millions have been spent to block up more than 250 miles of mine shafts to prevent part of the new Borders Railway route from collapsing.
At least £10 million has so far gone towards the preventive work, largely at the site of the new Shawfair train station just north of the City Bypass, where contractors have discovered hundreds of shafts left unfilled.
The work was revealed as bosses on the project provided an update showing the £350m rail project was on time, and on budget, with the tackling of some of the most challenging aspects well under way.
That includes the work to fill in the disused tunnels, which at one time formed part of the Monktonhall Colliery – one of the old Scottish super pits that shut in 1997.
Such has been the extent of remediation works that tunnels collectively covering a distance further than that between Edinburgh and Manchester have needed to be shut off, with more than 500,000 tonnes of earth already removed from the site in the largest use of manpower on any part of the project.
Network Rail spokesman Craig Bowman said: “The more modern pits were a lot deeper, but what we’ve found is some of the more ancient ones, the ones that aren’t necessarily mapped out. There’s been mining work happening here for 400 years.
“They used to just dig straight down and they didn’t do an awful lot to fill them in. They’d maybe stick down a tree into the mine shaft and cover it up and that was it. That’s where the main remediation focus has been on these ancient mine shafts.
“This is virgin territory for the railway. There’s never been a railway through this area. It’s the only area along the route that doesn’t follow the old Waverley line.
“The shafts have been filled with a grout, which is a mixture of ash and concrete, to essentially prevent any future collapses.
“Unfortunately, there is past history of ground giving way underneath railways, particularly in the vicinity of old mine shafts. The East Coast Main Line was affected about 20 years ago. We’re preventing that from happening again.”
Shawfair is one of seven stations being built along the 35-mile railway. Six miles from Edinburgh Waverley, the site will be at the heart of a new town centre originally earmarked as one of Scotland’s largest housing proposals, with a capacity of 4000 homes.
Centred around the villages of Danderhall, Millerhill and Newton, the new community is tipped to include 800 affordable homes and 30 hectares of business and medical parks.
A new road – nicknamed “Boomerang Bridge” because of its shape – will be built to access the train station.
Mr Bowman said trains will stop at Shawfair once the line opens, despite the likelihood of no housing around the transport hub at that time.
He said: “There’ll be a significant car park there so people will be able to use it as a park-and-ride facility. We’ve seen the success of those on this side of the city already. You’re talking 15 minutes into the city centre from here, which is attractive.”
Residents in nearby Danderhall will be able to walk to the station in about ten minutes.
Construction of the biggest of the route’s new bridges and the carving of a 130ft-wide tunnel has required a major two-lane diversion off a section of the city bypass being opened last month.
Creating the diversion alone has cost Network Rail about £1m. The altered section of the city bypass is scheduled to be reinstated in May next year.
Contractors have chosen to complete most of the high-risk parts of the project, including the Shawfair and bypass excavations, early in the project.
The gamble has paid off, with dry conditions across the summer meaning 85 per cent of all earthworks are complete.
Stuart Mackay, communications manager for principal contractor BAM Nuttall, said experiencing one of the best summers in years had been a major bonus to delivering the project on time and on budget.
The Borders Railway is due to be completed by summer 2015, with the first passenger trains to be running along the line by autumn of that year.
Testing along the route will start late next year.
Mr Mackay did not go as far as saying the project was ahead of schedule, but said: “We’ve gained some time, but we’ll put the resources into other areas. Every project faces challenges and when we do face them, we’ll be able to use some of that buffer.
“It’s a very challenging programme with quite tight timescales. Had the summer been poor weather, we would have had to come up with a new strategy for next year.”
Another major expense along the 30 miles of new track has been installing drainage to prevent landslides.
Wet and marshy ground around Tynehead has been vulnerable to landslips because of natural springs, with drainage channels and rock embankments built between the Gorebridge and Stow stations over the past month to prevent a disaster on the line.
Compensation for landowners for swathes of countryside either bought or temporarily used to build the railway have been funded out of a pool of roughly £55m set aside outside of line construction costs.
The Borders Railway will be the longest domestic railway to be built in Britain for more than 100 years and is expected to be used as a model for future projects.
Rail enthusiasts and pro-transport groups from south of the Border have already contacted Network Rail in large numbers to keep up with developments.
Newtongrange and Gorebridge have been tipped to be the big winners from the rail route’s revival.
Journeys from Edinburgh Waverley to Tweedbank will take 55 minutes when the line opens. Trains will run half hourly during peak periods, although the single track nature of large sections of the route will limit train frequency from being increased.
Cycle hoops are being built at all stations to support the pedal power movement. Midlothian Council is also investing funds into rebuilding parts of the National Cycle Network that are being replaced by the Borders Rail route.
Traffic disruptions in the countdown to Christmas will be concentrated around the historic Newbattle Viaduct – otherwise known as Lothian Bridge – and Hardengreen roundabout, close to Bonnyrigg.
The 22-arch viaduct, built in 1847, spans the River Esk and a ravine, north-west of Newtongrange.
Traffic restrictions for those heading under the viaduct on the A7 will be in force for a week from November 18-25.
The closure will allow workers from Loanhead firm Forth Stone – among 450 local suppliers used for the project – to complete masonry works to repair the structure.
Temporary traffic signals will be in place, with the trunk road shut completely overnight and vehicles diverted along Newtongrange High Street.
Carrington Road, running under the Victorian structure, will also be shut for five days from November 25.
Network Rail Scotland project director Hugh Wark said: “Once complete, these works will link the north section of the Borders route to the rest of the line.
“They mark a clear milestone in the construction of the Borders Railway which, once it opens in 2015, will be enjoyed by passengers for years to come.”