Several firms in Japan, including Kawasaki Heavy Industries, have been developing a number of new types of TBMs using novel cutterhead configurations. These machines are designed differently for a variety of reasons, as reported by Yasanori Kondo at a BTS meeting last year (T&T September 1999), including the possibility of excavating unusual cross-sections, such as rectangular, elliptical or horseshoe sectional shapes for tunnels, without any of the normal problems associated with these unusual shapes.
One of these machine types is the DPLEX, which stands for the developing parallel link excavation shield method, developed over recent years by Kawasaki Heavy Industries.
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Among the advantages claimed for these machines is a reduction in the cutting radius taken by the cutters and a consequent reduction in the necessary cutter torque developed by the drive motors. The oscillating action of the cutterhead leads to a reduced travel distance for many of the cutting tools and a much more even distribution of wear among the cutting tools. Kawasaki estimates that the reduction in wear for the cutting tools is approximately one third of that on a standard rotating cutting head.
The cutterhead is a framework which is mounted on four support shafts. Each shaft is in turn eccentrically mounted on a drive motor and the support shaft is 400mm from the centre of rotation of the motor. As the drive motors turn, the cutterhead oscillates in a circular movement. Each cutting tool therefore moves the same distance and on a path of 800mm diameter.
Three benefits of this arrangement are immediately apparent:
Larger machine development
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By GlobalDataFour DPLEX shields have previously been manufactured by Kawasaki prior to this contract but these had been of smaller size, being less than 4.7 m in diameter, and had been used in more favourable ground conditions, consisting of relatively easy layers of silt, and sand mixed with gravel up to a maximum size of 250mm diameter.
Following this sensibly cautious approach to the development of a new design, a much larger machine is being put thorough its paces. The new machine is 7.67m in diameter, which is considerably larger than before. It is working in much more challenging ground conditions, comprising gravel layers containing boulders up to 500mm in diameter, tough conditions for any shield excavation. It uses earth pressure balance principles to provide face support. In fact the oscillating movement of the head is ideal for the mixing of material in the head with conditioning agents before removal by screw conveyor. In practice it was found that this, combined with the strategic location of the injection ports, meant a reduced quantity of conditioning agent was used.
The construction of the openings in the cutter frame, and the positions for the screw conveyor, the mixing blade and the conditioner injection ports are carefully designed to enable 500mm boulders to be accepted and removed.
Calculations for motor torque and the power requirements had to be carefully carried out, especially with the handling of the large boulders. The development work carried out on the four earlier smaller machines was crucial in these calculations.
This very much larger machine also threw up some interesting manufacturing problems. The tolerances and interface between the four rotating drive shafts is very important, and it was necessary to develop a new bearing structure which would allow it to be dismantled for transportation without loss of tolerance.
Project conditions
The machine was supplied to a group of three Japanese contractors, Shimizu, Kumagai Gumi and Maeda, which is building a new rainwater storage tunnel in the Hyogo Prefecture near Kobe in Japan. They chose this type of shield to enable them to excavate the long drive length without having to stop for cutter changing in the difficult gravel and boulder filled ground.
The 7.67m diameter tunnel is 1,150m long and contains curves of 110m radius. To tackle these curves, two special over-cutters were installed and the body was articulated to allow +3º of movement.
The calculated natural ground pressure during the drive was 0.7 bar (0.07N/mm²). A minimum figure for face pressure was set at 1 bar. In practice, the earth pressure at the cutting face was held stable at an average of 1.2-1.4 bar. These pressures were measured by a pressure cell located at the centre of the cutterhead.
Machine performance
Advance rates of 25-30mm/min had been planned because of the difficult gravel and boulder conditions, but in practice it was found that rates of 35-50 mm/min were achieved. Actual thrust used was varied between only 17,000 and 18,000kN. This represents approximately 368–390 kN per m² of face area. The motor torque utilised was also lower than installed by approximately 1,600-1,800 kN-m.
Industry contractors have been watching closely the performance of the DPLEX shield machine.
Kawasaki has now accumulated important data from actual tunnelling experience with the DPLEX shields, at both small and large diameter operation. Further development continue to broaden the range of applications for this new type of shield.
Related Files
Diagram showing rotation paths of the head cutter picks
Schematic layout of the head
