THE TINY Western Pacific republic of Palau is considering a £10.6M out of court settlement offered by companies involved in the contract to strengthen the Koror-Babeldoab bridge just before it collapsed in 1996 with the loss of two lives.
A US Department of the Interior source confirmed this week that the President of Palau had authorised his officials to begin round table discussions to finalise the deal with the firms involved.
It is also understood that the Japanese government has already offered to make up the difference between the final settlement figure and the estimated £15M cost of a replacing the crossing between the republic's two main islands.
When it opened in 1978, the Koror-Babeldoab concrete box girder bridge had a world record span of 241m. But over the years it sagged more than 1,100mm at the centre, and in mid-1996 a £1.3M operation to partially recover the sag was carried out.
In September the same year, just months after the work was completed, the bridge collapsed without warning. Precise causes of the collapse remain obscure (NCE 17 October 1996).
Writs for damages exceeding £60M were issued in April 1997 by the Palau government against main contractor Black Micro of Guam and its parent EE Black of Hawaii, specialist post-tensioning subcontractor VSL (Guam) and its parent Bouygues, consultant Louis Berger Group and its Seattle- based subsidiary Berger/ ABAM, geotechnical sub-consultant Dames & Moore and bonding company American Home Insurance.
At the heart of the dispute was the strengthening method put forward by VSL and approved by Berger/ABAM and Dames & Moore. This involved locking together the two balanced cantilevers that formed the original bridge. Post-tensioning tendons were then run from one abutment to the other. Flat jacks at the central hinge exerted additional compressive forces on the top web of the box girder before the central movement joint was grouted up.
But an independent analysis of the stresses in the box girder before and after the strengthening operation carried out by Imperial College, London, indicated that the additional post-tensioning was unlikely to have overloaded the concrete in the web. The official investigation carried out by San Francisco-based TY Lin is understood to have reached much the same conclusion.
Instead, it is now thought sudden release of the additional unbonded tendons produced enough energy to cause most of the damage visible above water.
A joint report on the disaster currently under preparation by New Civil Engineer, Imperial College and the British Cement Association will assess the possible causes for the failure and the implications for similar bridges.