Mini and microtunnelers

Engineers are increasingly faced with the problem of laying underground networks, whether in urban areas or across natural or man-made obstacles. The solution is not always simple, given the environmental constraints involved.

Most of the time, fluid networks are installed on their own site, i.e. each network is laid in isolation in the ground or overhead so as not to interfere with other networks.

Experiments were carried out to reduce maintenance costs, by having several networks coexist in a single structure. The first of these was carried out in Paris, where the sewer network built progressively since 1860 was designed not only to be accessible for maintenance, but also to accommodate the water pipes supplying subscribers. These sewers were later used by other public service concessionaires to set up telephone networks or to transport information by compressed air (pneumatic transport).

Today, with the exception of services that transport water in all its forms (drinking water, non-drinking water, chilled water), this cohabitation has become very difficult due to the constraints inherent in equipment maintenance personnel, leading concessionaires to seek a degree of independence by installing their facilities in an independent profile.

The underground networks needed to serve users have become progressively denser, taking up more and more underground space until the first few meters are saturated.

Among the various recent construction processes, the use of demini and microtunneling machines is a solution that enables the construction of a non-visitable underground structure, without trenching, in all types of soil, at depths ranging from 2 to 20 m, with good layout accuracy, bypassing existing structures. Only the relatively remote temporary access shafts reveal the existence of the construction site.

In addition, political decision-makers are attaching increasing importance to trenchless construction methods for underground networks, as major earthworks generate nuisances that are less and less tolerated by the public and the local community, who bear the social costs. These nuisances, which are difficult to quantify, have many impacts. They include

• pedestrian traffic problems, diverting surface circuits and increasing the risk of personal injury accidents;

• vehicle traffic jams and diversions;

• noise nuisance, which for mini and microtunnelers is localized to the reduced construction site footprint;

• air pollution problems caused by combustion engines (the mini-tunneler uses electric power).