Research On Mud-water Fracture Phenomenon Of Shield Tunneling In Clay Stratum

For the mud-water shield,the stability of the excavation surface has always been the key to its safety control.Once the excavation surface is not reasonably and effectively controlled,the risk of mud-water splitting on the excavation surface will be greatly increased.In order to study the splitting damage of mud-water shield in clay stratum due to excessive mud-water pressure,this paper uses the self-prepared artificial clay to carry out the material triaxial splitting model experiment,and deeply explores the confining pressure,unconfined compressive strength and mud viscosity.The effect of crack initiation pressure on the law of crack initiation is verified by comparing the experimental results to verify the accuracy of the numerical model.Finally,combined with the north extension tunnel project,the crack development path and the extension pressure required to maintain the crack to continue to expand after the initiation of the stratum are deeply analyzed by the stratum factors,and the risk control measures during the construction process of the mud-water shield tunneling through the reservoir are proposed..The specific research results are as follows:（1）The horizontal orthogonal experiment was carried out to analyze the influence of different clay material components on the fluidity,setting time and consolidation strength of clay slurry,and the physical and mechanical parameters（shear strength,resistance to resistance）of artificial clay materials were measured.tensile strength,shear toughness,tensile toughness).（2）The splitting experiment was carried out based on the self-developed triaxial splitting instrument,and the influence of confining pressure,mud viscosity and unconfined compressive strength on fracturing was investigated.The splitting pressure increases linearly with the confining pressure and the unconfined compressive strength,and the crack initiation pressure increases with the increase of the mud viscosity,but this effect is only significant when the mud viscosity is small,and when the mud viscosity reaches a certain value When the degree of crack initiation（about 25s）is reached,the influence of the mud viscosity on the crack initiation pressure will no longer be significant.;The semi-empirical experimental formula of crack initiation pressure used to describe mud-water splitting in clay stratum is obtained by fitting the test results:P_f=σ_c+(-13.43e^{-v/5.499₊1.173}q_u（3）A XFEM model of mud-water splitting expansion was established to simulate the mud-water splitting test of the blind hole cylindrical specimen,and the influence of various factors on the crack initiation pressure in the model test was verified,and the pressure changes during the mud-water splitting process were analyzed.Destruction patterns of curves and cracks.The water pressure continued to increase before the cracks occurred in the sample,and the mud-water pressure suddenly decreased when the cracks occurred.By comparing with the experimental results,the accuracy of the simulation results and the applicability of the model are verified.（4）Taking the Beiyan Tunnel as the engineering background,the mud-water splitting model of shield tunneling was established to simulate the mud-water splitting and extension process of the shield when it penetrated the reservoir.It is found that the mud-water pressure is the largest when the formation is fractured.With the upward extension of the fracture,the mud-water pressure gradually decreases.After the fracture is penetrated,the mud-water pressure will be equal to the water pressure of the tunnel vault.In addition,the formation splitting pressure under different water depths and thicknesses of overburden was calculated.（5）During the construction of the mud-water shield,in the areas where the thickness of the covering soil changes greatly,especially at the surrounding dam of the reservoir,the mud-water support pressure should be adjusted in real time according to the changes of the thickness of the covering soil and the water pressure,and the viscosity of the mud-water should be increased and the water pressure should be increased.The method of driving speed prevents the splitting damage of the formation.