The bridge will be thrown across the destroyed spans of the old bridge over the Bijela river in central Bosnia, one of two which carry the Mostar road, the only all- weather route into Sarajevo.
The bridge is due to begin its five-day journey to Bosnia on Monday.
But it could be weeks before commando-trained Army Engineer divers plunge into the icy waters at the confluence of the Bijela and Neretva rivers to see if the piers can take the weight of the new bridge and traffic, and the new year before the vital road can again carry heavy vehicles.
The rehearsal marks the first real progress towards securing and reopening the crucial supply route, demanded in the Independent's campaign launched on 26 July.
Last week, as the plan began to take shape, there was more bad news. Local ceasefires remained fragile, and a second bridge 800 metres (870yd) to the north-west and previously intact, known as 'Son of Bijela', was blown up two weeks ago, creating an even more formidable gap.
French engineers from Konjic, to the north, have been trying to reach the second bridge, but have so far failed. The United Nations has asked the Slovaks for an engineer unit to repair the second bridge, but the Slovaks have not so far responded.
The Bijela bridge lies north-west of Mostar, at the junction of two steep gorges, in the Spanish UN sector. Yesterday another Spanish patrol pushed up the road between the warring Bosnian Croat HVO and mainly Muslim Bosnian BiH armies which, according to British soldiers who have accompanied them on previous patrols, face each other from the 500-metre heights on either side of the Neretva.
Negotiations continue between the UN and the opposing sides. On 15 October an agreement that UN forces would be able to carry out the complex operation unmolested was signed between the UN commander in Bosnia, Lieutenant-General Francis Briquemont , the HVO Chief of Staff, General Milovoj Petkovic and the BiH commander, General Rasim Delic. However, like all Bosnian agreements, it is unclear how far this can be enforced and General Briquemont visited the Spanish battalion at Medugorje on Thursday to pursue the matter further with HVO commanders.
There is no clear evidence who was responsible for destroying the 'Son of Bijela' bridge, although there would appear to be no advantage to the BiH, which would like to see the road to Sarajevo opened.
Free access up the Mostar road, a good-quality motor route, remains top priority for General Briquemont, who commands the UN Protection Force (Unprofor) in Bosnia, and the British Brigadier Robin Searby, who is responsible for all Unprofor's re-supply and logistics. But even if the physical damage to the otherwise excellent road can be repaired, fighting between the BiH and HVO could still close it, as frequently happens on the other route to the north-west - the mountain road through Lipa, Prozor and Gornji Vakuf.
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