A top down approach

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For most of its route, the railway between Dundee and Aberdeen snakes through the undulating foothills of north-east Scotland or runs along the coast. This was demanding terrain for railway builders who had to construct many embankments, cuttings and viaducts. It continues to present an asset management challenge to Network Rail, attracting serious investment to ensure its ongoing integrity.

At Carillion’s Bishopbriggs offices near Glasgow, Network Rail’s Project Manager Billy McKay explains some of the difficulties faced. For example, 2010 saw the completion of projects to strengthen, blast and paint Buckie Den and Den of Cowie viaducts – with £2 million spent on each – whilst at Carmont cutting, four miles west of Stonehaven, £1.8 million is currently being spent on stabilisation works. These are being undertaken by Carillion as part of its Scotland civils framework contract with Network Rail. This is a six-year ‘design & build’ arrangement that comes to an end this month.

Carmont cutting’s makeover

Network Rail’s Project Engineer Danny Beveridge explains how Carmont cutting had been suffering from drainage run-off from the fields which overtops it at a number of locations, partly due to an increase in the area of farmland and reduced effectiveness of the field drains. This has resulted in soil and small rocks landing in the cess below. Something had to be done as earthworks examinations were also assessing the cutting as poor, so a two-part project to stabilise the cutting was developed early in 2009.

The first stage, carried out in January 2010, involved the removal of vegetation and loose material. This enabled Arup to develop a detailed design for the second-stage stabilisation works. As a result, the start of 2011 sees the cutting undergoing a makeover to clean it up, cover it with netting and place soil nails.

Carmont cutting penetrates a slope so has two faces of differing heights. The Down side is, on average, 15 metres high – 12 metres of this is a rock cutting topped by a 3m earth slope at 45 degrees. The Up side is more modest – an average 6-metre high rock cutting close to the end of the slope.

Carillion’s sub agent Keith Robertson explained that the stabilisation works involve 14,000m2 of rock netting with 1,000 top and bottom anchors, as well as 160 rock anchors to secure potentially loose rocks. The top of the Down-side face requires 450 soil nail pattern anchors and a 500-metre crest drain.

In addition to the normal constraints associated with work on an operational railway, access to Carmont cutting demanded a 2.5km haul road through a farm. Keith explains that this involved upgrading 1.5km of farm track as well as laying a new 1km route across a ploughed field. Work to construct these started in December 2010. To enable the project’s 44-tonne excavator to reach site, a stone-arched bridge over a small stream had to be strengthened. The farmer had already extended it with steel beams and concrete but, unsurprisingly, there was no design data for this work so it was not possible to assess the structure’s loading capacity. As a result, an over-span consisting of more steel beams had to be provided.

Access to the farm is subject to an agreement negotiated by Network Rail’s Operational Portfolio Manager. Railway civils work is particularly reliant on good neighbour relations and Billy McKay was very conscious of this in his dealings with the farmer, who had been very cooperative. As an example of this, the handover meeting with the farmer and his agent that will take place shortly before project completion will be done whilst plant is still on site to ensure land is left in an appropriate condition.

Rail access

These days, the need to ‘work smarter’ in order to minimise service disruption and make best use of available engineering access is always at the front of a project team’s mind. So the Carmont cutting works have harnessed a long-reach excavator supplied by Hodge Plant to provide access from the top of the cutting. Moreover, although there is no design requirement to do so, the Up-side rock netting goes over the crest of the cutting and is secured in place by top anchors – this means that their placement requires no railway protection.

Much of the work is being done in a green zone behind a fence although with work taking place 10 metres or so above the track, “above and behind” might be a more appropriate term. A Vortok safety barrier has been erected alongside the Down line and, for much of the time, arrangements have been in place to ensure that equipment did not overhang the demarcation boundary created by the fence. Where this was not possible, work was done using T2 or T3 protection.

Even with these measures, the work at Carmont required 29-hour disruptive possessions on the last three Sundays of January 2011, blocking the route between Dundee and Aberdeen. To get the best use from these, some track renewals and signalling work also took place, together with the de-vegetation and scaling that comprise the first stage of a cutting project near Montrose. At Carmount, the possessions were used to install the bottom anchors as well as removing spoil and vegetation using on-track plant supplied by Paul John Plant Ltd. This necessitated a 2¼-mile round trip to Carmont level crossing.

Early in 2010, two of the three possessions planned for stage one had been lost due to bad weather. As a result, the scaling and de-vegetation work done at this time was focussed on enabling the designer to assess the cutting, leaving the remainder for the stage two period in 2011. So you might ask why is such civil engineering work done through the winter? Billy agreed that it would clearly be preferable to be on site over the summer as poor weather impacted on the job and created difficult conditions for the workforce, but he advised that disruptive access was unlikely to be granted during the summer; the bird nesting season also had to be considered. A further constraint was that, although 54-hour disruptive possessions had been booked for the stage one work in 2010, only 29-hour possessions were now available.

Getting the job done

Most of the vegetation and scaling work was carried out using rope access from the top of the cutting by specialist subcontractor Geo-Rope. Once this was done, drilling operations got underway to install the soil nails and rock anchors. Carillion’s Keith Robertson explains that this involved a Krupp rock drill mounted on a Ripamonti drill boom attached to the long-reach excavator. A grout plant at the top of the cutting was required to grout the anchors in place for which normal environmental precautions were taken to ensure no pollution of the drainage system. The net was then rolled into place and secured by the bottom anchors.

Following the last disruptive possession on 30th January, the remaining work continues in the safe haven of the fenced green zone with daytime T2 protection when required. Completion of the stabilisation work is planned for May 2011 after which the cut-off drain will be installed to prevent water overtopping the cutting. The site should be demobilised and the farmer’s land reinstated later the same month.

Whilst the works involved in Carmont cutting are very much ‘of the earth’, the arrangements surrounding them illustrate the very different landscape engineers now operate in. The project team has risen to the challenges of today’s access constraints thanks to a top-down approach.

Article courtesy of the rail engineer magazine.

1 COMMENT

  1. Happy 183rd day!!! Stupendous Thursday post, Joanie! Thanks a ton!!!NorCal Archies are aws#&me!oe8230;..Love the recap, food (yum!), souvenirs, wanna have me some!Wonderful interview by Arin…..Spanish songs and Christmas songs…..wonder if he recorded them before leaving?Cutest rapper ever…..David, of course!Karin rocks!

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