The companies SADE and SOGEA were jointly contracted by SIRTTEMEU (the regional authority for wastewater management) for the nearly three km project, which began in June 2007 and comprised two major operations: 750 metres of underground rehabilitation of the pipe sections, where an open cut technique would have been too problematic; and microtunnelling underneath one of the SNCF’s (French National Railway’s) main train lines in the country’s southwest.

Underground rehabilitation

Prior to installing the pipe, a diversion channel had been installed within the old sewer system to ensure service maintenance to riverside residents, as well as enabling staff to work during the day and in a dry environment, according to SOGEA Construction Manager Didier Paillot.

The channel comprised of 560 and 630 mm HDPE pipe sections installed along one of the existing sewer walls, and a group of three pumps, one of which was a back up, assured constant wastewater conveyance.

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Glass-reinforced pipe was chosen for the two lengths of 500 metres and 250 metres. The 2 metre pipe sections were lowered, transported along the tunnel and then bolted into place within a framework. Rubber seals ensured the pipe was water tight.

Because of considerable rainfall, the work teams had to evacuate the tunnel, and the pumps were no longer sufficient to ensure waste transfer. With the installation and setting complete, the empty space between the new pipe sections and old sewer main was filled with mortar using a concrete pump. SIRTTEMEU said that the HDPE diversion channel remained in place in case a back up system was needed.

Sophisticated surveillance microtunnelling

Microtunnelling was chosen to create an underpass under the tracks of the train line that runs from Marseille 259 km west to Ventimiglia, just over the Italian border. The company SMCE Forage was contracted for the work and supplied the laser-guided microtunnelling machine with a mixed shield of 1,400 mm in diameter, as well as hydraulic spoil removal.

The crossing was a length of 80 metres at a depth of 3.5 metres, but the very close proximity of the pits to the railway made it a delicate operation. Surveillance of the tracks was ensured by a computer-piloted theodolite, which allowed continuous, real-time detection of any displacement from the determined target. The Cyclops system, provided and implemented by Soldata, was used.

During project operation, the trains were slowed to 100 km per hour. The SNCF fixed the alert threshold for track displacement at 10 mm, with a stopping threshold of 17 mm. Mr Paillot said that internally, there was an alert threshold of 5 mm, but the track was never displaced beyond 2 mm, and the surveillance continued for another month after the work had finished.

Safety, quality, environment

This delicate task was carried out within heavy urban density, and often on private property. Making sure that council and riverside residents were kept informed, post-project repair procedures were followed, as well as maintaining staff safety, and respect to the environment all meant it was no small feat.