Awarding Bachy Soletanche design and construction of the cofferdam box allowed the contractor to value engineer its task and end up with unusual, cost-saving inclined props as temporary internal support.
The excavation's diaphragm walling is topped by a 900mm deep perimeter capping beam.
Its main 65m long sides start at different levels, creating a 4m wall height difference across the box.
Early conceptual designs called for a row of 15 internal props spanning 30m horizontally across the top of the excavation.
One end of each 1m diameter prop was to bear onto the lower wall's capping beam with the other end tied into a steel waling beam fixed 4m down from the top of the opposite, higher wall.
'By inclining each prop upwards instead, with both ends tied into the capping beams, we removed the need for this 65m long waling altogether, ' explains Bachy Soletanche chief engineer David Puller.
Capping beam reinforcement had to be increased in places to accommodate additional bending moment and torsional loads.
But overall, claims Puller, the novel design resulted in cheaper, quicker construction.