Study On The Seepage-induced Failure Mechanism Of Shield Tunnel Face In Water-rich Weak Stratum

Under groundwater is a key disaster-inducing factor in urban underground engineering,and it is also an important factor affecting the stability of shield tunnel face in water-rich soft stratum.The fluid-solid interaction between the soil and groundwater is the main physical mechanism throughout the above process.In view of the insufficient consideration of meso-scale fluid-solid coupling effect in current theoretical analysis,numerical simulation,and model tests,a CFD-DEM model of shield tunnel considering the coupling effect between the soil particles and the fluid field was established.By comparing the characteristics of the failure process of the shield tunnel face under different conditions,this paper reveals the impact of seepage force on the face stability of such tunnels.A laboratory model test considering the combined seepage-erosion action is further carried out.The failure modes and the micro-mechanism of shield tunnel face failure in gap-graded stratum with different particular composition are discussed.The main works and results are as follows:(1)Calibration of the meso-parameters of DEM.Based on the MBD-DEM coupling method,a virtual direct shear test model is established.The influence of particular micro-parameters on the macroscopical mechanical properties is futher investigated.Then,the samples are calibrated according to the actual sand parameters.Finnally,the rationality of the results is verified by the accumulation test of the selected granular materials.(2)Study on the failure behaviour of shield tunnel face in dry and saturated stratum.A discrete element model of shield tunnel face in dry and saturated ground is established firstly.Then,six analysis cases with different overburden thickness are established and calculated.Based on the simulation results,the evolution of the effect support pressure on the tunnel face,the ground displacement around the shield tunnel face,the contact force network of soil particles and the evolution of ground porosity during the failure process are monitored and analyzed.On top of that,the impact of the groundwater on the tunnel face stability is summarized.(3)Study on the failure mechanism of shield tunnel face under seepage conditions.Based on the CFD-DEM coupling method,a numerical model of shield tunnel face under seepage condition is established.Then,nine analysis cases are set to consider the influence of different burial depth and hydraulic boundary conditions.The evolving patterns of the effective support pressure,the ground displacement,the failure modes of tunnel face,and the development of stratum porosity and are discussed in detail.On top of that,the impact of seepage force on the tunnel face stability is discussed and summarized.(4)Failure mechanism of shield tunnel face under the combined action of seepage and erosion in gap-graded stratum.In view of the designed model tests,the model box,the model shield tunnel,and the real-time monitoring device for fine particle flow rate are firstly designed and installed.The ground deformation induced by tunnel face failure is further obtained with the help of the Photolnfro.On this basis,the failure process of the tunnel face in diversified ground conditions with different proportions of the fine particles,varied overburden depth and water level are studied.Through this experiment,the evolution law of failure zone in front of the tunnel face under seepage condition is thus clarified.The effect of fine particle content on the stability of excavation face in gap-graded stratum is revealed from the mesoscopic level.