Thames Tideway Tunnel: Blackfriars Bridge Foreshore: innovative in-river construction

A highly constrained location with multiple physical challenges meant an innovative solution was required for the interception of the Fleet Main combined sewer overflow (CSO) at the Blackfriars Bridge Foreshore site. The old CSO outfall located under a low-headroom arch of Blackfriars Bridge meant a cast-in-situ concrete interception could not be constructed. This paper describes how an innovative alternative was employed, comprising a precast concrete box structure initially cast within a dry dock and then floated into position below the bridge. A physical model was constructed and various scenarios were tested to demonstrate that the theoretical assumptions could be relied upon in the real situation. The physical modelling meant that risks inherent with such an unusual operation were minimised and the float-out operation was planned with full knowledge of how the culvert would behave under varying conditions. The collaboration between the main contractor, permanent and temporary works designers, the project manager and other stakeholders shows that it is possible to overcome seemingly unsurmountable challenges with inventive ideas and good planning and preparation. The successful outcome of the float-out operation and placing of the floated culvert has helped contribute to the exceptional Thames Tideway Tunnel project.