Little moves in Amsterdam these days without the team of engineers planning the city’s new $985M underground metro knowing all about it. Every creak or groan from up to 1,600 city centre buildings is being continuously monitored, while ground movements 50m beneath the streets will soon be meticulously recorded every hour.
A mammoth six year settlement survey, being carried out by French monitoring specialist Soldata, has just started. And with 140,000 readings currently being fed to the computers of the city’s engineers every week, the $12.5M survey is claimed to be the most extensive for any tunnelling project.
Close scrutiny of Amsterdam’s infrastructure is seen as crucial in ensuring that driving the metro’s twin tunnels, directly beneath the historic heart of the Dutch capital, causes minimal surface settlement. Yet, curiously, the start of tunnelling is still three years away.
“It is vital to establish, from an independent contractor, how these buildings behave naturally over the course of a full year and long before we begin tunnelling,” explains Frank Kaalberg, design manager for Witteveen + Bos, Dutch consultant for metro client the Municipality of Amsterdam. “Our overriding aim during tunnelling is to cause no structural damage to any buildings.”
To achieve this goal of negligible settlement, in a city where most old buildings are continually subsiding naturally in the weak ground at an average 1mm every year, demands the cooperation of engineers, surveyors, computer software experts and tunnelling machine manufacturers.
Kaalberg and his team are now 60% through an eight year pioneering research project to design and build an “intelligent” tunnel boring machine to drive the metro’s 3.8km underground section. It will be a TBM designed to interact with, and respond to, 3D computer analysis of building and subsurface movements during tunnelling. The aim is both to predict and reduce ground settlement. Working with German TBM manufacturer Herrenknecht, the team is designing a full face EPB tunnelling machine capable of exerting minimal subsoil disturbance – and therefore minimal surface settlement.
Kaalberg is confident that two $9.8M intelligent machines will be off the drawing board and in the ground ready to start the twin 5.8m finished diameter drives by the end of 2004.
It is a technical challenge that must not fail, for the possibility of causing damaging surface settlement is politically just not acceptable. City residents are well aware of the potential for disruption.
The first time metro construction was planned, early in the 1970s, tunnelling technology was much less developed and the likelihood of considerable settlement ruled out bored tunnels altogether. Unfortunately, the chosen alternative for an east-west line – forming the tunnels by sinking pneumatic caissons – demanded such widespread building demolition that it triggered riots in the streets from annoyed inhabitants. Ensuring good public relations this time, for construction of the total 9km north-south line running right beneath the city centre, is seen as a major priority.
Extremities of the line will run at grade or in cut and cover. But the central 3.8km section will be routed through twin tunnels driven at an average depth of 30m directly beneath Amsterdam’s main thoroughfares lined with many of the city’s most architecturally important buildings.
Four of the nine stations will be formed in large cover and cut boxes lined with diaphragm walling and excavated top down just metres from multi-storey 400-year-old buildings. Most buildings at risk in the city centre are listed structures and include Amsterdam’s ornate central station, several foreign embassies and Queen Beatrix’s royal palace.
To minimise the risk, the facades on many of these buildings now sport rows of small prism targets, read automatically by robotic theodolite work stations, erected high on the corners of neighbouring structures also within the settlement zone. Correlation with more distant stable targets allows readings to be standardised.
When fully commissioned next spring, the 74 Cyclops work stations, each with its own small onboard computer, will constantly search out and position over 5,000 prisms, sending readings back every hour to the client.
Curious as Amsterdam’s residents are at the sight of strange instrumentation being erected on their most prominent buildings, they are currently even more bemused by the linked preparation work under way to ensure the survey’s success.
Trees are being pruned along the city’s central boulevards, not with chainsaws but scissors, cutting away sections identified by 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. Such activities are essential in order for the theodolites to find their targets day or night.
With 60% of the work stations already operational, over one million readings have been recorded so far, transmitted by dedicated radio links to central computer banks.
Such readings are far from static. “Soft, butter-like mud” is how Christophe Bourlart, Soldata’s operations manager, describes the city’s underlying clays that leave the tall, narrow buildings con- stantly on the move. Despite their average 15m-deep timber piled foundations, some buildings settle up to 5mm a year in ground where, says Bourlart, “test drills and sheet piles sink under their own weight”. That is why, a full year before station construction starts and three before any tunnelling gets under way, Soldata – in joint venture with local contractor Grontmij – is busy erecting the targets on all properties within a 100m wide band of the route.
Each $35,000 robotic work station continuously reads up to 100 targets on surrounding buildings. Their positions are relayed 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. Measurements from groups of theodolites, accurate to 0.5mm, are currently fed to the client in four-hourly batches, though these will be transmitted instantly when tunnelling starts in 2004.
From these coordinates, the north-south metro team is creating a three dimensional real time computer model of building movements.
“It is surprising how much natural seasonal movement buildings can exhibit,” explains David Cook, project manager for British consultant Mott MacDonald, supporting the north-south metro team and overseeing the monitoring work. “It is important to record this background movement to indicate that it is not the consequence of any tunnelling work.”
In parallel with the settlement survey, city engineers are 80% through a structural check of all 2,000 buildings along the route. About a quarter have weak foundations and are now being renovated with internal bored piles tied to a new concrete ground slab. But any deep lying underground disturbance could cause the whole building to move no matter how good its foundations. So Soldata, a subsidiary of major French geotechnical contractor Soletanche Bachy, is also charged with sinking some 100 boreholes, positioned to within 1.5m of the future metro route, to record ground movements below, alongside and above the twin tunnels as they pass.
Drilled up to 50m deep, the 200mm diameter boreholes will each contain some 20 inclinometers and extensometers positioned at different levels down the tube. Readings will start about a month before tunnelling begins, intensify as the TBMs pass and continue for a full year afterwards.
Soldata is also sinking another 50 even deeper 70m long boreholes around the perimeter of the cover and cut station boxes. The aim here is for similar instrumentation to record ground movements as the station’s 45m deep diaphragm walled sides are formed, and also later when the box within them is excavated top down. Any tendency for the diaphragm walls to move inward could affect ground stability immediately outside the box triggering movement to roadside buildings less than 4m away.
“Most of these boreholes are being sunk in the middle of street pavements and must stay accessible for several years,” Bourlart points out. “Our most complex task is not actually sinking them, but carefully reinstating the pavement’s thousands of small mosaic tiles which must be fitted back together like a giant jigsaw.”
Such attention to detail both above and below ground is seen as essential in the complex technical battle to minimise settlement.
During Soldata’s six-year contract a staggering 150M ground and building movement readings will be fed to the client. Most will be used in the production of advanced computer programmes, key to the intelligent TBM’s role of predicting and controlling ground disturbance.
“This combination of real time monitoring linked to an interactive TBM is potentially the world’s most sophisticated settlement control technique yet developed for any construction project,” claims Kaalberg. By the time this central metro section is complete in 2007, the only movement noticed by Amsterdam’s public should be the theodolites still perched on building corners above them.
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