Research On Ballast Motion Behavior In Shield Slurry System And Pipeline Service Life Prediction

With the rapid development of China’s economy and technology in recent years,shield machines were widely used in various underground tunneling projects.During the excavation,the shield will generate a large amount of ballast,so how to transport the ballast from underground to the surface safely and efficiently is one of the key aspects in the design of the shield slurry system.At present,there are few studies on the direction of quantifying the motion behavior of the large-size,non-spherical ballast in the slurry system.In this thesis,based on the solid-liquid two-phase flow theory,the multiphase flow numerical simulation technology and the pipeline hydraulic transport test were used as the research tools to investigate the ballast motion characteristics and the pipeline service life in the slurry system respectively.The research contents are as follows:（1）In order to realize the solution of large-size,non-spherical ballast motion in the pipe,the coupling method of the fluid dynamics and discrete element（CFD-DEM）was applied to describe the interaction process between ballast and slurry.The ballast physical properties and slurry rheological parameters in the slurry system were measured on-site,and the ballast bonded sphere model and slurry Bingham rheological model were constructed,respectively.To meet the solution accuracy and efficiency requirements,the simulation model grid size was determined as 25 mm by grid-independence analysis,and a full pipeline transport model was constructed to provide a numerical simulation basis for the subsequent study of the ballast motion behavior in the pipeline.（2）The experiment of the 10-50 mm ballast transportation in water condition was carried out by means of a slurry circulation test bench.A high-speed camera system was used to capture the motion behavior of the ballast in the pipe,and the machine vision technology was further used to quantitatively analyze the ballast motion information.Through the hydraulic test,the ballast in the horizontal pipe was mainly picked up at the bottom of the pipe by lifting,and the pick-up velocity of the individual ballast was 0.49-0.96m/s,and the pick-up velocity of ballast group was0.93-1.45m/s.When the slurry flow rate reached 2.00-4.00m/s,the ballast motion velocity was 1.12-3.58m/s,which lagged behind the slurry by 10.5-44.0%.The correctness of the numerical model was verified by the experiment and laid the foundation for subsequent study of the ballast motion behavior in the pipe of the slurry system.（3）The study of the ballast pick-up velocity in the pipe was carried out,and the criteria for determining the ballast pick-up status under different working conditions are proposed,and the image method and mass loss method are combined to obtain the ballast pick-up velocity in the pipe.The ballast pick-up process in the horizontal and vertical pipes was analyzed by numerical simulation in terms of ballast force size,distribution state,motion direction and mass pick-up rate to reveal the ballast pick-up mechanism in pipe.Further,sensitivity analysis of the factors influencing the ballast pick-up rate was conducted to obtain the main influencing factors.Taking the theoretical model of single flat ballast pick-up velocity as a prototype model,the model for calculating the ballast pick-up velocity in the horizontal pipe was obtained through the covariance fitting method.The model was used to predict the ballast pick-up velocity under the actual working conditions,and the minimum slurry flow rate of the slurry system was recommended to be higher than 3m/s in order to prevent excessive deposition of the 60 mm ballast.（4）The ballast conveying performance in the slurry system was studied,and it was found that the ballast motion in the slurry system had a hysteresis phenomenon under the stable conveying conditions.Then the evaluation indexes related to the ballast conveying performance were proposed from the energy consumption and initiative.The relationship between the ballast conveying performance and the ballast sliding velocity,ballast distribution concentration and ballast conveying efficiency was analyzed by numerical model.The law of each influencing factor on the ballast sliding velocity,ballast distribution concentration and ballast conveying efficiency was studied,and then a quantitative assessment model of the ballast conveying performance was obtained.The assessment model was used to evaluate the ballast conveying performance in the existing projects which revealed that the optimal slurry flow rate of the slurry system was generally in the range of 4～5m/s,when the system has the highest efficiency in the ballast conveying.（5）With the help of three-dimensional laser scanning technology,the wear mechanism of the pipe wall was analyzed,and the wear mechanism of the inner pipe wall is consistent with abrasive wear.The radial and axial wear characteristics of the horizontal pipe at the construction site were obtained by using the ultrasonic measurement.The pipe wear model was reconstructed in terms of wall forces and wear coefficients,respectively.The wear coefficient of the pipe wall was solved for different wear time and load,and the wear coefficient of Q235 steel under quartzite formation was K₁=9.7×10^-13.The pipe service life prediction model was obtained based on the wear model,a method was proposed to predict the pipe service life in the project.Taking the pipe service life and ballast conveying efficiency as optimization objectives,the multi-objective optimization algorithm（NSGA-II）was used to optimize the ballast conveying parameters,both the ballast conveying efficiency and the pipe service life was expected to be improved.108 Figures,30 Tables,142 References.