1. Field of the Invention
The present invention relates to a shield tunneling method and apparatus, and more particularly to a method and apparatus for thrusting a shield, which is adapted for use in jacking pipes into the ground.
2. Description of the Prior Art
Generally, according to the pipe jacking method, as shown in U.S. Pat. No. 4,311,411, a shield is provided at the foremost part of a pipe to be thrusted and the ground is bored by the operation of an excavator attached to the shield, then by the subsequent operation of a hydraulic thrust jack disposed behind the pipe a thrust is exerted on the shield and the pipes, so that the shield and the pipes are thrusted into the bored portion of the ground. The above excavator is disposed rotatably in the front portion of the shield and is driven by a drive unit disposed behind a partition wall extending across the interior of the shield. During operation of the excavator, the cut surface of the ground or the tunnel face is maintained in a stable condition by being pressurized with pressurized water, sludge, etc.
Such preboring of the ground by the excavator diminishes the thrust resistance of the succeeding pipes, but since the pipes undergo an earth pressure acting on their circumference, the thrust resistance increases with adding of pipes required as the pipe thrusting proceeds and hence with increase of the overall length of pipes to be thrusted. Therefore, the above thrust jack must be large-sized enough to produce a large thrust. The foregoing earth pressure not only is an obstacle to the thrusting of a pipe but also continues to act on the circumference of the pipes after embedded in the ground and impedes a stable maintenance of the pipes.
On the other hand, the excavator for excavating the ground which covers the front of the shield requires a large-sized drive unit capable of producing a large driving torque for driving its rotary cutter head. This drive unit must be disposed within the shield, but in the case of a shield having a small outside diameter, e.g., 300 mm or so, there is no room for mounting therein a large-sized drive unit.