Authors: Brendan Meagher BE MIEI and Keith Tully BEng CEng MIEI, BAM contractors
In September 2015, the longest domestic railway construction project in the UK in more than a century opened to commuters and tourists alike. The Borders Railway is a 50km railway line along the old Waverly Route, running from Edinburgh to Galashiels.
The Waverly Route, which originally ran from Edinburgh to Carlisle, was closed in 1969 following the publication of the cost-cutting Beeching Reports, ‘The Re-Shaping of British Railway’. Almost 50 years later this historic route will reopen to rail traffic after three-and-a-half years of reconstruction of this old Victorian railway line by BAM in partnership with Network Rail.
Overview
The reconstructed section of railway connects Edinburgh’s Waverley Station to Tweedbank in the Scottish Borders, which is approximately one third of the original line to Carlisle. The alignment of the route predominately utilises the old Waverly Route, with just five of the 50km offline.
[caption id="attachment_24717" align="alignright" width="300"]
The route connects Edinburgh with the Scottish Borders, servicing seven new stations from Tweedbank up to Shawfair[/caption]
The northern section of the line was realigned to facilitate a significant residential development plan for the townland of Shawfair. In addition, seven new stations will be serviced by the Borders Railway line all of which vary in size and layout. The new line connects into the existing rail network near Newcraighall which then provides the link into Edinburgh city centre.
The procurement of the project
The project was originally intended to be procured by Transport Scotland as a PPP, with four consortia qualifying to bid. In the summer of 2011, because of difficulties in achieving the Capex requirements set by Transport Scotland, BAM Contractors (Ireland) were invited by their sister company, BAM Nuttall, to value engineer the bid.
Subsequently, they were invited to join the consortium of BAM PPP, BAM Nuttall and BAM Rail (Holland) to progress the bid. It was becoming apparent, however, that the scheme was too small to be delivered as a privately financed and operated railway. In the spring of 2011 one of the three remaining consortia withdrew from the competition and the second remaining consortium withdrew in the summer.
With only one remaining bidder, the competition was cancelled in the autumn of 2011. Network Rail was then appointed by Transport Scotland as Authorised Undertaker and a new Design and Build tender begun. In early 2012 the BAM JV (comprising of BAM Nuttall, BAM Contractors and BAM Rail) were awarded an ECI Contract to develop the Design. This was followed by an Advanced Works Contract for site clearance and site investigation, with a separate Mining Remediation Contract awarded in the summer and the Main Works Contract award followed in October 2012.
The existing route
[caption id="attachment_24719" align="alignright" width="300"]
The existing corridor was in a poor condition with drainage and slope stability issues throughout[/caption]
The existing condition of the route varied considerably prior to construction. Some parts had been paved to form cycle paths while much of the route was being occupied by adjacent farms. Generally, the route was in very poor condition and the challenge to rebuild this old route was by no means small.
Earthwork slopes were overgrown with vegetation and had failed in some areas, while parts of the solum (the old track-bed) were underwater with drainage absent or blocked. The existing steel structures were in an equally poor state, with significant corrosion to primary and secondary beams and timber decks unsafe for pedestrians let alone heavy rail traffic.
The number of existing public services along the route, most of which were installed post the closure of the railway, also represented a significant challenge ahead of the demanding construction programme.
Mine remediation works
Edinburgh and the Borders region of Scotland have a history steeped in oil shale and coal mining so mine remediation works are not uncommon. The northern offline end of the route necessitated significant mine remediation works before main earthworks could commence. An area near Shawfair Station was once the site of the old Monktonhall Colliery.
This area was heavily mined for coal up until 1997. Mine shafts adjacent to the route in this area extended over 900m below ground, while shafts directly under the solum extended over 100m below ground. Historically once a mine shaft was exhausted it was backfilled with waste materials from the mine workings. Over time, flowing water within the mine workings can erode material from the toe of the backfilled shaft and create a void.
Under load from heavy earthworks equipment or indeed rail traffic there is a possibility that the material within the shaft will settle into this void. Mine seams also pose a risk of settlement under load particularly those near the surface of the rock head. To mitigate this risk of settlement or worst case a collapse, the shafts and seams were probed and pressure tested. This probing and testing then defines the extents of grouting required.
In the case of mine shafts directly beneath the trackbed a reinforced concrete capping slab complimented the PFA grouting. During the course of the mining contract BAM drilled more than 3,000 boreholes, cumulating in more than 400,000lm of drilling. A total of 27 mineshafts were discovered and remediated while over 10,000m
3 of PFA grout was pumped below ground. At the peak there were 25 drilling rigs on site. Ensuring the mine workings were identified and remediated was a key challenge in delivering this project.
Building new retaining walls along a narrow corridor
[caption id="attachment_24721" align="alignright" width="300"]
Hand-placing stone for part of the 10km of gabion walls[/caption]
While much of the bulk earthworks were contained within the 5km offline section to the north, the existing route also necessitated significant earthworks. In total, more than 550,000m
3 of material was excavated while more than 86,000m
3 of engineered fill was used to build and widen embankments.
Optimisation of the sweeping rail alignment was key to minimising the impact on the existing slopes but changes in track design parameters, train gauge profiles and operational safety standards since the closure of the old line meant that following the old alignment exactly was not a possibility.
As such, this necessitated lowering and widening of some earthwork cuttings which in turn resulted in the need for large lengths of retaining structures. In collaboration with Network Rail and local council planning authorities, gabion walls were identified as the primary earth retention structure on the route.
Other systems such as sheet piled walls, reinforced earth and retained earth solutions were also used along the route but gabion wall construction was the preferred and predominant system used.
In total, more than 10km of gabion walls were constructed with heights ranging from two to four metres. Aside from the significant volumes and quantities involved, one of the main challenges up and down the route was managing the works within the confines of a narrow corridor. This represented one of the biggest challenges to the construction team with extensive time-chainage programming and management required.
Rebuilding the bridges
In total there are 161 structures on the Borders Railway line, consisting of underbridges, overbridges, footbridges, culverts and tunnels. 126 existing structures were retained for refurbishment while 35 new structures were constructed in order to bring the line back into operation. The original Waverly route was first opened in 1849 so the scale of refurbishing the 160-year old masonry arches and wrought iron bridges was a significant challenge.
Early site investigation works were undertaken to better understand the condition of these structures to enable the design team to finalise the refurbishment works. The masonry arch structures were generally found to be in very good condition. Repair works were then undertaken to remediate defects such as open jointing, spalling of the stonework and in some case cracking. The existing wrought iron structures however brought with them much greater challenges.
[caption id="attachment_24723" align="alignright" width="300"]
Click to enlarge[/caption]
Corrosion to primary and, more so, the secondary girders meant that full blast and paint operations were required along with new hardwood decking and subsequent waterproofing. This was particularly challenging as many of the wrought iron structures crossed watercourses, bringing with it significant environmental risks that needed management during the works.
Repair works were also undertaken to two tunnels on the project, the longest of which was 180m long. One of the significant challenges in finalising the design on this tunnel was the extensive repair work carried out in the years before the closure of the line in 1969.
Coring of the tunnel walls and even a review of records in the National Archives were undertaken to better understand what works were previously carried and more importantly why these were required.
The logistical challenge
Aside from the various technical engineering aspects the project presented, there were also significant logistical challenges to be managed. The sheer length of this railway and the programme to build it meant that every works activity had a logistical aspect that needed no less attention than the works themselves. For the track laying, coordination of the delivery and subsequent installation of more than 96,000 sleepers was no easy task.
[caption id="attachment_24725" align="alignright" width="300"]
Track laying within the 180m-long Bowshank tunnel[/caption]
Similarly, the rail installation works demanded planning expertise to ensure that the 1km daily production rate was achieved. The project also required the installation of 100km of fibre ducting and cabling. The fibre cables provided the communications spine for the route both for operational rail systems but also for retail telecommunications.
Once installed, the fibre cables were connected into the Edinburgh Signalling Control Centre such that the newly installed route signals and detection systems could be monitored and operated as part of the overall rail network in the region.
Summary
In June 2015, BAM completed the Main Contract Works and the project was successfully handed over to Network Rail and the operating company for the route. The challenges faced on this £275 million construction project were as equally rewarding as they were demanding.
Rebuilding the 160 year old Victorian Waverly Route was undoubtedly one of the most prestigious civil engineering projects in the UK in recent times and will remain so for many years to come.