PULLING BACK
The pulling back technique is a technical solution consists in executing a blind hole microtunnel, that is without an arrival shaft. The drilling shield, pushed from the starting shift through the definitive tubes, advances along the project trajectory until reaching the end, where it is recovered by applying a “pulling back” procedure. When the perforation is finished, the drilling shield is freed from the exca-vation head and the extension kit, and it is then slid back into the reinforce concrete piping, by means of a winch installed in the starting shaft, where it is recovered.
This is possible thanks to a particular drilling cutter, equipped with a double shield: the first shield (in-ternal) has a diameter slightly smaller than the internal diameter of the reinforced concrete pipes, while the second shield (external) has a diameter approximately equal to their external diameter. In this way, the first shield can slide inside the tunnel and then it can be extracted from the starting shaft; the second shield and the cutting head, instead, remain in place.
