Study On Limit Failure Mode And Support Pressure Of Tunnel Face Of Underwater Shallow Shield Tunnel

During the construction of shield tunnels in underwater ground,it is necessary to control the water and soil pressure of the excavation chamber reasonably to keep the stability of the tunnel face.In this paper,model test,numerical simulation and limit equilibrium analysis are used to study the stability of the tunnel face of shallow shield tunnel in underwater sandy ground,clay sand ground and composite ground,considering the influence of seepage.The main contents and research results are as follows:(1)A model test system for excavation seepage stability of shallow shield tunnel in underwater ground is developed.The tunnel face model of the system consists of a front plate and a back plate.The back plate moving backward horizontally can simulate the situation of excessive excavation and insufficient support pressure of the tunnel face.Through the water pump in the water circulation system,the water discharged from the tunnel face is directly sent to the surface of the ground to realize the generation of stable seepage field in the ground.A joint measurement system is set up to measure the internal settlement of the ground,which is helpful to reveal the three-dimensional instability model of ground.(2)By means of model test,numerical simulation and theoretical calculation,the influence of seepage conditions and soil parameters on ground collapse mode and limit effective earth pressure of the tunnel face in sandy ground are studied.The model test results show that,when the relative hydraulic head difference between the tunnel face and the ground surface(the ratio of the hydraulic head difference between the tunnel face and the ground surface to the diameter of the tunnel)is small(less than or equal to 0.33),the limit collapse range of the ground will increase slightly with the increase of the hydraulic head difference.When the relative hydraulic head difference is large(greater than 0.33 and less than 1.00),the limit collapse range of the ground will increase with the increase of the hydraulic head difference.When the relative hydraulic head difference is very large(greater than or equal to 1.00),the limit collapse range of the ground has basically reached the limit and will not change with the increase of the hydraulic head difference.The limit effective earth pressure of the tunnel face increase linearly with the increase of the hydraulic head difference between the tunnel face and the ground surface.The numerical simulation results show that the limit collapse range of the ground and the limit effective earth pressure of the tunnel face will decrease with the increase of the internal friction angle of the soil.According to the results of model test,a pyramid–prismoid–prism failure model is established,which can consider the influence of seepage and the failure range in the upper formation behind the tunnel face.Through the force balance analysis of the failure model,the calculation formula of the limit effective support pressure of the tunnel face considering the influence of seepage is derived.(3)By means of model test,numerical simulation and theoretical calculation,the influence of seepage and ground conditions on ground collapse mode,limit effective earth pressure of the tunnel face and pore water pressure of soil in front of the tunnel face in clay sand ground are studied.The results show that the stratum limit collapse range increases with the increase of clay content without the influence of seepage,while the stratum limit collapse range decreases with the increase of clay content with the influence of seepage.Under the same ground conditions(the same clay content),the limit effective earth pressure of the tunnel face with seepage is significantly higher than that without seepage.Under the same seepage conditions(the same hydraulic head difference between the tunnel face and ground surface),the limit effective earth pressure of the tunnel face decreases nonlinearly with the increase of internal friction angle,and linearly decreases with the increase of cohesion.Under the influence of seepage,the increase of clay content and backwardness of the tunnel face will increase the pore water pressure near the tunnel face.(4)By means of model test,numerical simulation and theoretical calculation,the influence of seepage conditions,ground stratification and soil parameters on ground collapse mode,limit effective earth pressure of the tunnel face and pore water pressure of soil in front of the tunnel face in stratified ground are studied.The pyramid–prismoid–prism failure model established in this paper,which can consider the influence of seepage and the failure range in the upper formation behind the tunnel face,is applied to actual engineering.The results show that the limit equilibrium theory in this paper can reasonably predict the limit support pressure of the tunnel face in the actual project,and provide a reference for the control of the pressure of the excavation chamber in practical engineering.