Britain’s canals formed the country’s first high-capacity transport network. In their day, the raw materials they carried laid the foundations for the industrial revolution.
In some places, they also transformed passenger travel. Hundreds of thousands of passengers took the eight-hour Swiftboat journey between Edinburgh and Glasgow, hauled by galloping horses on Scotland’s lowland canals.
The alternative was a slower, bone-jarring stagecoach over the rough roads of the time.
Railway canal crossings
Canal engineers faced significant challenges but at least had the advantage of a country almost free from man-made obstructions. Roads carried minimal traffic and could be crossed by simple swing bridges. For this reason, the Forth and Clyde (F&C) canal was able to carry tall-masted ships bound from Glasgow to European North Sea ports across the centre of Scotland. Opened in 1790, this canal was the world’s first sea-to-sea canal.
Railway builders had the canals to contend with. In London, the need to cross the Union Canal resulted in a steep descent from Camden into Euston, a tunnel under the canal before Kings Cross and an elevated station at St. Pancras. The tall ships of Scotland’s F&C canal required railways to pass under it unless a railway swing bridge was provided. When the Scottish Central Railway to Perth was opened in 1850, its passage under the canal at Carmuirs required cuttings and two 40 metre long single-bore tunnels.
The F&C canal was closed in 1963 and re-opened in 2001 as part of a Millennium project which also re-opened the Union canal to Edinburgh and connected the two canals by the Falkirk Wheel which is adjacent to Carmuirs. Unfortunately, in 2002, shortly after the Wheel and its canal basin were opened, there was a slope failure, depositing tonnes of rubble which almost blocked the mouth of the tunnel. This was due to saturated ground and resulted in the line being blocked for 16 days.
Replacing the tunnel
EGIP, the Edinburgh Glasgow Improvement Project, will electrify the Edinburgh to Glasgow route and associated lines, a total of 220 route miles. As with all electrification projects, this requires route clearance to replace or modify large numbers of historic structures well in advance of the electrification works.
In December 2011, BAM Nuttall was awarded a £27 million contract for EGIP clearance works. Since then, it has undertaken route clearance work on 41 structures, including some heavily used road bridges such as the five-span Shore Road for which the company received a commendation at the 2014 Saltire Civil Engineering Awards. Carmuirs is the last EGIP clearance job and presents quite different challenges.
Unlike double-track tunnels that have centre space which can normally provide room for overhead line equipment, the single bore tunnels at Carmuirs do not have sufficient electrification clearance. For this reason, the solution originally proposed in the 2007 GRIP 1 report was to lower the track by 150 to 200 mm and provide slab track.
It was later decided to replace the tunnels with a canal aqueduct. This avoided drainage problems and also the need for slab track. Together with clearance work at eleven other structures, this will achieve W12 gauge on the line and allow freight trains to carry ‘big box’ containers to and from Grangemouth docks.
The aqueduct consists of 36 portal units placed on five cill units on each of the original tunnel abutment walls with wingwalls built in-situ over new bases. These units form a deck over which 34 key wall units are laid to form the canal channel. There is some dead space over this deck as the canal crosses the railway at a 50° angle. To minimise the loading, foam concrete is used for infill behind the portal frame. Outside the wall units, class 6N granular fill provides the base for the towpath and landscaped slopes. On either side of the canal, ducts are provided for existing and future services.
With the new structure and its increased electrification clearance, it is no longer possible to provide a one-metre thickness of puddle clay as required by Scottish Canals. To maintain the required 2.1 metres draft over the portal units, the aqueduct’s canal channel has a waterproof membrane bonded to the concrete face on which there is 400 mm of puddle clay laid on top of a Bentomat sheet.
Getting ready to start
BAM Nuttall’s senior site agent John Edelsten explained that a significant amount of work had to be done before work could start on the aqueduct.
Two site compounds were required. One, of around 4,000 square metres, immediately adjacent to the canal crossing, includes the base for cranes of up to 1,200 tonnes. The other, 150 metres from the canal, is of around 20,000 square metres. John advised that there was initially some doubt that a compound of this size was required.
However, it was proved to be necessary with the requirement for trial assembly of portal units on the cill beams and storage of general demolition material, removed masonry and puddle clay as well as site accommodation and parking.
The first key milestone was the provision of a temporary footbridge. This was put in place by a 450 tonne crane in August last year as a replacement for the canal towpath during the works. This bridge also carries high voltage (HV) and low voltage (LV) electrical cables that have been temporarily diverted from the towpath. Also diverted were BSkyB’s fibre-optic cables although, as a permanent diversion was possible, these were not routed over this bridge.
An earth dam had initially been considered as a way of blocking the canal on either side of the tunnel – the same type of dam which had been used to block the canal for the repairs in 2002. However, with a much longer four-month canal blockage, it was felt that a more robust solution was required so dams formed of sheet piles were used. These are a cantilever design with eight metre deep piles that requires six metres of undisturbed canal bed on the dry side of the dam.
When the canal is re-filled, the piles will be extracted by a vibro-remover with no requirement to repair the canal bed as the puddle clay layer is self-healing.
The canal’s catchment is such that, at Carmuirs, there is a significant flow of water from its summit pond to the sea. Once the canal was blocked, this flow was something else for the temporary bridge to carry. Pipes and pumps were provided for a maximum flow of 400 litres per second – the actual flow at any time is regulated according to weather and Scottish Canal’s requirements.
Works of this scale required the normal environmental mitigation. As de-vegetation was carried out within the bird-nesting season, it was done in stages and subject to a watching brief by environmental specialists. The tunnel was surveyed for bats and, although none were found, cavities were filled in to prevent nesting immediately prior to the works. No badgers were found during the survey that had been undertaken.
For a railway project, an unusual requirement was the need to rescue fish trapped in the canal once it had been dammed either side of the tunnel. This was undertaken by EcoFish consultants, using a combination of electrofishing and netting under licence issued by Marine Scotland. This was done from a boat that was lowered into the blocked canal section by a heavy-duty fork lift.
As a result, approximately 750 fish, mainly roach, perch and pike, were rescued from the drained 120 metre long section of the canal and released downstream. A further mitigation measure is the fish mesh system on the temporary pumps that maintain the canal water flow.
Immediately north of the compound are thought to be the remains of a temporary construction camp for the nearby Roman Antonine Wall. There are no visible remains of this camp which required no archaeological mitigation measures as the project did not excavate any previously undisturbed ground.
After months of planning, the stage was set for the tunnel demolition. John advised that, amongst many other things, risk readiness reviews had identified the requirement for a trial assembly of the portal unit /cill beams and the provision of extra pumps to prevent protective mats floating away in the event of flooding. This possibility was raised as there are known to be existing drainage problems at this location.
The staged demolition sequence had been assessed on the basis of a finite element analysis of the tunnel undertaken by MHB Consultants – designers of the temporary works associated with the tunnels replacement. Demolition contractor was S Evans and Son from Widnes.
Prior to the demolition, the headwalls and towpath had been removed as well as the overburden to within 300mm of the tunnel barrels. This represented around 8,000 tonnes of material, about 35% of the total.
Demolition took place during a 54 hour possession on 25 and 26 December. After the last train on Christmas Eve, a signal north of the tunnel was removed and protective mats laid over the track. During Christmas Day, the tunnel arches and wingwalls were demolished and all spoil removed to expose the tunnel barrels which were demolished overnight with the lower tunnel walls left in place. These formed the abutments on which a total of 10 cill beams were placed on 26 December. After all material had been removed from the track bed and the signal by the tunnel reconnected and tested, the possession was given up ready for the first train on the following day.
The tunnel demolition was a carefully choreographed operation involving around six excavators. As well as demolishing the structure, these machines passed the 16,000 tonnes of excavated spoil to tippers for transport to temporary storage at the adjacent larger compound as landfill sites were not open over the Christmas holidays. This spoil included sandstone masonry from the tunnel portal which is to be re-used by the local landowner.
One legacy from the 2002 cutting failure was that, as part of the tunnel repair, the cutting retaining wall north-east of the tunnel was of reinforced concrete.
As a result, this required much more effort to remove than the other masonry retaining walls.
The possession over 1 / 2 January required a 1,200 tonne road crane, one of the largest in Europe, supplied by Mammoet. It was used to install the lower wing wall bases and numbers 36 portal units, each weighing between 25 and 40 tonnes. As the smaller compound by the canal was not large enough to accommodate them all, most of the units were placed in the larger compound where a 200 tonne crane lifted them onto lorries for transport to the 1,200 tonne crane.
The precast concrete units were supplied by Macrete of Northern Ireland which had also supplied units for many of the EGIP project’s reconstructed bridges. Delivery of the 46 large heavy units was arranged to a tight possession timeframe over the Christmas and New Year period – an achievement in itself.
After the New Year possession, work was double-shifted due to poor weather and the amount of work required to ensure that the canal would be re-opened on 27 March. Before then, around 1,000 cubic metres of foam concrete backfill was put in place, wing walls were constructed in- situ, the canal walls units installed and waterproofing undertaken. During the weekend of 21/22 February, Mammoet’s 1,200 tonne crane again returned to site to install the 33 key wall units to form the canal wall. The few lifts that would impact on the operational railway were undertaken during normal overnight possessions.
John Edelsten felt that some of the biggest challenges were the interfaces with other concerns. There was a significant amount of utilities work and the project also liaised closely with Scottish Canals both to ensure that the design met its standards and to facilitate inspections at hold points before the canal is re-filled. During maintenance of the Falkirk Wheel the canal water flow had to be carefully regulated as the outlet pipe for the Falkirk Wheel basin is adjacent to one of the sheet pile dams.
John commented that the Carmuirs tunnel work had generated a lot of interest from the passing public who had great views, not the least from the temporary bridge. BAM Nuttall has provided good information boards to keep everyone advised of progress including overhead photographs taken by drones.
Railway and canal revival
165 years ago, the contractor for Carmuirs tunnel and other original bridges on the Scottish Central Railway was Thomas Brassey, in partnership with Robert Stephenson. Their structures were well built and could have remained in place for many years to come had it not been for the need to modernise the railway. As part of the EGIP programme, BAM Nuttall has provided modern replacement structures, of which the Carmuirs aqueduct provides W12 clearance for container trains and overhead electrification due to be energised here in 2018.
Just 200 metres from the new aqueduct is the Falkirk Wheel, opened in 2002 to connect Scotland’s two lowland canals. It replaces eleven locks which were closed in the 1930s. The Wheel is one of Scotland’s most visited tourist attractions and plays an essential part in the revival of its once-derelict canals.
The new Carmuirs aqueduct may not have the tourist pulling power of the Falkirk Wheel. Yet, for those with an interest in modern transport, it is good to see these adjacent modern structures as part of a railway and canal revival.