FEARS ABOUT water penetration have triggered radical changes to the design of the record breaking 1.5km Marmaray immersed tube rail tunnel under Turkey's Bosporus Straits.
The original indicative design developed by Parsons Brinckerhoff (NCE 3 February) relied entirely on waterproof concrete construction and had a complex double doughnut cross-section.
But joint venture contractor Taisei-Kumagai-Gama-Nurol has gone for simplicity and ease of construction.
Twin bore rectangular concrete elements up to 135m long will have additional waterproofing in the form of a steel 'trough' under the base and up the sides, with an adhesive flexible waterproof membrane on the roof.
Provision is made in the design for easy injection of sealing compounds into cracks if the tunnel develops a leak.
This is thought necessary as the maximum head of water on the tunnel is 60m, more than 50% greater than any immersed tube tunnel built to date.
The steel trough is used as permanent formwork to cast the base and half the wall heights in the drydock casting yard. With its steel joints bulkheaded off, the element is then floated out and concreting completed from a work barge. This option minimises the depth of the drydock, which is on solid rock.