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Surveillance operation

SITE INVESTIGATION

One of Britain's most intensive ground movement surveys is checking that only trains and commuters are on the move at two of London's main line stations. David Hayward reports from King's Cross.

Pierre Salas is seldom far from his mobile phone.

There is one call that, if it comes, he must not miss. The computer triggered message would tell him that something is moving at King's Cross Station.

It is one of London's most hectic transport interchanges, with up to 20,000 people every hour swarming between mainline trains, four levels of underground tube station and traffic choked streets.

But Salas is not concerned with the movements of travellers, or trains. It is any small displacement in the infrastructure that surrounds and supports them that he needs to know about.

As project manager for structure monitoring specialist Sol Data, the French engineer is in charge of the most concentrated building and ground movement survey under way in the UK. Over the coming months there is little made of brick, concrete, iron or steel - both above and below the congested multi layer terminus - that will move more than 1mm without Salas and his monitoring team knowing all about it.

Hundreds of measurement targets are being fixed to the brick facades of the area's main buildings, including grade one listed Victorian structures. The distinctive double arch King's Cross train shed, the even more impressive St Pancras Station next door and the seven storey Great Northern Hotel sandwiched between them, are all being discreetly covered with 60mm diameter mirrored prisms. These allow automated theodolites, mounted equally discreetly on nearby structures, to continuously record any small movements of facades or walls.

Any deviation in the maze of tube tunnels beneath the complex will similarly be recorded by targets and theodolites attached to the walls of the Piccadilly, Northern and Victoria Underground lines.

The reason for such intense monitoring is that, over the next few years, this already densely inhabited underground terrain will be further invaded with an array of new ticket halls, pedestrian concourses, subway tunnels and lift shafts as the whole passenger flow network is improved.

Steve Macklin, senior engineer for lead designer Arup, says: 'The joint drivers for the upgrade are recommendations following the 1987 King's Cross station fire and the soon to arrive Channel Tunnel Rail Link. Our aim is to increase passenger capacity, improve fire safety standards and provide better disabled access.' Phase one of the two stage £300M revamp, for London Underground's project manager Infraco SSL, is now under way. A new two level ticket hall is being built in front of St Pancras, which is destined in 2006 to become the CTRL's northern terminus.

Existing tube stations and King's Cross ticket hall are being upgraded.

The second phase, which has just started, includes another new ticket hall excavated 30m deep by the side of King's Cross. A further passenger interchange level will be needed and an even more complex array of pedestrian subways, sloping escalator tunnels and lift shafts built to link new and existing ticket halls beneath both stations (see box).

Sol Data's £2.6M contract involves establishing the monitoring regime for this second construction phase when underground activity will reach a maximum. The company is hopeful of this month being awarded further work to survey all above and below ground movements until at least six months after construction ceases in 2006.

'The main challenge in such a congested area is to establish the most practical and economic sight line layout for our theodolites and targets, ' explains Pierre Salas. 'Our aim is for each theodolite network to continuously read up to 100 targets including fixed reference points.' Up to eight £25,000 Cyclops theodolites, each with an onboard computer, will monitor above ground structures, with a further six erected in underground stations. Over 1000 targets are being fitted to building facades, station walls, platforms and even track sleepers in both surface and underground stations.

A proportion of the 50 fixed reference targets, mounted on structures outside the influence of any underground activity, will be read by each Cyclops as it revolves to take a reading of a target in its network every 12 seconds. The three dimensional position of each target is then computed using co-ordinates fed by radio link to a 100 gigabyte bank of seven computers in Sol Data's nearby site offices.

The problem of theodolites inside King's Cross not being able to read outside reference targets is solved by fixing double prisms either side of the station's glazed front facade. Cyclops units inside the station will read the inner target while others outside monitor the outer prism just millimetres away. This allows overlap between the two sets of readings to offer a fixed reference for the station theodolites.

In underground stations such fixed reference targets are not practical. Here theodolites, attached to station tunnel crowns, will monitor targets on the track and around the circumference of the surrounding tunnel.

This enables engineers to determine any relative distortion of the tunnel lining and changes in track geometry.

When completely operational the contractor's site computer room will be receiving a staggering 20,000 readings every hour.

Each, when converted into usable co-ordinates, is available roughly one hour after it is taken, though Salas emphasises that, if needed, this can be reduced to 30 minutes.

Such a high volume of information could easily have become a liability rather than an asset.

So the entire study area is split into a dozen major interest zones - typically key building facades - with acceptable movement levels estimated for each, dependant on the phase of construction activity beneath.

This means engineers need not really get excited until movements of, for example, more than 2mm are recorded, allowing them to considerably reduce the number of measurements distributed to the client and relevant contractors.

Even so, Sol Data is already producing half a dozen weekly reports detailing graphs and charts of potential hot spots, before any of the main underground disturbance has started.

Each area has trigger values - ranging upwards from 5mm initial movement for a warning and a further 5mm for action.

If a warning trigger level is reached, a computer generated alarm signal is sent day or night to the core survey team. This is the important phone message to Salas, which sets in train a clearly defined process of consultation and action to determine what is happening and why.

'It could be nothing more than a pigeon sitting on a theodolite or adverse weather conditions, ' says Salas, emphasising the caution needed.

Given such a sophisticated system the possibility of technical problems is also not overlooked.

As backup to the automated readings, Sol Data has brought in a team of surveyors, Plowman Craven Associates, to take repeated manual levels of all the structures referencing them back to some 600 fixed points.

However, movement is inevitable, claims Arup's Steve Macklin. One new lift shaft will be barely 200mm from a Northern Line tube tunnel and subways will be dug just a few metres from building foundations - typically concrete strip footings supporting corbelled brick walling.

And with simultaneous construction activity - such as tunnelling from the base of a piled ticket hall cofferdam still being excavated - likely to produce accumulated ground disturbance, Macklin estimates several hot spots could settle up to 30mm.

Such subsidence is likely to prove 'acceptable' and Arup's contour map of predicted ground movements indicates only two areas where close monitoring will be required.

The important central 120m long spine wall of King's Cross is a multi arch brick support enclosing the hidden cast iron columns from which the station's imposing wrought iron arch roof spans either side. But Arup's settlement analysis for a new 7m diameter concourse tunnel driven beneath the wall still leaves it secure.

Similarly, a section of the nearby Great Northern hotel's curved facade will be affected by the vast 9m diameter barrel of an inclined escalator tunnel excavated 6m beneath one corner of the hotel.

Any resulting settlement here could demand hotel foundations being supported.

Such detailed predictions are the outcome of an intensive study by Arup of the entire 1.3ha area.

Two years of multidisciplinary brainwork has included three dimensional analysis of likely movements stretching ahead some 120 years.

This long term analysis - including predictions of how changing strain paths would effect soil stiffness - suggests that eventual settlement will increase by some 20% but will always remain at acceptable levels.

Despite all the construction activity, there will be no serious movements in buildings and tunnels.

'But this major monitoring exercise is all about risk management and ensuring confidence among numerous third parties, ' Macklin stresses.

'Disrupting trains above or below ground would result in punitive penalties, and damage to listed structures would not be acceptable. The real time ground monitoring is essential in reassuring owners and operators.'

The big squeeze

The 1.3ha area beneath St Pancras and King's Cross stations is already crammed with multiple layers of tube tunnels and passenger concourses, including escalators and lift shafts.

Disused steam train tunnels, sewer culverts, major gas mains and ancillary services further congest the ground space.

But, over the next four years, considerably more of the same must be squeezed into the limited vertical space between the two 'untouchables' - surface train tracks and underground tunnels.

This includes an estimated 20 new pedestrian tunnel sections - totalling 200m - along with four large shafts and a 30m deep ticket hall excavation.

Yet throughout this £300M project train services must not be disrupted and the grade one listed train sheds of King's Cross and St Pancras - plus the curved facade of the Great Northern hotel between them - must suffer no structural damage.

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