Canal Tunnels – Exercising foresight

In 2004, Great Britain won nine gold medals at the Athens Olympics. In 2005, Tony Blair won a third term as Prime Minister and then there was the World Cup in Germany in 2006 – but we won’t dwell on that. Writes Colin Carr

Whilst the world was focussed on these and many other events the Channel Tunnel Rail Link (CTRL), as part of the Thameslink Programme, was quietly digging underground, constructing two bored tunnels between the East Coast main line (ECML) at Belle Isle junction, just north of Kings Cross, and the St Pancras low level station at Canal junction. As the two tunnels pass about 15 metres under the Regents Canal, they are known as the Canal Tunnels.

Each tunnel was constructed with a six metre diameter bore and fitted with a pre-cast concrete lining, and they are both more than 660 metres in length. At the King’s Cross end there is a 100 metre cut-and-cover concrete box which leads up to an open area which, in total, forms a 1km length of new twin track railway.

Thameslink progressing

Since they were built, the tunnels have remained dormant. Elsewhere, the Thameslink project has been progressing steadily – reconstructing Blackfriars Station, building a new viaduct through Borough Market and developing the London Bridge Station and railway layout are just a few of the many schemes that make up this incredibly complex £6 billion project.

However, one of the key benefits that will be realised when the Thameslink project is completed will be the ability to run 24 trains per hour between Blackfriars to St Pancras Low Level which is known as the ‘core area’. This target will include 16 trains coming from the Midland main line route and eight trains from the East Coast main line, hence the need for the Canal Tunnels now to be fitted out and integrated into the operational railway.

In addition, the new Thames link Class 700 rolling stock is due to begin arriving in 2015, and is expected to be used on both the existing Thameslink and Great Northern routes. One of the Thameslink depots will be at Hornsey on the ECML. Therefore, the tunnels will be essential to enable the new trains to utilise the link for stock movements.



Contract awarded

Consequently, in August 2012, an announcement was made by Network Rail naming Carillion as the principal contractor for fitting out the tunnels and connecting them into the main lines. Balfour Beatty Rail would be responsible for the 25kV overhead line electrification (OLE) installation work and Carillion would install the slab track, associated emergency walkways, signalling and telecomsequipment, fire services and pumps and other associated safety equipment throughout the tunnels as well as being the overall site management.

Kevin Sullivan, Network Rail’s project manager responsible for this work, recently showed The Rail Engineer around the site. He explained that the work was progressing very well and that the construction of the slab track work in both the tunnels was now complete. Also, the installation of OLE equipment by Balfour Beatty Rail, using its innovative reduced-depth overhead conductor beam electrification system, was going to plan. Kevin explained that they were now concentrating on connecting the new emerging Up and Down Canal Tunnel lines into the existing mainline routes.

Collaborative working

Carillion appointed ARUP for all the design work. The tunnels are only six metres in diameter, which is a challenging space to fit everything in and the aim was to maintain W6A gauge. In addition, the tunnels will connect two different railway systems together so there is a need to design out any potential interference. For example, it would not go down well if, upon connection, there were track circuit failures on the ECML. As Kevin outlined, there has to be effective collaborative working between all parties and to ensure that this happened. “We have instigated weekly design integration meetings between all disciplines and Network Rail, a process that has proved to be very successful and invaluable,” he explained.

To ensure compliance with noise and vibration commitments, the track was designed using the Sonneville Low Vibration Track (LVT) system supplied by the Swiss manufacturer Vigier. This system is a duo block, slab track system with a rubber boot and concrete block pad. It has been tried and tested on other systems throughout the world but it is the first time that it has been used by Network Rail. The rubber booted blocks are cast into concrete exposing the concrete block pad which is designed to hold in place the rail and the pandrol E clip housings and insulations.

Challenging tolerances

The final positioning of the rails is very precise with only 1 to 2mm tolerance for gauge, rail incline and cant. As Kevin pointed out, this could only be achieved on a dedicated engineering site, as opposed to a track possession. The design life chosen for the concrete surrounding the rubber boots is 50 years and the design life for the sub base concrete is 125 years.

The signalling and telecommunications design is integrated with the Thameslink programme’s High Capacity Infrastructure (HCI), which ensures that the signalling system will deliver the targeted 24 trains per hour capacity in the core area.

A GSM-R radio system is being installed throughout the network to replace the Cab Secure Radio system currently in use. To ensure this will work well inside the tunnels, a ‘radiating cable’ or ‘leaky feeder’ is being installed throughout.

Walking through the tunnels, it is obvious that there is a steep gradient dipping down 1 in 34 toward the centre of the tunnel bores. This encourages any rain or seepage water to gather at this low point. Also, there is a 150mm fire main that runs through both tunnels with regularly spaced hydrants that can be accessed from the constructed walkway. So, if there was a fire and the fire main was utilised, the tunnels could be subject to flooding. Therefore, to cope with such a potential high volume of water, a 60 metre long sump has been installed incorporating fixed pumps that will pump water up the gradient to another intermediate sump which, in turn, has the capacity to pump water up into the existing East Coast main line drainage system.

The fixed walkway provides a continuous platform to enable day-to-day maintenance to take place and to provide a passageway in case of emergencies. A lighting system has been installed throughout the tunnels with lights spaced at every four metres above the walkway.

On the opposite side of the tunnel, there are two GRP troughing routes dedicated to signalling, telecoms and other mechanical and electrical equipment ensuring that everything looks neat and tidy and well ordered.

Continuous working

Work continues around the clock in two 12-hour shifts. Carillion works the day shift installing the slab track and other equipment, then the site changes over to Balfour Beatty Rail activities, using rail mounted access platforms to install the Conductor Beam OLE equipment. Kevin said that this system works very well and that each contractor has tried hard to ensure that its activity does not impede other work that has to take place.

So, the fitting out work in the tunnels appears to be progressing very well and targets are being met. The next challenges for the team were the connection work at each end of the tunnel into the main lines. The junction at Belle Isle is conventional ballasted track whilst, at the St Pancras Low Level station end, the junction is on LVT concrete slab track.

Kevin described the project as being unique in that there is a fascinating and challenging engineering project in the tunnels, a site that they have control of 24 hours a day. However, at the two end connection points of the site, there is a railway so it is necessary to work in possessions, being aware of hazards such as adjacent line working where traffic is running. This introduces a whole set of different risks and challenges.

At Belle Isle Junction, existing structures have had to be demolished and new ones installed. The existing sheet piled wall of the ECML railway has to be removed to make way for the new connection. The junction is situated in a fairly confined location between Copenhagen Tunnel and the Gaswork Tunnels just outside Kings Cross station.

In preparation for the connection of the new junction, a set of switches has had to be moved six metres north to accommodate realignment of the North London Incline. In addition, 140 metres of plain line has had to be renewed and a crossover was repositioned 30 metres north of its original position during an Easter 2014 blockade.

Connecting into the main lines

So, the site is nearly ready to receive the new double junction which will be installed using a Kirov crane to lift track panels that are being constructed alongside the running railway. After the new junction is in place, it will be connected to plain line track that will be laid on a transitional formation of ballast. This will then lead onto a formation of glued ballast, then onto slab track, before running into the Canal tunnels. The completed work will then be ready for commissioning with control located in the Kings Cross Panel.

The Canal Junction end of the site is now all LVT slab track, installed some time ago. It is where the Moorgate lines emerge from the lower station at St Pancras, switches and crossings had to be relocated and aligned and replaced by plain line. Once this work is complete, the junction can be commissioned onto the new Three Bridges Rail Operating Centre (TB ROC) which will then incorporate the whole of the new railway. The target date for the completion of this work is early 2015.

The commissioning of the tunnels is yet another step in this most fascinating project known as Thameslink, and one has to admire the foresight exercised in 2004 which ensured that the two tunnels would be ready and in place. The fitting out work will be completed this year, the Hornsey Depot will be completed in 2015 and the Siemens Thameslink Class 700 trains will begin delivery and then in 2018 services will run through the Thameslink core. Meanwhile, there is plenty to do, both within well-defined engineering sites and alongside the operational railway, which will provide more unique opportunities for the engineering teams involved.




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