Study On Slurry-induced Fracturing Pressure And Extension Path During Shield Tunneling In Clay Layers

Shield technology has played a great role in the infrastructure construction of subway,highway,railway,water transmission and power pipeline in China.Slurry shield is an important branch of modern shield method,and it has been widely used in the world.The slurry shield method is almost used in large-scale soft soil tunnels across the river and sea in China.Due to the limitation of geological conditions,engineering cost and other factors in the construction design of cross river sea shield tunnel,the thickness of covering soil is usually small.It is of great significance to prevent hydraulic fracturing under the condition of small overburden to ensure the safety of underwater shield tunnel construction.Thus,theoretical analysis,model test and numerical simulation are combined to study the slurry-induced fracturing pressure and fracture extension path of clay layer.The main work of this paper is as follows:(1)Based on orthogonal tests,the material ratio of artificial clay with different strength is developed.Through the soil test and the improved fracture toughness test,the physical and mechanical parameters of artificial clay are measured,which provides the parameter value basis for the study of slurry-induced fracturing.Furthermore,based on the self-developed triaxial fracuring equipment,the clay samples were fractured to reveal the influence of loading conditions,sample size,sample strength and slurry viscosity on the fracturing pressure.The results show that the fracturing pressure is less affected by axial pressure,increases linearly with the increase of confining pressure and shear strength,while increases with the increase of thickness diameter ratio and slurry viscosity,but the increase rate becomes slower.By summarizing the variation of fracturing pressure,the calculation formula of fracturing pressure is put forward,which provides a theoretical basis for the setting of slurry pressure.(2)The large-scale fracturing test equipment was developed to observe the fracturing process and fracture morphology of samples,and the fracturing tests were carried out on large-scale clay samples.The slurry pressure and extension path in the process of fracture extension were analyzed,and the influence of slurry viscosity and stress state on the fracture extension pressure and extension path was revealed.The results show that when the viscosity of slurry is small,the fracturing pressure is basically unchanged during the fracture extension,while it increases gradually with higher slurry viscosity in the process of fracture extension.The variations of the extension pressure and the initial fracturing pressure are basically the same.When samples were fractured,the fracture extends upward at a certain angle from the bottom of the blind hole,and the fracture angle is greatly affected by the stress state.(3)Based on the basic principle of fracture mechanics,the second development of ABAQUS extended finite element is carried out,and the embedding of tension-shear combination failure criterion is realized.Through the simulation analysis of clay slurry-induced fracturing model test and field test by establishing the numerical slurry-induced fracturing model,the accuracy of slurry-induced fracturing model was verified,and the shape of slurry-induced fracture and damage type of clay layer were studied.The results show that the slurry-induced fracturing of clay layer is mainly shear failure,and the angle between the fracture and the maximum principal stress is about45°+?/2.(4)Based on the tension-shear combined failure criterion of,the slurry-induced fracture extension model during shield tunneling was established.The slurry-induced fracturing process of shield tunneling in clay layer was simulated,and the variation of slurry pressure in fracture during fracture extension is clarified.The influence of water pressure,thickness of covering soil,shear strength and slurry viscosity on fracture extension pressure and extension path was also revealed.Furthermore,combined with the extended finite element and cohesive element,a three-dimensional slurry-induced fracturing numerical model was established to simulate the three-dimensional dynamic extension process of the fracture,and the three-dimensional extension path and shape of the fracture are explored.The influences of tunnel diameter,thickness of covering soil,slurry viscosity and slurry injection speed on the three-dimensional micro morphology of the fracture were also revealed.(5)The slurry-induced fracture extension model was used to simulate the occurrence and extension process of the stratum fracturing when the slurry shield tunneling through the underwater trenches.The influences of trench depth,water pressure and trench position on the occurrence and extension process of the stratum fracturing were analyzed.The variations of fracturing pressure,extension pressure and the amount of slurry injection are revealed.The relevant measures to prevent the occurrence and extension of slurry-induced fracture in the trench area are proposed.