Having a geotechnical firm as main contractor has eased construction of a huge quayside wall for the French port of Le Havre. David Hayward reports.
France's largest container port at Le Havre is about to double in capacity, courtesy of a vast new diaphragm wall that is creating another quay. At the same time, new breeding grounds, wetland zones and mudflats are being formed to protect rare birds, plants and frogs.
Six years of debate, extensive ecological studies and a public inquiry - a rare occurrence in France - were needed before a single piece of construction plant was allowed to invade the reclaimed northern estuarial shoreline of the River Seine.
Here, Le Havre's port has developed into one of the most successful on Europe's western seaboard. Port engineers are keen to encourage their most lucrative visitors, container ships, by building six new berths by 2007.
But the Seine estuary is attractive for other visitors too. It provides a habitat for 20,000 migrating birds, rare orchids, shrimp shoals and thousands of toads and frogs. Four special protection zones surround the site.
More than E48M ($54M) has been spent on environmental safeguards. No-go habitats have been further protected, nature reserves created and site run-off settlement lagoons established, while frogs and toads have been relocated to new ponds a few kilometres away.
Such expensive precautions make life a bit less stressful for Jean-Luc Gobert, project manager for main contractor Soletanche Bachy. He can now worry less about the endangered heron-like Butor EtoilÚ bird - although a pair nesting close to his construction offices forced a change in the programme - and more about completing a 6m long package of work along the concreted 1.6km diaphragm wall every day.
'Construction moves along the wall like a conveyor belt, ' says Gobert. The French geotechnical firm is now two thirds through its $157M contract to build four of the six new berths.
'Each of the dozen operations - including capping the top, inserting ground anchors, excavating both sides and dewatering - is on the project's critical path.'
The straight 1.2m thick wall, sunk 40m down through alluvial soils 400m inland from the estuary's shoreline, will convert into four, 350m long berths to host the world's largest container ships.
The wall's construction in 6m panels allowed Soletanche Bachy to show off its latest computer guided Hydrofraise cutting rig. The $2.8M Evolution Two, with its four lower cutting discs able to steer the frame to a full depth verticality tolerance of less than 0.3%, achieved excavation rates of 12m 3/h - double that of a conventional grab.
'The alluvial sands, silts and clays proved far more abrasive than expected and we had to replace cutters every week instead of monthly, ' Gobert recalls. 'In the end everything was totally worn out.'
Yet speed was essential and client, the Port of Le Havre Authority, allowed the site team only two hours to install panel reinforcement before concreting. Gobert says this time specification had been based on previous best practice figures, but for walls at least a third shorter.
His solution was to fabricate a single 40m long rebar cage for each panel and employ a network of three large cranes to manoeuvre the towering 60t assembly into an excavation demanding clearances of just 200mm.
'If something had gone wrong during the lifts there was little we could have done about it, ' he recalls, with a smile only possible because all 267 rebar cages are now installed and panels concreted. 'All lifts were achieved without any problems and each operation proved more than three times faster than fabricating the reinforcement in sections.'
It took just over a year to complete the main wall structure at the programmed rate of one 6m panel a day. But that was only half the challenge, as Soletanche Bachy then returned to the start advancing sequentially with follow-on construction activities.
The client's outline design had a 1.5m thick wall, stabilised behind by two rows of inclined ground anchors extending back about 45m to tie into a parallel sheet piled wall. At tender stage Soletanche Bachy value engineered the cost of the diaphragm wall down 25% to $34M by realigning and lowering the ground anchors to reduce overall wall thickness to just 1.2m.
Passive, rather than active and tensioned, anchors were chosen so the client does not have to revisit the quay wall to check for possible stress loss.
But the operation involves what any foundation contractor could be apprehensive about - major exposure of the diaphragm wall both sides.
Excavation is needed behind to position the anchors, and in front to create the new quayside by grabline and dredging operations across several hundred metres of fill to the shoreline.
With up to 17m deep sections of wall totally exposed, Gobert is proud of his team's work. 'It's an excellent finish - too good for a quay wall, ' he jokes.
No spoil from the 12M. m 3earthworks leaves site; the excess forming future construction areas and dredged material pumped into settlement compounds.
The 17m deep excavation behind the wall needed to position the two rows of anchors is itself a major logistics exercise.
The client wants only competent soil backfilled above the anchors, so much of the excavated material is stockpiled and regraded, with only high quality fill replaced. Accurate location of the two rows of maximum 110mm diameter anchors is critical.
Angled at up to 10% from the horizontal, and tied to the wall either 6m or 16m down, all 2,700 anchors are laid out with the help of global positioning satellites. Ground slopes around the anchors are levelled to an accuracy of less than 100mm with the help of lasers.
Earthworks and non-geotechnical construction account for 40% of Soletanche Bachy's contract value; a fact that, in the UK, would made it unlikely for a geotechnical firm to be appointed as sole main contractor.
But, with an annual global turnover of $900M, Soletanche Bachy is among France's top six general construction companies.
'This is one of our largest contracts worldwide and it is not unusual for us to oversee a lot of non-geotechnical work, ' Gobert explains. 'And here the project's critical path lies firmly through the wall, so we are well equipped to sort out any problems.'
It was just such a geotechnical challenge that threatened seriously to delay the project when dewatering to allow excavation around the wall.
The tide-affected water table is close to the surface, and the plan had been to dewater the alluvial sands and silts down to 20m using close centred wells both sides of the wall.
But ground porosity proved very variable, and in places the soil was 100 times more permeable than expected. Planned pumping rates of about 2m 3/h/m run of wall rose rapidly to 17m 3/h/m before Gobert realised he was fighting a losing battle.
'Tests indicated that rates could rise to 40m 3/h - a volume totally impractical to pump, ' he says. 'We needed to come up with an alternative plan and fast.'
The chosen solution was to blitz the central 700m area of the wall with three times the planned pumping capacity, while protecting the two ends from water ingress into the excavated area by sinking a deep secondary cut-off wall. The two 40m deep slurry wall sections were formed across each end of the diaphragm wall and extend back 450m either side.
The technique has worked, and the network of 220 central pumps is keeping the excavation area dry. But construction of the slurry walls has extended contract completion by six months to December 2004.
Gobert argues that having a geotechnical expert as main contractor, in daily dialogue with the client, has prevented a crisis from becoming a disaster.
'This problem could easily have doubled the delay had we not taken the risk and started forming the cut-off wall immediately, even before contractual approval, ' he says. 'We just knew it was the right solution and we did not stop work once.'
Port of Le Havre project director Paul Scherrer agrees. 'Our close working relationship helped produce a fast and effective solution, ' he says.
Few industries are more competitive than container cargoes, and Le Havre, one of Europe's largest container ports handling over 17Mt annually, needs to expand to stay in the race.
As both the first port of call for eastbound traffic from the US and the last westbound leaving Europe, the 3,000 ships that visit every year need ever larger berthing facilities serviced by vast dockside cranes.
The port's $670M, first stage sixyear expansion programme to 2007 calls for six new berths and a doubling of annual container capacity to 34Mt.
Soletanche Bachy's quay wall will provide four berths and the contractor is preparing the ground for another two berths either end of the wall.
Second stage expansion plans indicate a further six berths by 2010, but Port of Le Havre project director Paul Sherrer stresses the programme has yet to be finalised.