DN 1800 Microtunneling to Protect the River Seine

In France until the start of 2010, GRP pipes have been applied in microtunneling projects for diameters up to DN 1600.

The rainwater collector and storage project that would limit the amount of overflow caused by heavy rain falls is part of the Seine River de-pollution plan and was launched by the local authority Syndicat d’Assainissement de la Boucle de Seine involving DN 1800 CC-GRP HOBAS Jacking Pipes. This was, therefore, a national premiere regarding the utilized pipeline’s size and undoubtedly represents a precedent for future trenchless undertakings with GRP.

Only a few kilometers outside of Paris, at the Place des Fêtes in Bezons, a rainwater storage basin had been erected to hold 9500 m³. Having established a solid national as well as international reputation as a constructor of tunnel boring machines and as a tunnel-boring specialist with outstanding records, the Toulouse-based company CSM Bessac (subsidiary of Soletanche Bachy and therefore member of the Vinci Group) was entrusted with the installation of the 380-m-long transfer collector that feeds into the storage basin at a nominal flow of 5 m³ per second.

The geological and hydrological conditions, as well as the pipeline route that includes a horizontal and a vertical curve at a 900 m radius made the project complex. Project implementation required particularly meticulous preliminary studies which were conducted by the engineering office Cabinet Merlin who 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 in 12 to 15 m depth and runs between a layer of limestone, a layer of alluvial deposits and around 10 m beneath groundwater level. Furthermore, a DN 4000 sewer main is one of several obstacles that needed to remain unharmed in the route’s proximity.

The jacking station was built inside the 25-m-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-ton-loads from the station to the basin’s structure.

Groundwater posed a further challenge: Its level lying 10 m above the pipeline, it was estimated to generate a load of almost 30 tons 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. As remedy CSM Bessac designed an ingenious non-return system with two hydraulic jaws that firmly gripped and held the pipes in position.

Also the 380 m S-curved route 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 3 m 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 tons and can be jacked around curves down to 680 m 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.

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

With the successful completion of this technically audacious project and by offering sustainable high-quality products, HOBAS has once again proved its leading role in the national GRP market for trenchless applications and the successful project will certainly inspire others.

This article was supplied by HOBAS Engineering GmbH, Bois Colombes, France.
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