Support of the gallery by means of forepoling
The method consists of inserting steel tubes inside sub-horizontal holes made ahead of tunnel’s face. Structures in the form of pre-shaped arc are obtained in this way as support for the excavation. This system finds its ideal application in heterogeneous loose soils containing boulders and large blocks of rock (debris of avalanche). The installation of these tubes is done by means of special rigs who are very stable and are equipped with long mast. The machine is placed in the center of the arch and only the mast is moved in any position of perforation, without moving the machine itself. The drilling can be done directly with the steel pipe or dragging the same within an outer protective pipe or using a down-the-hole hammer placed inside of the tube itself. Lengths are possible drilling of up to 30 meters, but the optimum value lies between 14 and 18 meters, in this case one piece tubes without junctions can be utilized.
The distance between the tubes depends on static factors and the geology and is generally between 30 and 60 cm. The tube diameter is usually between 100 and 180 mm. The tubes are usually fitted with valves and are cemented by the introduction of mechanical single or double packer. Possible deviations of drilling are strongly dependent on soil type.