Close control of ground movements is key to the success of CTRL's construction, particularly while tunnelling beneath London, the River Thames and the North Downs.
'Building the London Tunnels is like taking a 20km slice through London, ' says Rail Link Engineering settlement team manager Keith Bowers.
'Inevitably, we were going to cross every type of infrastructure, ' he adds. In the course of running from Barking in east London to just north of St Pancras, the twin bored tunnels will pass beneath 3,000 buildings, 60 bridges, 600 utilities and under or over eight tunnels (six London Underground lines and two mainline railway lines) and waterways.
'A significant proportion of the job (around half), is under or close to existing railway infrastructure, ' Bowers says. The London Tunnels follow existing transport corridors as close as possible to minimise disruption and risk of structural damage through settlement.
'The settlement team has two main responsibilities, ' explains Bowers. 'First, we have to predict what ground movements may occur as a result of tunnelling and how this will affect infrastructure. In certain cases, movement will be unacceptable - we then have to work out what we can do to deal with it, either by mitigation measures or design changes.' The team's other responsibility is monitoring, mainly of infrastructure. 'We have to choose the appropriate monitoring systems for the project, so that it is not swamped in data but that there is enough to fully inform the construction team, ' Bowers says.
Assessment and monitoring is a three-stage process, which successively focuses on structures most at risk. This was specified in the development agreement. 'It is a continuation of those developed and used on London's Jubilee Line Extension and the Crossrail project, ' Bowers says. The CTRL process has evolved from this to some extent, 'although there are differences because of specific aspects of this project', he adds.
For instance, the tunnel corridor was set slightly wider than the expected zone of influence of construction. Owners within that corridor can obtain a legal deed that is basically a statement spelling out what is happening, what monitoring will be carried out, what mitigation there will be, if it is needed, and an assurance on repair. 'This means we have specific agreements with Railtrack, London Underground and other big infrastructure owners, as well as householders, ' says Bowers.
'First we carry out a quick and simple assessment - which gives a conservative prediction of likely movement, ' Bowers explains. This 'greenfield' assessment predicts ground movements due to tunnel construction, ignoring soil-structure interaction and giving the widest zone of influence.
The tunnels' centre lines are drawn on a map and settlement contours plotted. Any structure or property falling on or above the 5mm settlement contour is identified and a defects survey carried out - 'mainly to counter any false claims for damage', says Bowers. A monitoring proposal is then prepared.
Even then, concern is low, he adds. 'Any property at low risk can be put to one side, allowing focus to switch to those structures more at risk.' A stage-two assessment is triggered when a structure lies on or above the 10mm settlement contour. This varies according to the type of structure. For buildings, which are mostly masonry structures, the starting point for modelling laid down in the CTRL Act is Boscardin and Cording's:
Building response to excavationinduced settlement.
'But more recent work has caused changes to best practice.
As the project is committed to using best practice or best endeavours, we are using experience from previous projects to ensure this happens, ' says Bowers.
Buildings are assigned damage categories based on strain predictions. Category two buildings have the risk of slight damage and category three have the risk of moderate damage. 'If a category three is predicted, then a stagethree assessment is automatically triggered, ' says Bowers.
Parallel processes have been developed for other infrastructure. RLE worked closely with Transco (gas) and Thames Water to come up with damage categories for their utilities. For railway structures, site specific assessment is realistically the best way of working out risk, Bowers says. And for industrial properties, any specialised plant is examined to define potential problems and how to deal with them. Derelict buildings carry a different set of problems.
But as tunnels are deep, the settlement trough is shallow and wide so the effect on structures is slight, Bowers says. Even the temporary dewatering being carried out for tunnelling in Thanet Sand will only cause minimal settlement spread over hundreds of metres or even kilometres. 'Very few structures are category three - most are category one and some category two, ' Bowers says.
'By the time we get to stage three, we are dealing with exceptions. After this stage we may conclude that protection work is needed.' Some utilities will be protected because any damage could have serious consequences, he adds.
'Inevitably, there are a few hot spots, ' he says. One major example of a category-three structure is the A13 Alfred's Way Bridge in Barking. 'The tunnels will pass through the bridge's piled foundations so something had to be done.' Work will involve retrofitting of the foundations.
The mitigation strategy is primarily based on tunnel boring machine (TBM) design 'to minimise movement at source', Bowers explains. The philosophy is to spend money on the TBMs, rather than on techniques such as compensation grouting. 'We have a high expectation of their performance, ' he says.
'We have done quite a lot of work to see where ground movement occurs on a TBM.' There are five main areas of movement: at the face; around the shield due to over-cutting; at the 'tailskin' (back of shield); that due to tunnel lining ring flexure and long- term consolidation effects.
Most of the movement occurs around the tailskin, Bowers says.
'On the JLE, grouting was carried out through completed tunnel lining but due to time delays, a void still formed.' The tunnels will be built by pairs of earth pressure balance tunnel boring machines. These are supplied by Canadian manufacturer Lovat (for Contract 250), by German firm Wirth (for Contract 240) and by Japanese firm Kawasaki (for Contract 220). 'All the machines are being built to the same specification, ' explains Bowers, 'although they will vary slightly according to the geology they have to deal with'.
He adds: 'Machine specification is designed to pick these off.' Earth pressure balance and the rapid tunnelling rate of 100m every week, deals with face movement and ports in the shield allow fluid injection, for example, bentonite polymer, to fill the void around it.
Grout is injected along the tailskin in such a way that the machine cannot move forward until the void has been filled - a zone of unsupported ground is not allowed to form. 'This is state ofthe art, ' says Bowers. 'The technology is well ahead of previous London tunnels.' Lining deflection is minimised by using a stiff lining and employing strict building controls and consolidation effects are not a major problem because the tunnel is being built in dewatered sand.
The primary monitoring tool will be precise levelling. 'This generates a lot of data but not huge amounts, ' Bowers says.
RLE spent time with contractors to establish monitoring frequencies, interpretation procedures and data reporting.
A suite of borehole instruments, including inclinometers and extensometers, will also be used. These will be targeted at early parts of the drive to help understand the tunnelling operation, at zones of geological change - where tunnel drives change between sand and clay and tunnelling modes change as a result, and also targeted around specific structures.
'The philosophy is to try and find methods that ensure control and management of risk, rather than trying to be formulaic, ' says Bowers. This means that certain types of instruments will not be relied on and the most appropriate instruments will be used.
'We have gone for a blend of instruments on this project. We focus on what the risks are, look at the best way of monitoring them and pick the best system, including high and low tech, ' he says.