Albert Embankment Temporary Cofferdam 3
Robert West was appointed by the joint venture of Ferrovial Construction and Laing O’Rourke joint venture (FLO) responsible for delivering the central section of the Tideway project to design temporary structures to enable the construction of the permanent works. These permanent works would connect the Clapham combined sewer outfall (CSO) at the upstream side of the Vauxhall Bridge with Cofferdam 2, located downstream of Vauxhall Bridge.
Three temporary structures were required. These consisted of:
- A launch chamber to provide a watertight barrier around a working platform to facilitate the construction of the permanent culvert pipe jack tunnel connecting Clapham CSO with the Reception Chamber at Cofferdam 3
- A twin sheet pile wall cofferdam to allow the construction of a tunnel between the launch chamber and Cofferdam 3
- A retaining wall to create a dry working area around Clapham CSO to facilitate the construction of the permanent culvert connected to the existing Clapham CSO.
The main constraint for the twin sheet pile wall cofferdam was the limited air gap underneath the Vauxhall Bridge which prevented the construction of the permanent culvert with an open excavation. Instead, the permanent culvert needed to be driven by a jacking machine from the launch chamber. Two sheet piles creating a trench and filled with concrete provided protection for the tunnel. In addition, a reinforced concrete slab, designed to withstand the weight of the water, was placed on top of the mass concrete to protect the personnel working in the tunnel from the tide. A three metre thick reaction wall was constructed within the launch chamber to provide a strong point for the jacking operation.
The launch chamber needed to be dewatered and excavated. The water imbalance was approximately 10 metres. Our engineers took advantage of the box shape of the chamber to counteract the water imbalance of one wall with the one opposite, by inserting two levels of bracing frames.
As the Clapham cell is dewatered the engineering solution required in the launch chamber changes so a further solution was required. Our engineers designed a structural system to transfer the imbalance to the rigid points (Vauxhall Bridge abutment and the existing river wall) by means of transfer beams and concrete tie-ins. The construction sequence was of vital importance to ensure that all the bracings were installed before proceeding with the dewatering of the Clapham Cell.
Due to the proximity of the Victoria Line and surrounding residential areas, piling had to be undertaken at night whilst the Underground was closed and using a silent method.
To create the retaining wall the effluent at the Clapham CSO was temporary diverted to the Thames. The diversion consisted of a steel pipe connected to the first segment of the permanent culvert. The first segment of the permanent culvert needed to be built in the dry season with a strict eye on the weather forecast and the water level inside the CSO.
During the construction, London clay was found at different levels from the ones which had been anticipated in early design stage. Our engineers worked closely with the site team to adapt the design to the conditions found on site.
Our engineers enjoyed the challenge of the continually changing structures as implementation progressed – it was a real jigsaw! They enjoyed seeing the project from start to end, providing a solution which allowed construction in a safe way and at the same time responding quickly to new constraints as they emerged.
Overall, co-ordination of the global picture – keeping in mind the constraints and how the structures interact with them, ensuring their stability throughout and keeping workers safe during construction was a great achievement.