement induced by water and sand gushing in tunnel is proposed,and the formation loss rate MLR is used to evaluate the soil loss.The range of surface deformation increases with the increase of MLR.The process of surface deformation can be divided into soil settlement stage caused by seepage,loose expansion stage and ground grooving stage.In the ground grooving stage,there is an obvious inflection point in the displacement curve,and the soil has significant instability characteristics.The sand gushing first increases sharply,then decreases gradually,and finally tends to be stable.The increase of water gushing first increases and then tends to be stable.In addition,the greater the water pressure,the faster the soil loss,and the less time it takes for the surface to form a groove.The smaller the fault dip,the smaller the drag force of fluid on particles,the smaller the soil loss rate and the smaller the displacement growth rate.(4)The mechanism of land subsidence induced by water and sand leakage in water-rich strata is revealed,and the influence laws of leakage point width,water pressure and clayey thickness on land subsidence are obtained.With the occurrence of water and sand leakage,the displacement of each measuring point responds.The displacement growth rate of the measuring point close to the center of the tunnel face increases continuously until the soil collapses.The thicker the overlying clay is,the upward development speed of the cavity formed by soil loss gradually decreases,the ground grooving speed decreases,and the displacement decreases.With the increase of water pressure and leakage point width,the sand leakage rate is obviously accelerated,and the surface grooving speed is increased.(5)The mechanical model of collapse failure of overlying clay in the process of water and sand gushing is established.Then the criterion of overlying clayey collapse induced by water and sand gushing is deduced based on the unified strength theory.The effects of clay thickness,soil strength and cavity diameter on clay layer collapse are studied by laboratory test,and the accuracy of theoretical analysis is also verified.Finally,the of the maximum surface settlement in the process of water and sand gushing is proposed based on the random medium theory and ellipse theory.