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CLOSE CALL

UNDERGROUND CONSTRUCTION - Installing 47m deep diaphragm walls only 3m from some of Amsterdam's oldest buildings is no mean feat, as Andrew Mylius discovers.

Riots broke out in Amsterdam when buildings in its historic heart were razed to make way for the first metro lines 25 years ago. So fierce was the public outcry that construction of additional lines looked highly unlikely.

But the last couple of decades have stretched public transport to the point where new capacity is needed. Accordingly, the first work on a new e 1.5bn (£990M), 9.5km long north-south metro line got under way early last year.

The North-South Line is designed to relieve pressure on tram services between the Buikslotermeerplein district north of the Ij harbour, through the city centre with its concentric rings of canals, to Amsterdam's financial quarter in the south.

This time, the alignment of the twin tunnel line and its eight stations steers clear of private buildings, rigidly following the street and canal system.

A mix of construction techniques are being used along the route. From Buikslotermeerplein to near the Ij the line runs at street level, ducking into a short section of cut and cover tunnel before crossing the harbour in an immersed tube.

South of the Ij, the line will be tunnelled underneath Amsterdam's Centraal Station using an audacious method involving the installation of piled retaining walls beneath the station's basement floor.

Ground between the walls and beneath the floor slab will then be removed; because the water table is only 700mm below ground level, excavation will be carried out underwater.

Reinforced concrete caissons will then be floated into position and settled on the excavation floor before the annulus is backfilled.

Bored tunnels will be driven deep beneath the city centre, from Centraal Station through to RAI Europaplein Station, one stop before the line terminates at Zuid/World Trade Centre. At RAI Europaplein the line resurfaces to street level.

Work is now under way on construction of the TBM launch pit in front of Centraal Station and deep stations in old Amsterdam at Rokin, Vijzelgracht and Ceintuurbaan.

UK contractor Cementation Foundations Skanska has teamed up with Belgian firm Franki Geotechnics to tackle diaphragm walling for three of the stations under a £26.5M contract (GE November 2003).

The station boxes consist of 47m deep, 1.2m thick diaphragm walls, propped at about 29m below ground level by a 3m thick jet-grouted base slab and at their tops by precast reinforced concrete roof slabs. Jet grouting will be carried out by Keller and the roof slab installed by German main contractor for the station works, Max Bogl.

In all 70,000m 2of diaphragm wall is being installed, says Cementation's head of major projects Julian Crawley. At close to half way through, the job has gone seamlessly so far, but he adds that it is far from straightforward.

Franki-Cementation's key challenge is working in such a tight urban location. All three stations will be virtually the full width of the streets under which they will lie. This means construction is having to take place in two passes at Rokin and Vijzelgracht stations, with half the street width left open to traffic.

Even more challenging, though, are the diaphragm walls, which are being installed within 2m-3m of shop and house fronts. The proximity to buildings up to 400 years old throws up some ticklish problems.

Amsterdam was built on hugely unpromising ground. Between the surface and the first layer of competent sand lies 12-13m of silty, sandy, peaty 'mush' There are further layers of soft clays and silts 10m or so further down.

Even using bentonite to support the trench walls during excavation of the diaphragm walls, it was doubtful whether the top 12m would stand, Crawley says.'A collapse would endanger the buildings nearby, but we've also got people walking along pavements just the other side of our safety fencing - we're talking inches from the trench.

'A collapse might not bring down a building but chances are it would take out the pavement. If a passer-by went with it, it would be a disaster. Clearly we couldn't allow the slightest chance of that happening.'

Franki-Cementation therefore started work by mixing cement into the top 12m of ground either side of the diaphragm wall trench to increase trench stability. About 50kg/m 3ofcement was added using a three-bit reverse cycle auger. Site preparation also involved extraction of old wooden piles from the line of the walls - a large diameter drilling machine was brought in to ensure that no broken stumps were left in the ground.

Metro designer ABNZL, comprising Amsterdam City Council engineering department and Dutch consultants Royal Haskoning and Witteveen & Bos, has designed the station diaphragm walls to unusually tight tolerances of 1:200.'Normally in the UK we work to 1:100 or 1:120, ' Crawley notes.

To achieve the required accuracy, at first it was planned that excavation of the trenches would be by hydraulic milling machines, with slurry pumped away for disposal. But pumping such huge quantities of spoil and bentonite along the city streets and canals posed huge risks. Crawley says: 'If we'd had a pipe burst it would have gone down badly with the public. It would almost certainly have shut the project down, at least temporarily.'

Franki-Cementation's alternative involved securing two of the only four hydraulic grab rotators in the world. These 20t monsters are manufactured by Bauer and mounted on Liebherr crawler cranes.

They are instrumented to give readings for vertical alignment in x and y axes, as well as depth readings in real time to their drivers. The accuracy comes from being able to rotate the grab through 180infinity each bite and from the fine control offered by the hydraulic control system, Crawley explains.

'The grab has interlocking teeth - say four on one side and five on the other, depending on the width of your trench. With a conventional hydraulic grab you're putting it into the ground the same way around each time.What you find is that the grab starts steering itself and develops a tendency to slew around.

'By rotating the grab on alternate bites you never give the grab the chance to steer itself.

We've got to deliver 1:200 accuracy but we're aiming for 1:400. Our best panel so far we achieved 17mm deviation over a depth of 47m.'

The walls are made up of panels lengths ranging from 2.8m to 6.4m: Panel length is determined by the distance of the wall from adjacent buildings - the larger the distance the greater panel length allowed.

Rotating hydraulic grabs are used to excavate the starter panels and the larger one-anda-half or two-and-a-half bite panels. Mechanical grabs are then used to excavate the remaining half cuts and for removing stop ends. This maximises efficiency, as the hydraulic grabs are constantly used to best advantage - on accurate excavation - with the mechanical grabs following behind, construction manager Paul Wiltcher adds.

Prefabricated T50 reinforcement cages weighing up to 51t are being delivered to site in three parts, from Belgium and Holland.They are joined on site and placed using a tandem crane lift. Glass-fibre reinforcement is being used on the station box end walls where the TBMs will break through.

Construction of the North-South Line started in January 2003 and is due to take eight years, with the metro in service in 2011.

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