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Seeking early settlement

A major computer aided ground settlement monitoring contract is under way in Amsterdam a full year before the city starts building its underground metro. David Hayward visited the capital to find out why.

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

Nearby, discussions are continuing with local police to reposition a street banner warning of pickpockets.

These two seemingly bizarre operations have an equally unusual common aim: To build the city's £700M underground metro system without damaging any of the 1,600 historic buildings close to its tunnelled route.

To achieve a goal of negligible settlement, in a city where most old buildings subside into 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 - an operation involving over 70 automated theodolite work stations, plus 3,000 other instruments producing, over six years, a staggering 150M readings - is already underway. And design work has started on an 'intelligent' tunnel boring machine (TBM) capable of interacting with three dimensional computer analysis of surface movements during tunnelling, both to 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, consultant for client the Municipality of Amsterdam. 'We aim to reduce surface settlement significantly and cause no structural damage to any buildings.'

The city's underlying clays are described as 'soft, butter-like mud' by Christophe Bourlart, operations manager for monitoring specialist Soldata, a subsidiary of French based geotechnical contractor Soletanche Bachy. The clays 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, ' Bourlart adds.

Bourlart is the engineer charged with monitoring not only the 1,600 buildings lining the 3.8km underground route but also checking ground 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 - the likelihood of considerable settlement associated with the less sophisticated boring machines of the time ruled out tunnelling altogether. But the chosen cut and cover alternative demanded such widespread building demolition that it triggered street riots from annoyed inhabitants.

That is why - a full year before construction of the total 9km north-south metro line starts, Soldata, in joint venture with local contractor Grontmij, is busy erecting 74 Cyclops work stations perched high on key building facades, plus 5,247 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 Queen Beatrix's royal palace.

Each £25,000 work station, comprising automatically controlled theodolite and small onboard computer, constantly reads up to 90 targets on surrounding buildings. Their positions, relative to a fixed more distant target, are relayed by radio link back to a 310Gb suite of 13 computers in Soldata's offices.

Each target is read roughly every 20 minutes with dedicated software correcting for time or weather variations.

Theodolites can read in the dark, but do not like obstacles - hence the tree leaf and banner repositioning exercises.

Measurements accurate to 0.9mm from a group of theodolites are then fed to the client in four hourly batches, although it will be possible to transmit these instantly when tunnelling starts.

From these basic co-ordinates the north-south metro team is already creating a three dimensional real time computer model of representative buildings.

Soldata is about to sink some 150 boreholes along the route within which groups of extensometers and inclinometers will record ground movements beside and above the twin 5.8m diameter tunnels as the TBMs pass.

By monitoring virtually everything along the route before, during and for a full 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 north-south metro team and overseeing monitoring work.

In parallel with this survey, city engineers are already 75% through a structural check of all buildings along the route. About 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. And it is to achieve Kaalberg's aim of no damage either to buildings or public relations that the metro team is looking to innovation.

The plan is to design an earth pressure balance tunnelling machine capable of exerting minimal sub soil 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 tail void area behind tunnel lining segments where greater control is needed.

Underground work was originally offered as a single contract but station tenders came in above budget and the client is reletting them as separate contracts. Tenders will be in next month with excavation due to start in spring 2002.

But already the preferred, though as yet unnamed, tunnelling bidder is developing a more flexible machine capable of accurately varying grout pressures and volumes behind the tail to balance the stresses on surrounding soil caused by tunnelling.

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

But there is a snag - the time delay between sub soil disturbance and its migration 30m up to the surface to trigger settlement in buildings directly above.

And here lies the really clever bit.

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 passed just 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.

'Our model should bridge that time gap allowing us to minimise damage, ' predicts Kaalberg. And by the time this central metro section is complete in 2007, the only movements noticed by Amsterdam's concerned populous should be the continually turning theodolites still perched on building corners above them.

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