Rising commodity prices, squeezed budgets and the introduction of ultra-stringent regulations formed the backdrop for design and construction of four new shafts on London's Lee Tunnel super sewer - among the deepest in the capital. Added to which, client Thames Water was looking for whole life cost savings.
The fundamental challenge was to prevent the shaft linings from cracking when full with sewage - 70m from bottom to brim. Conventionally, hundreds of tons of steel reinforcement would be used to resist bursting pressures. With JV contractor MVB (Morgan Sindall, VINCI Construction Grands Projets and Bachy Soletanche) we rethought the problem and designed a way of pouring cement grout behind the linings, putting them under pressure and compressing them during construction. This enabled almost all the steel to be eliminated. Meanwhile, above ground, the pipeline linking shaft to treatment works was designed with a slight fall, enabling sewage to flow under gravity, with no need for intermediate pumping.
Rethinking the shaft design saved 1500t of steel, cut construction time and reduced the risk of damage to the shaft linings due to steel corrosion over the 80+ year working life of the sewer. The gravity-fed flow transfer pipeline saves on the long-term maintenance and operating costs associated with running pumps. Commissioned in January 2016, Lee Tunnel has cut the amount of untreated sewage discharged into the Thames by 40%. Along with the yet to be built Thames Tideway tunnel, it will enhance the quality of life for all who live and work in the capital for generations to come.