ABSTRACT
In the context of the Line 16, Lot 2 project of the Grand Paris Express, the Joint Venture between We Build and NGE had to face an emergency relating to the inflow of water during the Break In phases of TBM7 in the circular shaft called OA401. Although the TBM Break-In had been planned to use a metal sealing ring equipped with two Phoenix rubber joints and a Bull flex inflatable join and annular void injections between shield and sealing ring, a water leakage of considerable magnitude occurred (approximately 500m3/h flowrate, at 0,15 MPa static pressure). With the support of the Client (Société du Grand Paris), and their geotechnical assistant SETEC/Terrasol, the project engineer Egis/Tractebel Engineering and WB/NGE’s consultant KF International, this situation was investigated and solved by implementing a sequence of mitigation works aiming to collect, reduce and ultimately stop the substantial water inflow flooding the shaft. The accident was due to the inflatable gaskets failure at the tail of the shield when entering the Bartonian aquifer, the solution to stop the flow was to seal the annular void of the shield and filling the cutting wheel with a bentonite and silicates composite mixture (Bentogel), until the stopping of the water inflow. The article aims to describe the strategy, the concept of the mitigation works that were implemented, and the controls adopted to progressively resolve the challenges encountered, which resulted in guaranteeing the safety of the workers, the stability of the surface at every stage of the project and mitigated the risks to the environment. Emphasis is placed on the elaboration of an unusual methodology, consisting in infilling a soft material with sufficient cohesiveness to temporarily balance the waterhead, while being able to shrink to allow the placement of the tamping mortar used for instalment of tunnel ring elements.
