MetroDescription and operation of the shields
The Earth Pressure Balance machines are equipped with cutter heads 6,080 mm in diameter; the total length of the tunnelling complex exceeds 100 metres; the weight amounts nearly to 900 tonnes. The cutting head can be divided into three blocks with the aim of facilitating the dismantling operation after the completion of the excavation. It is fitted with 17 twin-disc cutters and four single-disc gauge cutters, which can be extended to increase the excavated diameter to 6,100 mm. In the case of driving through soils, it is possible to completely replace disc cutters with rippers, which perform better in the soft environment (at the moment the cutterhead is fitted with rippers).
The EPB shield tunnelling principle is based on disintegrating ground at the tunnel face by means of cutting tools mounted on a rotating cutting head. The disintegrated ground (muck) passes through openings in the cutting head to the extraction chamber, where it is mixed with the previously disintegrated muck. The pressure force induced by thrust cylinders is transferred to the disintegrated muck by a pressure bulkhead. This is how the muck is prevented from uncontrolled intrusion from the excavation face to the pressure chamber.
The muck is transported from the extraction chamber by means of a screw conveyor to a belt conveyor found behind in the tunnel, where there is already the atmospheric pressure.
The cutting head, extraction chamber and cutting head drive unit are connected to form a single compact complex, supplemented by the hyperbaric part. This compact complex (the shield) is provided with pairs of hydraulic thrust cylinders installed around its perimeter, which extend, thrusting themselves against the last completed ring of the segmental lining, thus shifting the entire machine complex and the back-up ahead.
The cutting head is driven by six hydraulic motors with the power input of 1,200 kW. Sixteen pairs of hydraulic cylinders generate the maximum pressure on the excavation face of 39,000 kN.
The machine back-up is connected to this propelling part of the machine by a steel structure having the form of a bridge. The bridge-design is used with the aim of keeping the entire bottom section of the tunnel unobstructed for the transport and installation of lining segments. The lining is installed using a vacuum-assisted erector, transporting individual segments to proper positions.
Behind the bridge there are the control cabin and other necessary elements, such as a belt conveyor running along the whole back-up complex. The back-up is formed by individual service centres necessary for the operation of the whole system. They are installed on seven platform cars, namely platforms carrying hydraulic pumps, transformer stations, cable reels and pipes, a section for extending all lines in the tunnel, etc.
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