The Permanent Way is a diverse concept. There is the traditional formation, cess and drainage; there are embankments and cuttings, bridges and viaducts and, when necessary, there is the option, albeit an expensive and always a challenging one, to create a pathway by constructing a tunnel.
Very few tunnels are straightforward, each one having its own unique challenges and character. The excellent book by Thomas A Walker on the construction of the Severn Tunnel helps readers to understand the challenges engineers faced when tunnelling in the nineteenth century. A walk through Standedge Tunnel that links the line across the Pennines between Manchester and Leeds with the older canal tunnel below, or even a study of the mesmerising 3D maps of London’s underground stations and services, cannot fail to impress.
Many urban transport systems are heavily dependent on tunnels to provide an adequate pathway through the myriad of culverts, sewers, cable routes and piled foundations. Crossrail, probably the biggest engineering project in Europe, must be one of the most complex and challenging tunnelling undertakings today.
The Channel Tunnel provides the only high speed route from the UK into Europe. Plans for HS2 are being developed which will extend this route from London to the north of the country. To ensure that it is less invasive on the landscape and more acceptable to local communities, a significant proportion of the proposed route will be provided through tunnels.
Significant proposals are being developed involving railway tunnels across Europe and tunnelling is now becoming ever more popular in the armoury of options for a railway engineer and that is why THE RAIL ENGINEER attended a recent Tunnelling Conference held at the Institution of Civil Engineers.
The scene was set by Matt Sykes, ARUP’s leader for tunnel design, who focussed on the huge potential that tunnelling offers the civil engineer not only in this country but throughout the world. In Shanghai alone, at this moment, there are 39 Tunnel Boring Machines (TBMs) in the ground with a further 20 or more, ready to be put into operation.
As well as the major UK rail projects, Crossrail, HS2, London Underground upgrades and Light Rail, there are many others; power station new builds, cable tunnels, Thames Tideway Tunnel, Lee Tunnel and, taking just one example from abroad, the fascinating immersed tunnel project linking Denmark with Germany.
Investing in the future
The opportunities are clearly there and that is why the new Tunnelling & Underground Construction Academy (TUCA), based in Ilford, London, has been created. It is a key initiative designed to promote excellence in underground construction which will help to address the need for an adequate skill supply of young, innovative engineers to ensure that every opportunity is taken to maximise this potential flow of tunnelling work.
To highlight the emerging opportunities for young engineers, it was mentioned that when Brunel was only 20 years old he designed and built the first tunnel under a navigable river, the Thames Tunnel. He then went on to design and construct the then longest tunnel in the world, Box Tunnel, when he was 30. Will this next tranche of tunnelling projects offer similar opportunities for innovation for our emerging young engineers?
New build railways
Many projects were discussed at the conference, several of which were railway related.
Leading off with HS2, Tony Walker, railway business director for Mott McDonald, outlined the progress that was being made with this scheme. The first phase is the development of a Hybrid Bill that will be put to Parliament so that a final route can be agreed by 2014. This Bill will include the route selected, land required, initial design and environmental impact for a high speed railway to the Midlands.
Within this first phase will be significant lengths of twin bore tunnel designed to accommodate trains running up to speeds of 400km/hr. To address the pressure wave gradient changes that will emerge from trains entering a confined space at such speed, porous portals will be constructed along with tunnel shafts every 2km. Although details are a little vague at this time, tunnelling between Euston and Old Oak Common and other locations including Amersham and Northolt offer in the region of 36km of tunnelling for the first phase.
Opportunities for young engineers
The overall tunnel mileage for this project will be significant. Last September, work started on the second phase of the project, extending the route to Heathrow airport to Manchester and Leeds. The plan is to complete phase1 by 2026 and stage 2 by 2033 – a significant period of work to excite any budding young tunnelling engineer.
During the phase 1 process, engineers will have to reconsider the concept of whole life costing for a tunnel. Many tunnels built in the Brunel era are still providing great value for money whereas some younger tunnel structures have caused problems. Quite rightly, HS2 is demanding a far more rigorous review of maintenance costs so that a truer value can be attributed to the overall cost of tunnel construction when compared with other options. It is possible that tunnelling could come out of this exercise in quite a favourable light.
A concise update of the Crossrail project was given by Paul Glass, technical director for the BAM Ferrovial Kier joint venture (JV). On the 118km route, 21km of the railway is within 7.1m dia. twin bored tunnels. There are three tunnelling contracts and the JV has the west end Royal Oak Portal to Farringdon Station section. Two tunnel boring machines (TBMs) are being used, one known as Phyllis and the other Ada, each about 150m long and weighing over 1000 tonnes.
A tight squeeze for Phyllis
Phyllis is now happily underground but it was a tight squeeze. The TBM had to be moved 400 metres into the portal at Westbourne Park. The journey started with Phyllis sitting on a transporter, passing under a footbridge which had to be jacked up in a possession. To progress further, the TBM was lowered onto skids to travel under gantries with a 12mm clearance, then once into the portal it was jacked down into the launch pit ready for boring. Subsequently, the machine has navigated bridge pile foundations, sewer culvert inverts, water mains, gas mains, the A40 flyover and the Hammersmith & City Line.
Once Phyllis had reached the Paddington Box, Ada followed and they are now both progressing under Hyde Park. Every movement and pressure change on both TBMs is monitored closely at all times. This includes face pressure, propulsion rate, cutter torque, belt weight and foam injection as well as grout volume pressure. Trends are being analysed continuously and checked against pre determined acceptable limits.
Precision instruments have been provided by specialist companies. One company, Soldata, is providing 75 Automatic Total Stations working in groups for up to 4 to 5 years within the areas that could be influenced by the TBMs. They are also providing water settlement cells, inclinometers, rod extensometers, tilt and crack meters. This is supported by a round the clock data management service.
Crossrail is using prisms to monitor rail movement and shape arrays for culverts. To date, ground movement has been contained to within 3 to 4mm and volume loss is 0.34%. They are both well within the specified tolerances and the 50mm per minute progress target for the TBMs is being achieved.
At Tottenham Court Road station and Bond Street, major changes are underway. Due to the congestion of tunnels in these station areas and the ever changing shape and incline of the tunnels over short distances, accommodating escalators, walkways, staircases, and service shafts has meant that sprayed concrete techniques are becoming very popular.
However, in some instances there is no option but to use old tried and tested mining techniques.
Keeping neighbours on side is a high priority and, unfortunately, some of the techniques used do have a tendency to create noise that can travel into adjacent properties. This calls on the skills of other suppliers such as Bruel & Kjaer which were keen to share their expertise in monitoring urban and industrial noise at the event.
Moving further afield, an intriguing project shortly to go out to tender is the fixed link tunnel between the German offshore island of Fehmarm with the Danish island of Lolland in the Baltic Sea. The proposed tunnel link will be approximately twenty-one kilometres long and it will be made up of nine elements, each 217 metres long and consisting of two sections carrying a single railway line and two sections carrying a dual carriage roadway.
Each element will be constructed in a specially made harbour then floated out to sea, flooded and lowered into a dredged trench in the seabed. Water will then be pumped out of each element and they will be sealed together and covered. This will take place 35 metres below the sea level and will pose a significant engineering challenge.
The technique has been used before, but not to this scale. Invitations to tender for a design and construct contract will be issued early in 2013 and the project value is estimated at £4.6 billion. No doubt a strong consortium will be required to deliver such a complex undertaking.
For many experienced railway engineers, the thought of a railway tunnel conjures up an image of dark cold weekends, soot and grime, loose brickwork and falling stonework, bulging linings and sickly diesel fumes. That image will not disappear but there is now another world of tunnelling which involves new build, ingenuity, practical endeavour and prestige. It’s a real opportunity for any budding, young, engineer.