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Grout Expectations: restoring the Borders Rail route

Construction of the longest passenger railway to be built in Britain for over a century has involved extensive ground treatment to stabilise abandoned coal mines in the Scottish Borders

In 1969, the Waverley Line, connecting Edinburgh with Carlisle through the Scottish Borders, was closed as part of the widespread cuts to Britain’s railway network proposed in the Beeching Report. Now, nearly 40 years later, the Borders Railway project, being delivered by Network Rail, will partially restore this once important route, with trains running again between Edinburgh and Tweedbank in summer 2015.

While the project focus is an existing rail route, reopening is not as simple as relaying some track and sorting out a new rail timetable – as well as civil engineering challenges, the scheme also calls for major grouting work.

Construction on the £294M contract being undertaken by Bam Nuttall is split into three sections: Area North, between Shawfair and Gorebridge; Area Central, between Gorebridge and Bowshank Tunnel; and Area South to Tweedbank.

The original railway served not only passengers but was also an important freight route, transporting coal from the Midlothian coalfield to Edinburgh. Ironically, it is coal mining that presented the first major challenge to the project, with disused workings beneath the route presenting a risk to both construction and operation of the new railway.

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The criteria for ground treatment is based on the 10:1 cover ratio principle, so for every metre thickness of workings, there has to be 10m of cove

These are being stabilised using bulk grouting to prevent migration of settlement to the ground surface. Cementation Skanska won the contract to carry out treatment of two abandoned seams beneath the 1.5km long Area 1 of Area North, with work starting in October 2012.

“One seam is at about 18m to 19m depth and the other is between 41m and 42m,” says Cementation Skanska operations manager Gareth Hales. “Mining finished in the early 20th Century and it was originally thought the mines were still open or filled with water. In fact, most of them had already collapsed.

“The criteria for ground treatment is based on the 10:1 cover ratio principle, so for every metre thickness of workings, there has to be 10m of cover. Any less and we had to grout,” he explains. “Beneath bridges, stations and other structures there has to be a minimum of 30m cover.”

“The key challenge on this project was to ensure the grouting was consistent and always met the treatment criteria. Although it was repetitive work, quality was vital.”

Gareth Hales, operations manager, Cementation Skanska

The grouting programme, designed by URS, has been carried out on a 3.5m by 3.5m grid beneath the track, with perimeter holes at 1.75m centres to prevent grout migration out of the treated area. Beneath structures the grid is reduced to 1.75m centres to further reduce risk of movement of these sensitive structures. The majority of the grout boreholes were vertical, although some had to be angled, between 5° and 35° from vertical, to deal with surface features such as bings (spoil heaps) and to grout beneath structures.

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Historic coal mining has resulted in the need for grouting to protect the new rail route

Cementation used up to 10 rigs, mainly Casagrande C6, Soilmec and Klemm machines, to drill the 2,500 boreholes in Area 1. Boreholes were an average 37m deep, with 76mm diameter casing used through the underlying made ground and glacial till down to rock head. Holes were continued via open hole methods at 48mm diameter, through the sandstone and mudstone bedrock and into the mine workings.

Drilling was carried out with water flush and loss of water, combined with cuttings, indicated when the coal seams had been reached. The PFA cement grout, which has a typical strength of 0.7N at 28 days, was then injected through 25mm diameter tremie pipes, supplied by two grout mixing stations on site, allowing several holes to be grouted concurrently. Following completion of grouting, one further borehole was drilled in every 100m2 of treated area and pressure tested for quality control.

Work in Area 1 finished at the beginning of March 2013 and had gone so well that by that time, Cementation had already begun treating Area 3, 1.5km further along the line.

Cementation had to drill around 4,500 boreholes along this section, but only needed to use six rigs during the 21 week phase. Main treatment works on Area 3 finished in June and since then Cementation has turned its attention to grouting mine shafts in Areas 2 and 3. Typically 2m square and between 20m and 40m deep, these are still open, although covered at the surface.

“While we knew roughly where they were from historic records, probing was used to confirm their exact location,” Hales says. Once pinpointed, Bailey Bridges up to 35m long were used to span the shafts and provide safe access for drilling and grouting operations. Shaft grouting was carried out in sequence through a number of pipes (typically three for a 2m square shaft), to create a wedge of grout.

Cementation has treated 11 shafts, to add to the grouting of vulnerable areas along 3km of the route, both beneath the track and at nine bridges, two stations (Shawfair and Eskbank) and the approach roads to four overbridges, without a hitch. This is down to excellent quality control and working practices, Hales says.

“The key challenge on this project was to ensure the grouting was consistent and always met the treatment criteria. Although it was repetitive work, quality was vital to ensure we didn’t have to go back to do any remedial grouting, which to date has not been an issue.”

 

Reinstating the Waverley Line

Despite fierce local opposition, the Waverley Line closed in 1969, with rail systems removed by 1972. However, an independent feasibility study commissioned by the Scottish Executive the turn of the 21st Century suggested there was justification for a line to the central Borders which would bring economic, social and environmental benefits to the region and beyond.

It took another five years before it was announced that part of the line would be reinstated, with the passing of the Waverley Railway (Scotland) Act and it took a further six years for construction to begin but in summer 2015, trains will once again run between Edinburgh and Tweedbank.

The new railway will follow the original route of the Waverley Line, although it will stop at Tweedbank, rather than Carlisle. Of the 49km of line, just 5km will use existing track – the section from Edinburgh Waverley station through the south of the city to Newcraighill Junction.

There will be 10 stations, including seven new ones: Shawfair, Eskbank, Newtongrange, Gorebridge, Stow, Galashiels and Tweedbank. Additionally, existing structures, including 95 bridges, as well as cuttings, embankments and retaining walls, will be refurbished to bring them up to current standards.

Initially, diesel trains will run every half hour at peak times, with the journey between Tweedbank and Edinburgh taking less than an hour. While the original line was twin track, the new railway will be single track, with three loops for trains to pass and scope for future expansion to twin track and electrification.

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