Source: {"pile_set_name": "USPTO Backgrounds"}

The present invention relates to tunnelling apparatus, i.e., with apparatus for driving a tunnel in the ground.
Tunnelling apparatus of the general type here in question is already known from German Pat. No. 2,431,512. This type of equipment has a generally annular tunnelling element at the front end of which there is provided a working chamber the forward end of which is open and which is formed by providing a transversely extending wall which subdivides the tunnelling element into the forward or leading working chamber and a rearward or trailing chamber. Mounted in the working chamber is a material-removing device which extends through the front open end of the tunnelling element and removes material from the soil face just ahead of the tunnelling element. A pressurizable flowable medium can be introduced into the working chamber in order to support the soil face against collapse. In the type of equipment disclosed in the aforementioned German patent, it has been found that from time to time it may become necessary to gain access to the working chamber, either by the admission of a worker into the chamber or by the introduction of tools into the chamber, because obstacles may be encountered in the soil by the material-removing device which the device itself cannot remove or reduce to smaller manageable portions, respectively. This may, for example, be large rocks, buried tree trunks, tree roots or the like. In order to gain access to the working chamber, it is necessary to withdraw the supporting medium which may be thixotropic fluid. After this is done, an operator may gain access to the chamber through an appropriate port. This presents no particular problems if the tunnels are of large diameters, for example, the type of tunnel required for the contruction of subways. In the case of small tunnels having a diameter of only about 0.8-1.5 meters, such as is for example required for large waste-water mains, the relatively large space requirements of the material removal device within the working chamber make it impossible for a worker to enter the chamber, for lack of space. Nevertheless, the problems which occasionally make it necessary to gain access to the working chamber are just as acute in the case of small-diameter tunnels as in the case of large-diameter tunnels.