Microtunnelling is a trenchless technology which allows to lay underground pipelines without the need to create open trenches, minimizing the landscape and environmental impact. This makes the tech-nology particularly suitable for carrying out interventions in urban areas, for the execution of river, rail, and road crossings, as well as for laying pipelines in sites of high naturalistic and archaeological interest.
The excavation is carried out by means of a closed and full-section milling shield (or microtunneller), remotely controlled, which moves forward into the soil, followed by pipes to be laid. The excavation proceeds according to a predefined design track, from a starting shaft, where all the work equipment is installed, to an arrival shaft, where the shield is recovered. In particular applications, the shield can be recovered inside a shallow excavation or, in case of sea outfalls, underwater from the seabed.
The thrust force required to advance the shield is provided by the main thrust station, a frame equipped with hydraulic jacks installed inside the starting shaft, and it is transferred to the microtun-neller by the pipes laid behind it.
The excavation method is based on the use of a fluid that, depending on the characteristics of the soil, can consist of water or a mixture of water and bentonite, which have a dual function: to support the excavation face during the advancement, ensuring the balance of the pressures acting on it, and to act as a means of transport for the excavated material, allowing it to be removed and kept away from the excavation face in the form of slurry (a mixture of disintegrated and fluid soil). The circula-tion of the fluid takes place by means of a closed hydraulic circuit, integrated in the milling shield.
The microtunneller is driven from the outside, by means of a control console located inside a con-tainer placed on the surface, near the starting shaft. The console allows to check and adjust the pa-rameters concerning the excavation activity. The microtunneller guidance system is based on a con-tinuous detection of the milling shield position through a laser beam, which hits an incorporated pho-tosensitive target. The information is transmitted to the computer of the control console, which de-termines the exact position, and any necessary corrections to be made. These corrections are made by means of three/four hydraulic jacks, which can be activated individually. These jacks act on the head of the milling shield, modifying its orientation. In this way, it is possible to obtain tolerances of ± 3 cm vertically and ± 10 cm horizontally.
The full section excavation, with mechanical and hydraulic support of the excavation face, the con-tinuous control with laser pointing system in order to check the directionality, and the simultaneous control of the drilling and progress parameters by the operator, guarantee a safe and precise execu-tion.
The drilling system also allows the laying of the pipeline even under the water table: the closed drilling head and the use of a special header ring in the starting shift guarantee the hydraulic seal under leaves up to 30 m.