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Check system missed 0resund segment fault

INADEQUATE QUALITY assurance procedures appear to have caused the failure of a vital support beam on the 0resund tunnel segment which sank last month.

0resund Tunnel Contractors said last week that quality checks before the segment was cast have revealed that shear reinforcement for the temporary upstand beam was absent. But there is no record of the casting yard team taking any form of corrective action - because the beam was considered as temporary works and therefore not covered by the quality management scheme.

Designed to support one of the watertight bulkheads that transform the immersed tube tunnel elements into 57,000t 'concrete submarines' during float-out and installation procedures, the upstand beam on the 13th of 20 units failed towards the end of the float-out operation. This allowed the sea to force its way past the bulkhead and sink the element prematurely, leaving it resting 3m east of its intended position (NCE 20/27 August).

The joint between the beam and the tunnel element floor was supposed to be strengthened with hairpin links cast into the main concrete structure. Normally these would have been bent flush with the rest of the rebar during casting, and then shielded with an aluminium cover so as not to obstruct the formwork.

0TC project manager Per Nielsen said the contractor could have inserted reinforcement after discovering that some of the hairpin rebar was missing. But the contractor may have failed to bend the hairpins back before the beam was cast.

He said that the flooded element would have to be pumped dry before a full examination of the failed beam could reveal whether the rebar was there or whether warnings triggered by the original quality check had gone unheeded.

Nielsen expects water to be pumped out of the element this week. 0TC plans to raise the element from the seabed later this month by pumping water from ballast tanks, and towing it further east. This will allow the contractor to re-level the gravel bed before the element is repositioned.

Last week the contractor finished casting a second reinforced concrete upstand beam behind the failed one to add extra strength before repairing the bulkhead. It has also cast a similar strengthening beam behind the upstand beam supporting the bulkhead in the corresponding section of element 12, in position on the seabed.

Upstand beams for the rest of the tunnel elements have been redesigned with extra hairpin reinforcement.

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