Seine protected by microtunnelling project

The use of the largest diameter GRP pipes in France is helping to reduce pollution in the River Seine.
Alexandra Dobnigg of Hobas Engineering explains how the scheme triumphed

Until 2010, GRP pipes had been applied in French microtunnelling projects up to diameters of DN 1600. However, a rainwater collector and storage project to limit the amount of overflow caused by heavy rain has involved the use of DN 1800 CC-GRP Jacking Pipes from Hobas.

Given that it was a national first regarding the use of 1,800mm pipelines, Hobas believes it represents a precedent for future trenchless undertakings with GRP. The application is part of the Seine River de-pollution plan and was launched by the local authority Syndicat d’Assainissement de la Boucle de Seine.

A 9,500m³ rainwater storage basin has been erected a few kilometres outside Paris, at the Place des Fêtes in Bezons. Having established a solid national, as well as international, reputation as a tunnel-boring specialist, the Toulouse-based company CSM Bessac (a member of the Vinci Group) was entrusted with the installation of the 380m-long transfer collector that feeds into the storage basin at a nominal flow of 5m³/s.

The geological and hydrological conditions, as well as the pipeline route that includes a horizontal and a vertical curve at a 900m radius, made the project complex. Implementation required particularly meticulous preliminary studies, which were conducted by the engineering office Cabinet Merlin, which was also in charge of documentation.

Further studies and calculations regarding the construction works were carried out by CSM Bessac and Hobas France. Apart from keeping disturbances to the environment down, the motives to opt for a trenchless solution were primarily the great installation depth, groundwater and several existing structures above and around the pipeline route.

The pipeline was installed at 12 to 15m depth and runs between a layer of limestone, a layer of alluvial deposits and at about 10m below groundwater level. A DN 4000 sewer main is one of several obstacles in proximity to the route that needed to remain unharmed.

The jacking station was built inside the 25m-diameter storage basin. Its thrust wall, that serves to provide a reaction against which to jack, incorporated a special slab leading back to the basin wall, so as to transmit the 800 tonne loads from the station to the basin’s structure.

Groundwater posed a further challenge, lying 10m above the pipeline, it was estimated to generate a load of almost 30 tonnes on the boring head. Especially when the first pipes were jacked into position, the pipe string could have been pushed back out while the jacking machine was loaded with a new pipe. To remedy this, CSM Bessac designed a non-return system with two hydraulic jaws that firmly gripped and held the pipes in position.

Curved route
The 380m S-curved route also posed a challenge to the contractor and at the same time gave Hobas Jacking Pipes the opportunity to prove what they are made of. The 3m pipe sections DN 1800 (De 1940), SN 64000, PN 2.5 were smoothly installed and their leak-tight flush couplings provide a tight line. Hobas pipes accept high jacking forces equaling up to 800 tonnes and can be jacked around curves down to 680m radii, without having to adapt the force. This is possible due to the material’s elasticity that allows the forces to be evenly transferred from pipe to pipe over the entire wall-thickness of the pipe ends.

CC-GRP Jacking Pipes have a long record of application in France and have been quality certified for trenchless projects by the Centre Scientifiqueet Technique du Bâtiment. Following the market’s increased need for larger GRP pipe diameters, this certification was extended to the full range of Jacking Pipes, from DN 250 to 3000, in October 2010.

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