Culvert Float Out at Blackfriars
Love solving problems? This is a fascinating challenge for the team at Robert West.
A culvert is required as part of the Thames Tideway Tunnel programme at Blackfriars to divert the flow currently discharged into the river to the new sewer. The constraints at the site - an existing Combined Sewer Outfall (CSO) underneath Blackfriars Bridge and the presence of the Waterloo and City Line Tube tunnel - prevent the construction of the temporary cofferdam in situ.
The solution designed by Robert West consists of pre-casting the culvert within the Blackfriars Cofferdam and then floating it out to its final position where it will be set down. The float out journey is approximately 100 metres long. The culvert itself is 100 metres long, seven metres wide, seven metres high and it weighs 3,500 tons.
To assist the culvert in its journey some parts have been removed to reduce the weight. Once the structure is in position the complete section will be cast in situ sheltered from the tide by steel parapets which extend the legs of the U-section above the water level.
The constraints for the floating out manoeuvre are also very tight: if the manoeuvre is carried out at high tide it would result on an impact against the soffit of the Blackfriars Bridge, if it is carried out at low tide, the structure would hit the riverbed.
The culvert will be floated into position pulled by mooring lines and winches. Once it reaches the final location, it will be set down with the dropping tide until it rests on the riverbed. Some valves will then be opened to allow water ingress ballasting the culvert to prevent it floating again.
The tolerance in the final position of the culvert is very precise (only 150 mm) - it cannot touch Blackfriars Bridge, Blackfriars Pier or the river wall. A roller fender will assist with the exact positioning. Auxiliary structures have been designed to assist with the final positioning, with guiding systems where the culvert’s nose will slot into maintaining its position while the water level decreases.
Other considerations for the floatation process are:
- the shape of the culvert – it is asymmetric and slender (unlike the shape of a ship)
- a short time window of two hours for floatation to avoid contact with the foreshore
- a gap must remain between the culvert and Blackfriars Bridge.
It would be difficult to return the culvert to the safety of the cofferdam once it has begun its journey as the foreshore is uneven resulting in there only being certain points at which it can return. It is therefore vitally important to do all that we can to ensure that the float out is successful on the first attempt.
To prepare for such a knife-edge journey for the culvert our engineers have undertaken calculations using their mathematical skills and Orcaflex. Computer based simulations have been undertaken and a scale model (1:15: seven metres long!) prepared. Finally, the scale model has been tested by operators using joy sticks to direct the culvert and introducing varying factors such as using fans to create wind, river currents and magnitude being simulated and the measurement of tension on the wires.
The float out of the culvert is due in 2020, when we hope for favourable weather conditions… and no seagulls landing on one side!