Discrete element model for performance analysis of cutterhead excavation system of EPB machine
Introduction
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, 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.
Section snippets
Performance indicators
The cutterhead system of EPB machine is mainly composed of the cutterhead, the soil chamber and the screw conveyor, the function of which is cutting ground, supporting excavation face and discharging soils (Fig. 1). The performance is related to the structure parameters, the operation parameters and the soil parameters. The design of cutterhead system is concerned of the cutterhead structure style, the opening location and size, the tool selection and layout, the arrangement of muddlers in the
DEM model for performance analysis
The DEM model of cutterhead excavation system tried to reproduce the tunneling ground (Fig. 2) from the north station of Jiaomen and the south station in the line 4 of Beijing metro. The ground is composed of rounded pebbles cemented by sand and gravel. The diameters of most pebbles are between 20 mm and 60 mm. The maximum diameter is 180 mm. The content of particles whose diameter is larger than 20 mm is more than 55%. The EPB machine (Fig. 3) constructed by Mitsubishi from Japan was used in the
Performance analysis results and discussion
The DEM model of cutterhead system excavating is shown in Fig. 4. With the advancement of system and the rotation of cutterhead, the soil particles flew into the chamber constantly through the opening in the cutterhead. When the calculation time reached 1200 s, the screw conveyor began to rotate. The chamber and the screw were filled with particles when the calculation time was 1320 s. Then the rotation speed screw conveyor was adjusted to 4 rpm, and the tunneling speed was turned to 30 mm/min. The
Conclusions
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
Acknowledgements
The authors are grateful to the support of the National Natural Science Foundation of China (Study on Dynamic Interaction between Cutterhead of Shield Tunneling Machine and Soils and Design Theory of Cutterhead System, Award No. 51105048).
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