Study On The Disturbance Induced By EPB Shield Tunnelling In Mixed Ground With Soft Sand Lying On Hard Rock

Earth pressure balanced(EPB)shield tunnelling has been widely used in metro tunnel construction in China.The engineering geological conditions of the ground that EPB shield passes through is an important factor affecting the safety of the shield tunnel.China is a vast country with various complex geological conditions and EPB shield tunnel construction faces many technical problems that need to be overcame.The mixed ground with soft soil lying on hard rock is a typical adverse geological condition widely encountered in metro tunnel construction.As the soft soil is weakly stabilized and the hard rock is difficult to be excavated,large ground loss leading to excessive ground settlement or even collapse may be induced by the disturbance of EPB shield tunneling.This thesis is based on the metro tunnel construction in mixed ground with soft soil lying on hard rock,and supported by the scientific research project “Key Technological Development of River-crossing EPB Shield tunnel of Fuzhou Metro” and the Innovation Foundation of Southwest Jiaotong University “Study on the microscopic mechanism of soil deformation induced by EPB shield tunneling”.Theoretical analysis,laboratory model test and numerical simulation are employed in parallel to study the soil deformation mechanism of mixed ground from both macroscopic and microscopic perspectives.The main results obtained are as follows:1.Laboratory test using ?800mm model EPB shield tunnelling system is carried out.After launched,the model shield passes through hard rock,mixed ground and homogeneous sand sequentially.The characteristics of ground deformation induced by EPB shield tunneling are studied by analyzing the surface settlement,the development process of ground loss from around the shield to the surface is obtained by studying the subsurface settlement.The influences of underlying hard rock on the movement of overlying sand are investigated by comparing with the model test results in homogeneous sand.2.The conventional discrete element method(DEM)triaxial test is improved by simplifying the confinement provided by rubber membrane in laboratory test as assigned radial support pressure acting at boundary soil particle.In this way,the specimen can deform freely in lateral directions.Based on the DEM triaxial test with flexible boundary,extensive parameter studies are carried out to study the influences of microscopic mechanical properties on the macroscopic mechanical responses.3.Three dimensional DEM(3D DEM)model consisting of shield cutterhead,cutting tools and soil bin and et al is established to model the EPB shield tunneling in mixed ground,and the over excavation of overlying soft soil in mixed ground is quantitatively explained for the first time.The influence of tunnel buried depth,soil properties,and ground loss on surface and subsurface settlement is extensively discussed,and the distribution of horizontal movement in mixed ground is also studied.4.Three dimensional DEM simulations are carried out to study the face stability of EPB shield tunnels in mixed ground with soft sand lying on hard rock,considering the dynamic excavation process.The influence of buried depth,soil properties and construction parameters on tunnel face stability when the EPB shield is advancing or stopped is clarified,and the face collapse process is studied from a microscopic perspective.The existence of soil arch above the tunnel crown is investigated by analyzing the horizontal stress redistribution.Moreover,the influence of ground reinforcement on tunnel face stability is also discussed,and the selfstabilized condition of the tunnel face when EPB shield is moving forward or stopped is proposed.