VITAL SHEAR reinforcement omitted from a temporary concrete bulkhead support has been blamed for the accidental sinking of a massive precast tunnel element for the resund crossing off the Danish coast, it emerged this week.
The 57,000t concrete element - 176m long, 38.8m wide and 8.6m high - sank after part of the concrete beam restraining the giant steel bulkhead sheared off and allowed water to flood in.
Disaster struck at 4.45pm on 4 August as the 13th element - named 12a for luck - was being positioned. The base of the element was 1.3m above the bed of the trench and 18.7m under water when one of the bulkheads failed.
Within a minute the element was resting at the bottom of the trench, 4m out of position to the east of segment 12. Two hours later it was completely flooded.
resund Tunnel Contractors project director Per Nilssen said the accident was likely to delay the tunnel construction programme by between four and six weeks.
The 3.5km resund tunnel forms the western end of the combined tunnel and bridge crossing now under construction between Denmark and Sweden. Each of the tunnel's 20 elements will be positioned in a specially dredged 12m deep seabed trench.
So far TC - whose members are John Laing, NCC of Sweden, Dumez GTM of France, E Pihl & Son of Denmark and Boskalis Westminster of the Netherlands - has successfully positioned 12 segments since the first element was floated out in August last year.
Tunnel elements are made up of eight segments stressed together. East segment is cast as a five bore honeycomb at a specially built casting yard just north of Copenhagen. Two bores will carry the carriageways of a dual two lane motorway, two railway lines and a small single bore will house a service tunnel and emergency exit.
Elements are then floated to site before being sunk into position. Re-usable steel bulkheads seal the ends of each tunnel bore during floating.
These bulkheads are supported at the top by temporary steel brackets tied into the roof and at the bottom by a 330mm high, 800mm wide insitu reinforced concrete upstand beams cast across the base of the bore. It was one of these beams, across the eastern entrance to the outermost, southern railway bore of element 12a, that failed. At the southern end of the beam there was a 450mm gap for realignment jacks which were to have fine tuned the alignment of the element when in position next to its neighbour.
However, some vital hairpin shear links connecting the beam to the base of the tunnel - as specified in the design by TC's in house consultant Symonds Travers Morgan - were omitted. Consequently a 1.15m section at the end of the beam next to the 450mm gap gave way as the pressure on the bulkhead increased underwater.
The hairpins should have been cast into the tunnel floor at 150mm centres along the front edge of the upstand beam. They would then tie in to lateral reinforcement before the beam was separately cast insitu.
TC's internal investigations have since suggested that at least eight reinforcement hairpins were missing from the failed section of the beam.
The contractor is now conducting an internal investigation to discover why no-one noticed that the hairpin links in the failed beam section were missing before the concrete was poured.
In a statement, client resundskonsortiet said the contractor was redesigning reinforcement 'to avoid similar incidents. For those tunnel segments not yet cast, the construction has been changed to increase safety.'
'There is no indication that the element is damaged,' said Nilssen. However, he said the extra weight of the flooded element caused it to compress the gravel bed lining the trench by between 250mm and 300mm more than calculated.
Nilssen said that TC plans to repair the damaged bulkhead support and reinstall the bulkhead. The element will then be raised from the seabed and placed further east roughly where segment 15 will eventually be positioned. The trench bed will then be re-levelled before the element is placed in its intended location.
The re-floating operation will involve pumping seawater out of the element, allowing it to suck in air via a 800mm diameter access pipe to the surface. TC said it was unlikely to raise the element to the surface, expecting to be able to manoeuvre it once it has floated off the bottom of the trench.