Mechanism And Influence Of Water-Soil Gushing Around Shield Tunnels Using Material Point Method

With the rapid development of China’s economy,the urban infrastructure construction is booming.As the main constituent of the urban infrastructure,the structural safety of the shield tunnel has become the hot topic of the whole society.Due to the structural characteristics of shield tunnels and the complex underground environment,soil and water gushing accidents often occur in tunneling engineering.There may be management and technical reasons for the accident.However,the key problem is that engineers have little understanding of the soil and water gushing disasters around tunnel.On the one hand,most studies about gushing accidents only focus on monitoring data and apparent phenomena;on the other hand,when faced with large deformation problems such as soil and water gushing,traditional numerical methods will encounter technical limitations such as grid distortion,cracking slippage and so on.These problems have hindered scholars’ research and understanding of the large deformation process,evolution mechanism and influence law of soil and water gushing in tunnel.Therefore,developing a simulation approach for large soil deformation caused by soil and water gushing in shield tunnels,exploring the evolution of soil hydro-mechanical behavior in the process of soil and water gushing,and grasping the impact of soil and water gushing on the surrounding environment and the tunnel structures,would have significance value for ensuring the safety of shield construction and maintaining the safety of tunnel operation.Relying on the National Natural Science Foundation of China(No.41772295)and Innovation Program of Shanghai Municipal Education Commission(No.2019-01-07-00-07-456 E00051),this paper adopts theoretical research,numerical simulation,field investigation to reveal the evolution of large soil deformation and the response law of tunnel structure caused by soil and water.The main research contents and results are as follows:(1)The basic theory of the Material Point Method(MPM)and the simulation method of porous media are studied.Two classical theories for analyzing the water-soil coupling are compared,as well as three types of governing equations.The governing equations and its discrete form which are suitable for the two-phase MPM are deduced,and the calculation process is summarized,which provides the theoretical basis for the numerical simulations of this paper.Then,based on the velocity correction of the contact nodes,a contact algorithm for soil-structure interaction is proposed.And other key algorithms such as moving mesh are introduced into the two-phase material MPM to improve the computational efficiency,accuracy and stability of the proposed method.(2)The analysis method of soil and water gushing in shield tunnel is established.Firstly,based on a series of model test with complete data,the accuracy and reliability of the analytical method for simulating soil and water gushing are verified.Subsequently,the influence of the geometry of the opening is studied based on a smallscale numerical model,and the results showed that it mainly affected the soil loss rate,but hardly changed the evolution of the soil stress field and displacement field.Then,a large-scale numerical model of soil and water gushing in the tunnel is established,and the development law of soil mechanical behavior during the process of gushing is studied.Finally,the influence of different gushing locations,tunnel depths and soil types on the large soil deformation caused by gushing is analyzed.(3)Accurate water and soil pressure acting on tunnel lining in each stage is obtained by using the analysis method for soil and water gushing.Firstly,the evolution of the distribution of water and soil pressure acting on tunnel lining caused by the gushing is analyzed.Based on the classical beam-spring model of the shield tunnel,the development law of the internal force and deformation of the tunnel structure is further studied.Then,the influence of gushing locations,tunnel depths and soil parameters on the response of tunnel structure is analyzed.Finally,based on the results of parametric analysis,some suggestions and guidance for future projects are put forward.(4)Aiming at the phenomenon of soil erosion in the process of gushing around the tunnel,a fully coupled seepage-erosion-mechanics model is established,based on the Nor Sand critical state soil constitutive model,to simulate the coupling process of seepage-erosion-deformation when fine particles in surrounding soil are eroded.Then,the coupling model is developed in the framework of the two-phase MPM,and the applicability and accuracy of the coupling model are verified by using the results of laboratory tests.Finally,a set of parameter calibration procedure is proposed for the coupled model parameters.(5)On the basis of the single-point two-phase material point method,a coupled seepage-erosion-mechanics model was introduced,and the analysis method considering soil erosion was established.Based on the test results of graded soil samples,the effect of soil erosion in the process of gushing and the applicability of the coupled analysis method are discussed.Finally,the analysis method is applied to a gushing accident in a metro tunnel.Combined with the monitoring data,the key issues including the development of soil loss and ground settlement,the deformation of the tunnel structure,and the timing of remedial measures are studied.