Culvert Float Out at Blackfriars

The solution designed by Robert West consisted of pre-casting the culvert within the Blackfriars Cofferdam and then floating it out to its final position where it was set down. The float out journey  was approximately 100 metres long. The culvert itself is 100 metres long, seven metres wide, seven metres high and weighs 3,500 tons.

To assist the culvert in its journey some parts were removed to reduce the weight.  Once the structure was in position the complete section was 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 were also very tight: if the manoeuvre was carried out at high tide it would result on an impact against the soffit of the Blackfriars Bridge, if it was carried out at low tide, the structure would hit the riverbed.

The culvert was floated into position pulled by mooring lines and winches. Once it reached the final location, it was set down with the dropping tide until it rested on the riverbed.  Some valves were then opened to allow water ingress ballasting the culvert to prevent it floating again.

The tolerance for the final position of the culvert was very precise (only 150 mm) – it could not touch Blackfriars Bridge, Blackfriars Pier or the river wall.  A roller fender assisted with the exact positioning.  Auxiliary structures were designed to assist with the final positioning, with guiding systems where the culvert’s nose slotted into place to maintain its position while the water level decreased.

Other considerations for the floatation process were:

• 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
• provision for a gap to remain between the culvert and Blackfriars Bridge.

It would be difficult to return the culvert to the safety of the cofferdam once it had begun its journey as the foreshore is uneven resulting in there only being certain points at which it could return.  It  was therefore vitally important to do all that we could to ensure that the float out was successful on the first attempt. 

To prepare for such a knife-edge journey for the culvert our engineers had prepared calculations using their mathematical skills and Orcaflex. Computer based simulations were undertaken and a scale model (1:15: seven metres long!) prepared.  Finally, the scale model was tested by operators using joy sticks to direct the culvert and introduce varying factors such as using fans to create wind, river currents and magnitude being simulated and the measurement of tension on the wires.