Study On Deformation Characteristics And Surface Settlement Prediction Of Large Slurry Balance Shield During Construction

The influence of shield construction process on ground deformation and the control and prediction of settlement are heavily concerned issues.Based on the analysis of on-site data from Xinzheng Airport to Zhengzhou South Railway Station Inter-city Railway slurry balance shield tunnel,the influence of shield construction process on surface settlement and its prediction were discussed.The main contents and results are as follows:(1)Through the analysis of the ground deformation monitoring data,the deformation characteristics induced by the shield tunneling were studied.The surface settlement during the construction was well controlled.The maximum surface settlement of more than 90% points were less than 10 mm.The reasons for surface uplift at some individual points were that mud cake stuck on the cutter and the cutter's seriously worn,which induced parameters such as the thrust of the shield machine large,causing excessive extrusion on the soil in front of the machine.The maximum settlement value was-17.7mm,caused by the interruption of the sand layer while the longterm shutdown tools change process.(2)Based on the analysis of shield construction parameters and settlement monitoring data,this paper studied the principle and effect of the auxiliary air pressure excavation method applied in the mud-water balance shield construction,and summarized the advantages of the auxiliary air pressure method and the precautions in the application.The auxiliary air pressure excavation method is to connect the excavation chamber with the air chamber,so that the upper part of the excavation chamber the air chamber filled with compressed air while their lower parts are compressed mud.In this status the pressure of the excavation face could be balanced by adjusting the air pressure.In this paper,the parameters such as tunneling speed,shield thrust and cutter torque during the application of auxiliary air pressure excavation method were analyzed.It is found that this method can effectively improve the excavation rate,reduce the mud cake of the cutter,and control the ground settlement better.It is confirmed that the auxiliary pressure tunneling method can be applied to the slurry balance shield construction in well-stabilized soil.Moreover,the shield parameters should be well controlled,otherwise surface uplift,collapse or instability of the excavation surface may occur.(3)In this paper,the FLAC3 D was used to simulate the construction of shield undercrossing the South-to-North Water Diversion Project,and the influence on the canal structure was studied.The numerical simulation results were consistent with the measured data.The structure of the canal was safe,and the numerical simulation reflected the actual situation well.The shield construction had significant impact on the vertical settlement,which occurred at the bottom of the channel above the tunnel and the value was-6.5mm,while the impact on the horizontal was relatively small.The maximum principal stress of the main canal structure was 1.9 MPa and the minimum was-7.2 MPa which were in safe for C50 concrete.(4)In this paper,the genetic algorithm was used to optimize the weight and threshold of BP neural network,and the optimized neural network model was applied to the dynamic prediction of surface settlement of shield tunnel,and a good prediction effect was obtained.Seven geological parameters such as compressive modulus,Poisson's ratio and density,and six construction parameters such as depth,tunneling speed and thrust were considered.The initial weights and thresholds of BP neural network were screened by genetic algorithm,and the appropriate crossover rate and mutation rate were selected through experiments.The dynamic prediction results show that the BP neural network optimized by genetic algorithm could predict the surface settlement of shield tunnel axis well.