Mechanism Model And Experimental Investigation Of Chamber Pressure Control For Earth Pressure Balance Shield

Earth pressure balance (EPB) shield tunnelling has successfully been adopted for urban tunnelling in recent years in very different ground conditions and at present it can be considered the most commonly used mechanized tunnelling technology, even challenging the role of the Slurry Shield both as far as machine size and the geomechanical fields of applicability are concerned. The screw conveyor's speed and discharge rate is controlled by the operator or computer software and is used to control the pressure at the working face and match the muck discharge rate to the advance rate of the EPBM, and effectively control ground deformation induced from shield tunneling. The control of face support is a major issue in EPB shield tunneling. Continuous support of the tunneling face must be provided by the excavated soil itself, which should completely fill the working chamber. The required support pressure at the tunneling face will be achieved through shoving the shield forward and regulation of the screw conveyor's rotation rate. The support pressure has to balance the earth pressure and the water pressure. The supporting pressure on tunneling face must be carefully determined and avoid both the collapse (active failure) and the blow-out (passive failure) of the soil mass near the tunnel face. The collapse and the blow-out are induced from smaller and bigger supporting pressure, respectively. Nowadays, the reference pressure on pressure bulkhead is commonly determined by operator experiences. The operator continually adjusts screw conveyor's speed to control earth pressure on tunneling face. The main problems occurred in shield tunneling include:1) How to optimally choose earth pressure on chamber bulkhead; 2) How to determine shield thrust force; 3) How to simulate earth pressure distribution in shield chamber; 4) Propose mechanism model of controlling earth pressure on chamber bulkhead; 5) How to estimate model parameters of control model; and 6) Present optimization and automatic control strategy for controlling earth pressure on chamber bulkhead. In order to deal with these problems, some investigations are performed:1) Based on Duncan-Chang nonlinear elastic constitutive model, the earth pressures on head chamber of shield machine are simulated. Model parameters of conditioned soils in head chamber of shield machine are determined based on tri-axial compression tests in laboratory. The loads acting on tunneling face are estimated according to static earth pressure principle. The soil-structure interaction in shield tunneling is investigated by analytical solution. The mechanism of working face stability is analyzed and the reasonable face earth pressure for EPB shield is deduced according to the active and passive earth pressure principles. The optimal thrust fore for EPB shield is proposed in different soil parameter and shield size cases. The comparison of the practical thrust forces of EPB shields with computed ones shows that the proposed computing procedure for optimal thrust fore for EPB shield can agree well with practical engineering examples. The effectiveness of the proposed computing procedure is validated. The variation of earth pressure on different section in head chamber of shield machine is depicted. Relationship between pressure transportation factor and openings rotating cutterhead of shield machine is proposed by using aggression analysis.2) The inverse problem of parameter identification is solved by minimizing an objective function of the least-squares type that contains the difference between observed and calculated strains. The tri-dimensional compression tests of soil are performed to supply experimental data for identifying nonlinear constitutive model of soil. The calculated strains are determined by linear approach simplification. The real-coded hybrid genetic algorithm is developed by combining normal genetic algorithm with gradient-based optimization algorithm. The numerical and experimental results for conditioned soil are compared. The forecast strains based on identified nonlinear constitutive model of soil agree well with observed ones. The effectiveness and accuracy of proposed parameter estimation approach are validated.3) Based on numerical simulation using finite element method, the reference model is proposed for controlling earth pressure of head chamber. The relationship between the strain and the accumulating amount of earth is proposed based on Duncan and Chang's model. The relationship between the increment of earth pressure and the angular velocity of screw conveyor is presented. Based on nonlinear constitutive model of soil and mass conservation law, the differential equation of nonlinear discrete system is deduced by solely adjusting conveyor rotating speed. The earth pressure change in the chamber comes mainly from two parts, which conclude the change of mass quantity of conditioned soil in chamber and change of earth pressure in working face. The relationship between the change of mass quantity of conditioned soil in chamber and shield tunneling rate and conveyor rotating speed is proposed. The relationship between the change of earth pressure in chamber and shield thrust force is presented. Taking account of coupled reaction between shield machine and soils, a new control mechanism model for controlling earth pressure acting on headchamber is presented. The new control mechanism model can rapidly perform pressure balance for shield chamber and automatically control earth pressure according to preset reference pressure.4) Some parameters of control model should be determined by using system identification because these parameters can not be easily estimated in laboratory or in situ in shield tunneling. The main parameters of control model include deformation modulus of conditioned soils, conveyor discharge efficiency and loosing coefficient of soils. An estimation approach using least squares method is presented for identification of model parameters of pressure control in shield tunneling according to observed system input and output. The randomly observed noise is numerically simulated and mixed to simulated observation values of system responses. The numerical simulation shows that the state equation of pressure control system for shield tunneling is reasonable and proposed estimation approach is effectiveness even if the random observation noise exits. The identification procedure for nonlinear control system of shield machine tunneling is proposed by using neural network. Bp neural network is applied for an identifier to estimate model parameters of time varying control system. The numerical simulation shows that the identification procedure is effective for nonlinear and time varying control system in shield machine tunneling.5) By combining system identification with optimal control as an integral body, the control strategy and algorithm in shield tunneling are proposed, which are based on neural network identifier and controller. The identifier and controller can effectively perform these functions to nonlinear, random and time-varying dynamic system. The optimal procure is developed for controlling earth pressure acting on headchamber. The effectiveness and preciseness of the new model proposed are verified by numerical simulation results. The numerical simulation shows that the proposed control model is effective and stable for controlling earth pressure of shield machine by adjusting controlled variable.6) In order to validate the effectiveness of control mechanism model, parameter identification procedure and optimal control strategy, the model tests in laboratory are performed. The observed test data include earth pressure on chamber bulkhead, conveyor rotating speed, shield tunneling rate and thrust force. Based on observing data in test bed, control model parameters are first estimated. And then the earth pressure on chamber bulkhead is automatically controlled during next tunneling process based on preset reference pressure.