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GROUND IMPROVEMENT - What do the Second World War beaches of Normandy and the present day coastline of southern Spain have in common- Damon Sch³nmann reports.

The famous Mullberry harbours vital to the D-Day landings were temporary structures built in secret and assembled in a matter of days. But the new docks being built at Algeciras on Spain's southern coast using similar techniques are being built in phases over several years and are clearly intended for the longer term.

The scheme to extend the port which sits within clear sight of the rock of Gibraltar, is part of a grander Spanish expansion. 'All the harbours in Spain are growing as every year there is an increase in the transportation of containers and goods, ' says Alejandro Segundo Gonzalez, group contracts manager of Geocisa, which in joint venture with Pennine Vibropiling is handling the ground improvement at the Algeciras site.

Currently under way is creation of a 46ha dock that will sit on a submerged plateau between two ancient rivers.An upcoming phase will include an adjoining but larger 60ha dock.

Running parallel and about 400m in front of both docks, is a breakwater over 1,600m long.

Main contractor Dragados is building the quay wall using concrete caissons, with those lining the north face typically measuring 50m long, 15.5m wide and 20m deep.

These units are built in the harbour at a huge floating casting yard and then, like the D-Day units, towed out to position and flooded so they settle on the seabed. Once insitu, Dragados is filling them with dredged sands topped with 250kg to 500kg limestone boulders.

As might be expected, the ancient river bed does not make a competent foundation material and the weight of the sunken caissons, and Dragados opted for Pennine's gravel column solution to be implemented in conjunction with its own geotechnical division Geocisa.

Pennine Vibropiling managing director Arwel Williams says: 'The riverbed will not resist the excessive settlement from the 20m deep quay wall. The seabed is currently 21m below sea level so they've dredged and then laid a 4m layer of gravel by barges.

'We are putting stone columns straight through that [layer] to control the settlement underneath the quay wall. You could not easily pile through 21m of water.' The gravel blanket infills the holes made by the vibroflot, which vary in length from 4m to 20m, until the column founds on the competent flysch below. Segundo Gonzalez says: 'The ancient river is now totally covered with brown clays and that is the soil we are improving with the columns. It is an area that will take two or three of the concrete boxes.' Work began in November last year and by the end of this month, 1,000, 800mm diameter vibropiles will have been completed.

Having won the £300,000 contract, PennineGeocisa is now eyeing the substantial follow-on ground improvement scheme as the port continues to expand into the bay. Initially this will be the stabilisation behind the caissons.

Williams says: 'Once they start filling in behind the dock wall, that's going to need treatment.

'When you put 21m of fill on top of clay, that's going to settle quite considerably so there is going to have to be some form of treatment there. If they have plenty of time they may use band drains and surcharge but if they're short of time they'll put stone columns in.' Segundo Gonzalez adds: 'This ancient river crosses the next dock they want to do, so they will find the same [strata] problem again.' Construction costs for the complete works are put at around E360M (£253M) with the contract value to Dragados being E100M (£70M).

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