It’s not only trenchless crossings that need geotechnical expertise. OPTIMUM has been supporting Eiffage over recent weeks on a different sort of mission: the demolition of an ageing ovoid sewer section to make way for a new storm overflow valve chamber.
This crucial work, just north of Paris, will ensure the Conseil Départemental des Hauts de Seine’s wastewater network is future‑proof and able to withstand the increasingly fluctuating — and unpredictable — water levels of the River Seine. The project was never going to be easy, but simplicity wasn’t the goal — security was.
One of the most delicate operations — excavating around and removing the old sewer section, then installing a temporary retaining structure for the new chamber — is now complete, despite multiple challenges:
- A 2.20‑metre‑diameter rainwater pipe running close by
- A jobsite 7 metres underground, below the watertable, just 120 metres from the banks of the River Seine
- Immediate proximity of Paris’ A86 super ring road
To secure the excavation, the team designed a temporary retaining structure combining slide‑rail and timber boards — an unconventional but adaptive solution shaped by the site’s specific constraints.
OPTIMUM engineers didn’t just “check” the design. Through a G3 mission, they provided key insight, ensuring construction stability and jobsite safety despite the proximity of the River Seine. Their work ensured the complex site remained a controlled environment — and kept the project moving forward.
- Validating the geotechnical hypothesis, operating methods and structural design
- Designing the groundwater lowering system to ensure a dry and safe working environment
- Controlling settlement testing and assessing the impact of the excavation / construction works on the adjacent rainwater pipe
The temporary structure is now complete, and construction of the permanent chamber can continue with confidence. A huge shout‑out to everyone involved : Conseil Départemental des Hauts de Seine, EIFFAGE Génie Civil, COTRASOL,
