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Completion of shaft for VL8 collector excavation

In Orly, teams from Soletanche Bachy France and Sefi-Intrafor are approaching completion of the final shaft for the VL8 collector. This shaft will allow the extraction of two tunnel-boring machines, which are currently beginning their excavation.

After various site preparation works carried out at the end of last year, the worksite teams were able to complete pre-treatment before the Christmas holidays. This pre-treatment work, including 30 boreholes 46m deep, required the grouting of 50m³ of bentonite-cement slurry into the ground to avoid possible sludge losses during construction of the diaphragm wall.

A 6m-high and 20m-long noise barrier was constructed for the worksite using acoustic panels, reducing noise for local residents by more than 15 decibels. A clever ballast system developed by the design office avoided the need for deep foundations to ensure its stability.

Once this stage was complete, work could begin on installation of the diaphragm wall, with assembly of a D550 grit chamber and a HC05 compact Hydrofraise.

Despite difficult access conditions, excavation work was able to begin on 26 January 2023.

The shaft, with an 8m internal diameter and 80cm thick, is made up of three primary panels and three enclosing panels with interlocking joints. The diaphragm wall is 45m deep and embedded in marl and gravel. The shaft is made up of steel cages containing permanent fibreglass reinforcements in two places to allow two tunnel-boring machines to pass through, one from Valenton and the other from Vigneux-sur-Seine.

Since the cramped nature of the worksite prevents continuous excavation, the HC05 Hydrofraise was shut down during the equipment and concreting phase. However rapid excavation enabled the worksite to be completed in four weeks, working in two shifts, for a total of 1,400m² excavated.

Once this phase was complete, demobilisation could begin, much to the delight of local residents whose view had been obstructed by 100m³ silos. Work on the ground injection – 5m high and covering an area of 50m² – was then able to begin for a period of four weeks, lowering the permeability of the ground housing the diaphragm wall’s foundations.

Finally, a pumping device will be put in place to allow earthworks for the civil engineering works package to be carried out in a dry environment. Excavation of the shaft and construction of the slab are due to be completed by summer 2023.

Good synergy between the teams made it possible to meet the deadlines and achieve the expected level of quality for this work. The Orly shaft therefore completes work on the VL8 collector shafts, which began in October 2020.

BESSAC, on the other hand, is just getting to grips with its project. The teams have completed installation work to prepare the worksite for the arrival of the tunnel-boring machine, including connection to the high-voltage network, construction of the crane embankment and the gantry stringers, installation of the surface networks (overhead and beneath technical trenches) and assembly of the crane. The naming ceremony for tunnel-boring machine “Kristell” was held on 5 June. It has since begun digging from the Valenton shaft to create a collector with an internal diameter of 3m and 3025 linear metres long.
The BESSAC tunnel-boring machine is due to complete the Orly shaft next spring.

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