Study On Balance Mechanism Of Chamber Soil Pressure Of Shield Machine By Discrete Element Method

The earth pressure balance (EPB) shield tunneling machine is the important technical equipment for infrastructure projects, resource development and national defence. The pressure balance control between the chamber and the ground at the excavation face is the key for tunneling safely and efficiently. Aiming at the problems that the satisfying mechanical model for the chamber pressure control is absent, in this work, the relation between the pressure on the excavation face and the pressure on the chamber board as well as the relation between the pressure on the chamber board and the working parameters are established, by the discrete element method (DEM) simulation, theoretical analysis and experiment validation, which not only provides the mechanical models for the chamber pressure control in tunneling, but also enriches the DEM model parameters determination method and extends the DEM engineering application.The first problem for DEM to simulate the work process of the chamber system of the EPB shield machine is the modeling of the conditioned sands. Taking the sands in a subway section as the original sands, and according to the proportion of the additives volume to the earth volume, the conditioned sands is reproduced and the slump test and the direct shear test are implemented in the laboratory. The equivalent discrete element model of the conditioned sands is provided, on the basis of which the DEM numerical model of the slump test and the shear test of the conditioned sands are established. By comparing the numerical results of the slump test and the direct shear test of the conditioned sands with different proportions to the corresponding laboratory test results, it is illustrated that the proposed DEM equivalent model can simulate the flow characteristics and strength characteristics of the conditioned sands on the condition that the proper DEM parameters is selected.The key for simulating the work process of the chamber system correctly is the determination of the equivalent DEM model parameters of the conditioned sands. The inversion method combining the genetic neural network and the discrete element simulation of triaxial tests is proposed for determining the contact model parameters of the conditioned soil. The aim is to make the error of the simulation curves and the laboratory curves of the triaxial tests minimum. The parameters solution is based on the genetic neural network. The training samples of network are provided by the DEM simulation. The complicated nonlinear relation between the input and the output samples is mapped by the genetic arithmetic. The simulation curves calculated with the inversed model parameters match the laboratory curves well, which illustrates that the presented inversion method of the DEM parameters of the conditioned soil is feasible. It provides the basis of the model parameters determination of the conditioned sands for the chamber pressure analysis model.Taking the shield machine tunneling in a subway section as the research object, the DEM geometrical model for analyzing the chamber pressure of the shield machine is established. According to the work principle of the chamber of shield machine during the tunneling, the simulation method of the excavation face pressure under different buried depths, and the modeling method of the dynamic transfer process of the conditioned sands in the chamber system which generate particles on the excavation face and delete particles on the outlet of screw conveyor are proposed. On the basis of this, the boundary condition of the DEM model of the chamber system is determined. The numerical simulation results of the observed pressure on the chamber board are compared with the field construction data, by which the feasibility of the simulation method is validated. The model provides the basis for further pressure analysis and control model building of the muck chamber system.On the basis of the DEM model of the chamber system, the pressure distributions of the chamber under the tunnel depth of 10m, 15m and 20m for the board cutting wheel and the spoke cutting wheel are studied. The mathematical model of the pressure distribution in the chamber is proposed. The relationship between the pressure on the excavation face and the pressure on the chamber board is established, the pressure transfer coefficient (the proportion of chamber board pressure to the excavation face pressure) in which is defined as the function of the opening ratio of the cutting wheel. The results provide the basis for the setting of the chamber pressure during tunneling.On the basis of the DEM model of the chamber system, by changing the numbers of the generated particles at the excavation face and the rotate speed of the screw, the different working conditions during EPB machine tunneling are simulated. The corresponding data of the pressure changing rate of the chamber under different excavation volumes per unit time (thrust speed) and the discharge volumes (the rotate speed of the screw) are obtained. The influence of the thrust speed and the screw rotate speed on the changing rate of the chamber pressure is studied respectively. The function of discharge rate and the changing rate of the chamber pressure is established, which provides the reference model of chamber pressure control for EPB machine tunneling.