Supplying drinking water to the town of Thonon
A daunting challenge, given the Allinges and Lyaud mountains, to be met with an MTBM!
A portion of the water supply pipe, dating from 1936, which links the catchment point in the natural springs of the Blaves galleries (Lyaud) to the Chavannes reservoir (Allinges) was collapsing and in need of replacement.
This is a particularly significant operation for the Thonon conglomeration and its users, as the network in question supplies 90% of the town of Thonon-les-Bains and ensures safe water for more than 30,000 inhabitants.
Given the local topography (a hill over the pipeline) and the need to intervene at great depths (more than 20m for some sections of the pipeline), the replacement of the old pipeline was carried out by boring with an average incline of 0.75% in a complex geographical context.
The works required the creation of two shafts: an entry shaft with piling reinforcement at 4m deep, and an exit shaft with secant piling reinforcement at 9m deep. Boring by microtunnelling enabled the laying of a 1,200mm-diameter concrete pipeline along 24m. Two intermediate thrusting points distributed the effort applied to drive the microtunnel boring machine (MTBM) At the same time, a by-pass solution was implemented pending the commissioning of the new network.
As the exit shaft was located in the catchment area, a by-pass pipe and a temporary standby pumping system (2 x 350m³ per hour) were maintained in place for the total duration of works.
The project also included a large water gate chamber, designed for the gravitational distribution of water flows, and various junctions.
Microtunnelling works were entrusted to SADE Special Works, and to the Grenoble Agency (Centre-East RD) for the laying of the by-pass, shaft excavation, building and equipping of the gravitational chamber, and the laying and connection of a 500mm-diameter cast-iron pipeline inside a 1,200mm-diameter sleeve.
This was the first time the microtunnelling technique has been used in the Thonon conglomeration. These trenchless operations helped to reduce to a strict minimum the impact of works to vehicle traffic.