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Monitoring marathon

TUNNELLING & UNDERGROUND CONSTRUCTION

A major ground monitoring contract is under way in Amsterdam a full year before the city starts building its underground metro.

Tree surgeons in Amsterdam are busy pruning along the city's central boulevards armed not with chainsaws but scissors and laser beams. Wind movements and seasonal growth are crucial factors and branches are carefully removed at night during an operation likely to spread over six years.

Nearby, discussions are continuing with local police about repositioning a street banner warning of pickpockets.

These two events have a common aim: to build the city's underground metro system without damaging any of the 1600 historic buildings close to its tunnelled route.

Achieving a goal of negligible settlement, in a city where most old buildings subside naturally in the weak ground by an average 1mm every year, demands the pioneering teamwork of surveyors, tunnel machine manufacturers and computer software experts.

Ground and building settlement monitoring is already under way. This operation involves more than 70 automated theodolite workstations, plus 3000 other instruments. Over six years, they will produce a staggering 150M readings. Design work has started on an 'intelligent' tunnel boring machine (TBM) capable of interacting with threedimensional computer analysis of surface movements during tunnelling to both predict and minimise surface settlement above it.

'This combination of real time monitoring linked to an interactive TBM is potentially the world's most sophisticated settlement control technique yet developed for a construction project, ' claims Frank Kaalberg, contracts manager for Witteveen & Bos, Dutch consultant for client the Municipality of Amsterdam.

'We aim to significantly reduce surface settlement and cause no structural damage to any buildings. '

Two factors make extensive underground construction a challenge for the metro engineers - poor ground and public hostility.

'Soft, butter-like mud, ' is how geotechnical engineer Christophe Bourlart describes the city's underlying clays that leave the tall, narrow, up to 400-year-old buildings constantly on the move despite their average 15m deep timber piled foundations.

'Test drills and sheet piles sink under their own weight, ' he says.

As operations manager for monitoring specialist Soldata, a subsidiary of French geotechnical contractor Soletanche Bachy, Bourlart is charged with monitoring the 1600 buildings on the 3. 8km route and checking settlement around it before, during and after the two sophisticated TBMs have passed through.

The last time metro construction took place in Amsterdam - for a short section early in the 1970s - tunnelling was ruled out because of the likelihood of considerable settlement associated with the less sophisticated boring machines of those days. But cut and cover required such widespread building demolition that it triggered street riots from annoyed inhabitants. Maintaining good public relations is a major objective this time.

That is why, a full year before construction of the North-South metro line starts, Soldata, in joint venture with local contractor Grontmij, is busy erecting 74 Cyclops workstations high on key building facades, plus 5247 small target prisms on all properties within a 100m wide band of the twin tunnel route.

Buildings at risk include the ornate central railway station, several embassies and even Queen Beatrix's royal palace.

Each £25,000 workstation, comprising an automatically controlled theodolite and a small computer, constantly reads up to 90 targets on surrounding buildings. Their positions, relative to a fixed, more distant, target, are relayed by radio back to 13 computers in Soldata's offices.

Each target is read roughly every 20 minutes with software correcting for time or weather variations. Theodolites can read in the dark, but do not like obstacles - hence the tree pruning and banner repositioning.

Measurements from a group of theodolites, accurate to 0. 9mm, are then fed to the client in four-hourly batches, although they can be transmitted instantly when tunnelling starts. From these basic coordinates the North-South metro team is creating a 3D real time computer model of representative buildings.

Soldata is about to sink 150 boreholes along the route to install extensometers and inclinometers to record ground movements beside and above the tunnels as the TBMs pass. The firm's £9M, six-year contract also includes a conventional levelling survey of ground and buildings.

When excavation for the four cut and cover station boxes starts next year, Soldata will also check the verticality of the 45m deep diaphragm wall sides.

By monitoring virtually everything along the route before, during and for a year after tunnelling, the Soldata-Grontmij team will provide a complete independent history of metro-induced settlement.

'We need to know how the buildings move naturally throughout a full year so we can adjust for seasonal changes, ' explains Henry Sam, project manager for Mott MacDonald, the consultant supporting the metro team and overseeing monitoring work.

In parallel with this survey, city engineers are 75% through a structural check of all buildings on the route. A quarter have weak foundations and are being strengthened with internal bored piles tied to a new concrete ground slab.

Settlement expected from a conventional tunnelling operation would cause considerable structural distress. To achieve Kaalberg's aim of undamaged buildings and undamaged public relations, the metro team is looking to innovation.

The plan is to design an earth pressure balance tunnelling machine capable of exerting minimal subsoil disturbance and therefore minimal surface settlement. Existing EPB machines using a bentonite slurry are already competent at equalising face pressures. It is in the rear, in the 'tail void area' behind tunnel lining segments, where greater control is needed.

The preferred, though as yet unnamed, tunnelling bidder (see box) is now developing a more flexible machine capable of accurately varying grout pressures and volumes behind the tail, balancing the stresses on surrounding soil caused by tunnelling.

The operator then needs to analyse and counteract the surface settlement that the TBM is causing directly above. Soldata will provide real time settlement readings which the client will convert into a 3D visualisation of building movements instantly displayed in the machine cab.

But there is a snag - the time delay between subsoil disturbance and its migration 30m up to the surface to trigger settlement in buildings above. To overcome this problem, the 3D virtual model will already have been fed with soil and TBM characteristics; the calculated movement of the building the machine is about to drive beneath; and the real reaction of similar buildings back along the route. This information is fed to the operator who estimates and dispenses the amount of grout needed before driving under the next building.

By the time this central metro section is complete in 2007, the only movements noticed by Amsterdam's citizens should be the theodolites, still turning on their perches on building corners.

The central 3. 8km section of the 9km metro route will run in twin 5. 8m diameter tunnels driven beneath city streets.

Four of the nine stations will be in large cut and cover boxes 30m deep and only metres from historic timber piled buildings.

Underground work was originally offered as a single contract, but station tenders came in above budget and the Municipality of Amsterdam is reletting them as separate contracts.

If the preferred, though still unnamed, tunnel tenderer is confirmed, tunnelling will start in mid-2004 and take 18 months.

Tenders for the underground stations will be in by October this year with excavation due to start in spring 2002 for completion by 2007. But the overall G2. 5bn (£700M) project will not open until all nine stations are ready in 2009.

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