Tunnelling takes place largely underground. That may seem to be an obvious statement, but it also means that the work is mostly out of sight. True, there are areas of activity at the portals, and there are trains or lorries taking the spoil away. But further along the tunnel’s route, there is nothing to be seen above ground.
Out of sight, however, doesn’t mean out of mind. While the general public could well be oblivious to what is taking place beneath their feet, engineers are paid to worry. One set will be worrying about the tunnelling itself.
Is it on schedule? Is it on the correct route? Is the spoil being removed correctly? Are the concrete lining segments available in sufficient quantities?
Another team worries about the overground. Is the tunnelling work affecting any structures above ground? Or, in London’s case, is it affecting other tunnels (underground railway, sewers, power) in the area?
However many calculations are made, the only true way to know is to monitor those structures and see if they move.
Crossrail is the biggest tunnelling project underway at present. Going under central London, it is not surprising that the programme of structures monitoring is equally large-scale.
One element of the work is the construction of the station tunnels at Whitechapel and Liverpool Street station 30 – 40 metres below the city (known as contract C510). This is being undertaken by BBMV, a joint venture of Balfour Beatty, Alpine BeMo Tunnelling, Morgan Sindall and Vinci Construction Grands Projets. Work on this £250 million contract began in April 2011 and is due for completion in 2015, three years ahead of the route opening.
Managing the risk of movement caused by this tunnelling was one of BBMV’s first challenges. It needed to know the baseline levels and positions of buildings and streets above contract C510, and up to 100 metres around it, ahead of construction commencing. And, of course, this information had to be recorded accurately and repeatedly at the start of the contract, during construction and, potentially, for a number of years after completion.
Instruments called ‘total stations’ are widely used on construction sites to measure and record positions. These measure the distance, angle and slope to a reflecting prism and so, when used repeatedly, can detect any shift in the position of that prism relative to the instrument. Automatic total stations can be programmed to monitor a number of prisms, and to do so at regular intervals.
More than 100 of these instruments were supplied for this contract by Topcon to automatically measure movement above ground and manually check tunnel alignment below ground.
For monitoring movement above ground, the challenge was not only in achieving the required accuracy and reliability of results, it was in accessing the instruments and the information measured. Many of the instruments had to be fixed onto the side of buildings from where they collectively measured the exact location of thousands of prisms fixed to other critical structures.
The total stations were installed on buildings such as schools, office and residential blocks and supermarkets. They were mounted at high level using Mobile Elevating Work Platforms (MEWPs). “Once we identified which buildings needed monitoring, prisms were located on these buildings and total stations installed on others,” said BBMV’s engineering survey manager Alastair Cruickshank. “One of our main concerns was always about logistics. How will we reach that total station location? Will we require a road closure? Can the total station see the prisms we need to survey?”
Crucially, once the total stations had been installed (they also need to be connected to a reliable electricity supply), Alastair and his team wanted to avoid having to revisit them due to the time involved in organising road closures, notifying building owners and the cost of hiring plant and trained personnel. One reason for revisiting an instrument could be if additional prisms were installed on new structures to be monitored, requiring the instrument to be manually programmed to measure the position of each new prism.
An automatic total station manufactured by Topcon provided a solution to this problem. The MS05AX model has a function called ‘matrix detection’ which can routinely scan for new prisms. It is a simple function, but one which was not standard on other total stations on the market.
“Using the latest matrix detection technology from Topcon, the automatic total stations can scan and locate the position of each prism – the instrument finds all the prisms,” Alastair recalled.
Without this function, the task of reprogramming each total station to measure every newly placed prism over four years would have significantly impacted on the overall construction programme. “Some locations are very difficult to revisit – the school can’t be accessed during school hours and another near a railway line can only be accessed at night. The roads around Liverpool Street are also very congested making daytime access difficult,” he added.
“Using the MS05AX, we saved a lot of time and we have a system which allows us to add more prisms in any order as necessary and there’s no concern over whether the total station has missed any.
“All other instruments on the market were very similar in terms of specification and price but matrix detection was the big difference – it was one of the main reasons for going with Topcon and it singularly was worth much more to us than the value of the instrument itself. It means we save many man hours which would otherwise be lost to resetting instruments to find prisms.”
Prisms are occasionally lost when buildings are repainted, for example, so the matrix detection function is still being used to scan for additional prisms 18 months after the start of construction.
Out on location, the robotic total stations have to withstand wind, rain and even the occasional indiscretion by pigeons. The total stations are encased in a thin wire mesh cage in some locations to deter pigeons from landing on them.
Each total station works by routinely and automatically spinning 360 degrees on its base and locating and measuring the angle and distance of up to 100 prisms within its sightline. So accurate are the instruments that, on occasion, millimetres of rainfall have been detected after a thunderstorm.
“The MS05AX is one of our highest precision instruments,” says Topcon (GB) technical support manager Peter Roberts.
“Typically, most instruments used on construction sites work within a 3-5mm range of accuracy. This instrument works to better than 1mm.”
Readings are sent to a central computer and can be scrutinised by an engineer to assess whether any movement has taken place. The instrument may be programmed to use different software, allowing the engineer to use whichever is most familiar to him or her, or can be run using Topcon’s software. On contract C510, monitoring software is provided by Sol-data.
Information from total stations is reviewed every 12 hours (although measurements are taken much more regularly – every 15 minutes, if necessary) and if any movement has been detected, established trigger values are consulted and the construction process altered if necessary.
Tiny amounts of movement may be acceptable, but even a small deviation might prompt a change in the tunnelling procedure to counter the movement. More movement, closer to a ‘trigger’ value, will generate an alert.
Eighteen months into the project and there are 60 total stations above Whitechapel and Liverpool Street Stations monitoring movement and 19 in operation within the station tunnels checking alignment. Ten are used for general surveying across the site.
Alastair Cruickshank is full of praise for the equipment, particularly in the tunnel environment where instruments have to perform reliably while subject to vibration and dust, and in the dark. Readings are consistent and the project team is regularly impressed with the MS05AX’s accuracy and robustness. “We know exactly when excavation has started because there is a movement of about 0.1mm,” he says.
Tunnelling recently started under the Hammersmith and City Tube Line near Liverpool Street Station where ground movement will be closely monitored to ensure the existing infrastructure and public are safe. There is also compensation grouting on this site to reinstate stresses in the ground which may be temporarily lost due to tunnelling. Ground movement before and after compensation grouting is also scrutinised to ensure the ground is responding as predicted.
So, in fact, it is possible to see tunnelling works from above ground, by looking closely at the sides of buildings.