With the aim of probing for design theory of cutterhead excavation system of earth pressure balance (EPB) machine, the parameters representing the system performance are concluded firstly. Then a 3D model for cutterhead excavation system of EPB machine is presented with the discrete element method (DEM) software PFC3D, which is capable of simulating the tunneling ground in site, the machine structure and the excavation operation. The performance parameters indicating stability of excavation face, soil discharging rate, cutterhead system torque and cutter wear are measured by running the DEM code. The results obtained with the DEM model are accord with situ data. It indicates that the DEM model is a promising method replacing the field experiment to analyze the influences of the structural parameters on system performances, which are essential for structure optimization design of the cutterhead system of EPB machine.
The full face tunnel boring machine (TBM) includes the soft ground TBM and the rock TBM. The soft ground TBM includes the EPB machine and the slurry machine according to different supporting ways of excavation face. Performance prediction of the rock TBM has already been studied. Rostami and Ozdemir, 1993, Burger, 2006, Karlheinz, 2009, Peter, 2009 and Abdolreza and Siamak, 2012 and Bilgin et al. (2012) are among the researchers who realized the pioneering works with respect to rock TBM performance. Contrary to rock TBMs, the soft ground TBM performance forecasting is a new science. It is rather difficult in capturing the complex interaction between the pressurized shield drive and the ground. Since the early 20th century, the soft ground TBM cutterhead design has historically been driven by iterative designs based on empirical data, observations over various projects and industry rules of thumb (Glenn and Mustafa, 2011). Nishitake (1987) in Mitsubishi Heavy Industries Ltd., described several types of EPB shields which can cope with large size boulders, and the existing machine is modified to deal with the problem. Dowden and Cass (1991) in Robbins Corp. reviewed the current designs of shielded type machines and their application, and discussed new developments which blended hard and soft ground tunneling technology. Burger (2007) in German Herrenknecht Corp. described the technical points of the cutterhead design of the slurry shield and the EPB shield machine.
Nowadays a lot of scholars try to find out the relations between the tunneling parameters, the soil parameters and the structure parameters of cutterhead system. Wang and Fu (2006) formulated mathematical models with respect to the total thrust, the soil pressure in chamber, the rotation speed of screw conveyor and the advance speed by theoretical analysis, and the formula is validated by in situ measured data. Zhang et al. (2005) proposed mathematical models of the advance speed and the torque of EPB cutterhead respectively. Yang et al. (2006) analyzed the thrust force, the torque of cutterhead and soil pressure in chamber in two different cutterhead opening rates of 30% and 70% by model tests. Shi et al. (2011) introduced an improved torque calculation formula taking account of cutterhead structures and cutting principle of tools. Lambrughi et al. (2012) developed a three-dimensional FEM numerical model of EPB machines simulating the overall process of excavation and construction of a tunnel. The influence on calculated ground displacements is investigated by means of a series of operation parameters analyzes. Manuel and Luis (2005) established a DEM model to study the problem of soil stability at the tunnel face, the thrust and the torque needed to excavate the tunnel. The research works described above give the factors affecting the cutterhead torque, the thrust and ground stability. However, the performance index of the cutterhead excavation system is not definitely proposed from the view of system design.
Recently, Ulrich and Marc (2011) pointed out the key performance indicators for estimating the performance of EPB tunneling based on the evaluation of data from a great number of projects. Glenn and Mustafa (2011) examined how discrete element analysis can be applied to cutterhead performance optimization. This paper establishes a DEM model of the cutterhead excavation system, taking an EPB machine tunneling in Beijing metro construction as prototype. The performance parameters representing the stability of excavation face, the soil discharge rate, the system torque and the cutterhead wear are shown by running the DEM model. Parameters of EPB tunneling are measured in situ to validate the model. The DEM model is an effective tool for performance prediction and design of cutterhead system.
Taking an EPB machine tunneling in the 4 line of Beijing metro as prototype, the DEM model of EPB machine excavation was established in PFC3D software. The model reproduced the excavation ground, the structure and the operation process of the cutterhead system. The performance indicators of the cutterhead system, which were the excavation face stability, the system torque, the cutterhead wear and the soil discharging rate were defined in the DEM model of the chamber system.
In DEM model, the calculated average pressure on the chamber board was 115 kPa, which was accord with the actual construction data. The measured cutterhead torque was about 3500 kN m, and the measured screw torque was around 100 kN m. The error between the calculated torques and the observed data in site was no more than 16%. The forces of all tools located on the cutterhead in X, Y and Z direction were measured. The tool forces in the y direction represent the thrust force for cutterhead tunneling. The composition of forces in the x and z direction reproduces the cutting force of tools. The calculated volume of incoming soils in unit time was 0.76 m3/min, and the observation value in site was 0.74 m3/min. The error was 2.7%. The above data illustrate that the DEM model of the cutterhead system is able to substitute the actual tunneling machine to measure system performances.
Changing the structure parameters and the operation parameters and performing a series of analysis, the relation between the structure parameters and the system performance will be found. So the DEM model is a useful tool to forecast the system performance and optimize the system structure.
