Danish tunnel pushes on

Oresund contractors last week claimed that they can finish the Danish immersed tube tunnel on time, despite an element being flooded and sinking last month.

The 57,000t tunnel element suffered only minor damage when it sank and is to be used as planned in the permanent structure. Oresund Tunnel Contractors (OTC) has refloated the 176m-long concrete element and next week plans to ballast it into final position in a seabed trench. Construction has been at a standstill since the accident.

OTC is building the 3.5km-long tunnel off the Danish coast, near Copenhagen, as the western link in a Danish-Swedish transport project. The tunnel is being built from 20 precast concrete elements by the contracting group, which includes John Laing Construction, but the 13th tube element sank during final ballasting operations in August.

Seawater punched-in one of the bulkhead doors and damaged another after the failure of a temporary concrete support. OTC assistant project director Niels Uldall said the base plinth support failed because of "human error" - a U-shaped rebar support was omitted during fabrication.

Uldall said the production system has been tightened up. "We've introduced some new 'hold-points' for physical checks at different stages of fabrication." He said the client's All Risks insurance would pay for the construction recovery work.

This week OTC focused on repairing the foundation bed that was damaged by the sinking element. The element had been fitted with some replacement bulkhead doors, dewatered and refloated to be taken some 200m away to await completion of foundation repairs.

OTC is using one immersion pontoon to place the tunnel elements. The pontoon has remained connected to the element because removal would have been difficult and unnecessary as the January 1999 installation deadline should be met, said Uldall.