TUNNEL SEGMENTS
SCHELDT TUNNEL

Owner: Lantis
Contractor: TM COTU
Asset: Immersed tunnel elements, New Scheldt Tunnel (Oosterweel Link)
Location: Antwerp, Belgium
Design service life: 100 years
Engineering: SANACON

Background & Challenge

The New Scheldt Tunnel, part of the Oosterweel Link in Antwerp, consists of prefabricated concrete tunnel elements immersed in the brackish water of the Scheldt. The specifications require a design service life of 100 years and included preventive cathodic protection for the outer face of the elements.

The contractor asked Sanacon to critically assess the durability requirements and to demonstrate, with a verifiable calculation, that the concrete design achieves the 100-year target.

The Solution by Sanacon

Critical review of the durability concept: The specified preventive cathodic protection was assessed against the relevant standards and the actual exposure. Below groundwater level, very little oxygen reaches the outer reinforcement, while the inner face is more exposed to CO2 and de-icing salts. Efforts were therefore focused where the risk really is.
Laboratory tests on the actual concrete mix: Rapid chloride migration tests were carried out on the CEM V mix used for the elements, at four different ages to determine the ageing factor. The surface chloride load was derived from Scheldt monitoring data and fib Bulletin 76.
Full probabilistic service life model: Chloride-induced depassivation of the reinforcement was modelled according to fib Bulletin 34 and 76, with a design cover of 60 mm, using 10,000 Monte Carlo simulations.

Key Results & Impact

100-year design life demonstrated: The probability of depassivation after 100 years is 5%, corresponding to a reliability index of 1.6. This meets the fib Bulletin 34 target of 1.3 (about 10%).
Protection where it matters: The durability target is met through mix design and concrete cover. Additional protective measures can be targeted at critical zones such as the joints, instead of the full outer surface.
Birth certificate and monitoring: A birth certificate approach (fib Bulletin 93) was proposed with control tests on produced elements and cover scans, plus a monitoring plan with predefined triggers and actions.