Written by Paul Insley, Senior Engineer, Balfour Beatty Rail
Approximately ten years ago, Channel Tunnel Rail Link (CTRL), on behalf of the Thameslink Programme, constructed 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.
Each tunnel was constructed with a six metre diameter bore, about 500 metres long and pre-cast lined, and they form part of a new twin track railway approximately 900 metres long, the remainder being cut and cover boxes and open sections.
The two tunnels pass under the Regents Canal and have subsequently been called Canal Tunnels. At each end of the Canal Tunnels are double junctions.
The junction at Belle Isle will be conventional ballasted trackform whilst at the St Pancras Low Level station end the junction is on a concrete track slab with resiliently mounted supports.
No services such as track work, lighting, power and emergency walkways were constructed as part of the original tunnel construction and, until 2006/2007, Network Rail had not obtained planning permission, legal powers and funding for the Key Output 2 works, of which this forms part.
The Thameslink Canal Tunnels are part of a scheme to enable Thameslink services from Peterborough and King’s Lynn to travel south of the Thames and return.
The Canal Tunnels project is a vital component of the overall Thameslink programme. It will deliver the infrastructure required to run up to 24 trains per hour per direction in the core area (between Blackfriars Junction to the south and Kentish Town to the north), permit longer 12 car trains to operate and allow more destinations to be served by Thameslink services.
As part of the advanced staging works, Balfour Beatty Rail was asked to install two NR60 D 13.5 slab track turnouts, designed to reduce vibration in nearby buildings at the northern end of the platforms.
This scheme is known as Canal Tunnels Junction. The existing Up and Down Moorgate plain line tracks had previously been fitted with Vanguard baseplated slab track mounted on approximately 10-metre-long reinforced concrete slabs.
Six of these slabs were to be removed on each line to allow the installation of the two new turnouts. All of this had to be done without any speed or operational disruption to the existing Thameslink services.
Design and Planning
Balfour Beatty Rail had to design and plan the job very carefully. Not only did they have to maintain Thameslink services, but the new track would have to meet stringent ground borne-vibration requirements due the projects’ proximity to existing and new residential building developments.
This requirement resulted in the design team, in partnership with Network Rail, selecting the Sonneville Low Vibration Track (LVT) system from Swiss manufacturer, Vigier.
This system is a duo block slab track system with a rubber boot and block pad and has been used extensively in tunnels worldwide.
The main works were planned to take place in four key 53-hour possessions, backed up with a small number of limited mid week 3.5 hour working windows. At the end of each key possession, the track had to be handed back in complete working order and with no temporary speed restrictions.
In addition, as the work was adjacent to the platforms at St Pancras, dust and noise were issues and as the site is underground, access and logistics were critical.
The original 10-metre long reinforced concrete slabs were at least half a metre deep and had been in service for a number of years, so careful structural design and system interaction was required. The first option considered was to reuse the existing slabs as part of the design, using directly mounted baseplates, but this was not deemed acceptable for this project.
Complete removal and re-cast was considered but to remove them entirely would have required the use of concrete breakers. Previous experience of similar works at Thameslink Clerkenwell suggested that this would not be practical given the time constraints and that a significant amount of manual breaking out would also be anticipated with the associated exposure of staff to hand-arm vibration, noise and dust.
Working in partnership with consultants Heierli Consulting Engineers, a plan was developed to slice the concrete slabs in half and lift off the top half to allow a new surface to be recast. A number of methods of removal were considered as part of the risk assessed design.
Using disc-saws was not seen as practical while implementing breaking equipment was too time consuming and would have created unacceptable amounts of dust and debris. Diamond wire cutting was identified as the best way to cut through the track slabs entirely.
This was supported with associated securing works to hold the track in place after each cut. Selecting this design meant that the risks of exposure to dust, hand arm vibration and noise would be minimal and it would also allow work to carry on in mid-week nights and weekends, with the track handed back for operations every day with no requirement for speed restrictions.
Diamond wire cutting is a specialist activity requiring few personnel but a controlled environment. It offers speed and significant noise and dust reductions over other methods. A cooling / lubricating water supply is needed, and exclusion zones have to be enforced during the cutting operation.
It is also a system that could be implemented in mid-week night possessions, leaving free time in the key 53 hour possessions to focus on heavy engineering works.
During the key possessions, the pre-cut slabs were lifted clear, and replaced with a one-piece slab that covered the footprint of the whole turnout. The straight through track of the Switches and Crossings (S&C) was replaced by plain line fitted on adaptor baseplates so that railway operations could continue.
The plain line trackwork and S&C track slab was designed and installed in a manner which left the infrastructure in a position that will facilitate a method of speedy installation for the switches, crossing and points system without greatly impeding the Thameslink services in any way at a later date.
The design and construction methodology used allowed the advanced programme construction works to be undertaken safely in a short number of key weekend possessions whilst meeting the stringent environmental design constraints of working in tunnels on one of London’s busiest commuter railway lines.