Chelsea embankment foreshore temporary cofferdam
THE BRIEF
The joint venture of Ferrovial Agroman and Laing O’Rourke that is building the central section of the Thames Tideway Tunnel appointed Robert West to design the Temporary Works at Chelsea Embankment foreshore (CHEEF).
The main structure consists of a temporary cofferdam, creating a new piece of land in the River Thames to enable the construction of a shaft for the Thames Tideway Tunnel. The cofferdam took the form of twin, sand infilled, steel walls tied together to form the main body, which will allow the Thames Tideway Tunnel project to progress work beneath ground. When completed, the temporary cofferdam will be removed. Robert West provided construction support throughout.
VALUE-ADDED SOLUTION
There were a number of engineering challenges which our team had to take into consideration in providing our solutions:
• the River Thames has a large tidal range which varies by six to seven metres each day between high and low tide
• barges berthing at the temporary cofferdam create berthing and mooring forces
• the cofferdam needed to be designed to resist ship impact from Thames fluvial traffic
• Part of the future Permanent Works are extended into the footprint of the Cofferdam, reducing the available width for the structure
• At this location, an additional excavation of approximately 10m in depth is required to enable the construction of ancillary Permanent structures
• The flow from the existing combined sewer outfall (CSO) is to be temporarily diverted to the river through the cofferdam
• A 160 tonne crane which is wider than the reduced cells will be working on the top of the cofferdam.
To overcome these challenges
• Mooring arrangement analysis was undertaken to accommodate the barges to be used on site and to assess berthing and mooring forces transferred to the structure
• The geotechnical stability and structural response were assessed through Finite Element Models
• The structural ductility against ship impact was analysed with a Non lineal Static analysis (pushover). It enabled the assessment of the energy that can be absorbed by the structure in the elastic-plastic range at the performance point (i.e. with limited damage)
• Deep piles were designed to provide stability to compensate the narrowness of the cofferdam
• Temporary bracing was designed for the additional excavation in preparation for the deep piles
• The flow diversion is formed of twin tubular pipes (Ø2000 mm) connected to the existing CSO and the cofferdam. A bespoke headwall was designed as a transition piece at the sewer. Additional waling beams and stronger piles were required to allow the pipes to protrude through the cofferdam
• A steel bridge spanning between the adjacent cells was designed to allow the crane to track from one side to another
FINAL OUTCOMES
After the completion of the works, the temporary cofferdam was sealed at low tide to minimize the volume of water to be pumped out.
The mooring arrangement analysis found a balance between flexibility of operation and forces transferred to the structure and therefore optimization of the structure.
The response of the Cofferdam is being monitored and is adequate when subjected to the water imbalance and tidal variation. Works in the hinterland are in progress.
The pushover analysis allowed the exploration of the plastic response of the structure under abnormal loading and its ductility to accommodate unexpected loads. This avoided the need for huge and expensive piles.
Our team demonstrated strong technical knowledge of engineering principles on a technically demanding project overcoming challenges successfully.
You can see the temporary cofferdam’s development at https://www.youtube.com/watch?v=oWsIDyunEdY
