The Glasgow & South Western Railway grew out of an amalgamation of smaller companies in the 1840s and 1850s to serve a roughly-triangular area of south-west Scotland. By 1923, when it became part of the LMS, it had over 1,000 route miles, a headquarters at Glasgow’s St Enoch Station and locomotive workshops in Kilmarnock.
Today St Enoch Station is gone – demolished to make way for a shopping centre. However, the Glasgow South Western Line still exists, linking Glasgow with Gretna Junction near Carlisle via Kilmarnock and Dumfries, with branches off to East Kilbride and Stranraer Harbour.
The line was heavily used as a diversionary route while the West Coast Main Line was being electrified in the 1970s. Later, its importance was reduced and some of its double track was singled. Recently, that trend has been reversed with work to extend the Lugton Loop and there are now half-hourly services between Glasgow and Kilmarnock. Otherwise its principal use is for freight and local passenger services.
Viaduct failings
As is typical on Scottish lines, the route features several viaducts. Two of these are at Crawick near Sanquhar, just north of Dumfries, and Enterkinfoot – a little further south. Both are Grade B listed. The latter is known as Enterkin Viaduct, not to be confused with Enterkine Viaduct near Tarbolton in South Ayrshire.
In 2009, concern was raised about the extent of cracking that could be seen in the arch barrel brickwork of both structures and the parapet at Enterkin was deformed. Network Rail’s framework team asked Carillion to carry out site investigations and develop designs to rectify these faults.
Crawick Viaduct is a six-span masonry arch structure crossing a river and a minor road. Enterkin is taller and more remote, making access difficult; it is also next to a designated SSSI. Carillion engineers studied historic documents and found that both structures were built from sandstone blocks with internal spandrel walls backing onto the arch barrels and stone slabs supporting the track. The original voids had subsequently been filled with concrete.
A visual inspection revealed longitudinal cracking behind the spandrel walls, some of which were up to 5” wide. Enterkin’s parapet showed a distinct bulge and was tilting out.
Finding solutions
Temporary banding was installed to tie everything together and brick arch specialists Cintec called in to advise on the problem. To determine the condition of the existing structure, core samples were taken and trial pits excavated to confirm that the voids were indeed filled. In addition, Enterkin’s parapets were laser-scanned whilst ground penetrating radar was also employed to determine how much ballast and clay fill was beneath the tracks.
The results of these investigations allowed Carillion and Cintec engineers to come up with designs for each viaduct for submission to Network Rail. It had been established that, over the years, the depth of ballast on the two structures had built up, increasing the loading which could have contributed to the current problems. Network Rail agreed to lower the track by 300mm at Crawick and 250mm on the Up line at Enterkin.
The main repair work involved similar designs being formulated for both viaducts. Cintec anchors would be used to tie the structure back together to form a single mass. These consist of a long stainless steel section in a fabric mesh sleeve into which a cementitious grout is injected under pressure. The woven polyester sleeve restrains the flow but expands from its normal diameter, moulding itself into the shape of the spaces within the walls. Grout milk also passes through the mesh to form a mechanical bond with the existing material. This style of anchor has been used successfully to repair bridges all over the world, as has been reported previously in the rail engineer.
Pull yourself together
The Cintec anchors were to be installed horizontally – tying the spandrels back into the main structure. Concrete slabs with pre-cast edges were cast the length of both viaducts and these were also tied in using the anchors. At Enterkin, the edges were L-section so that the parapet walls could be fixed to them, preventing any further movement.
Work started at Crawick early in 2010 and was completed by April. The team then moved on to Enterkin to install the anchors there. All the external anchors were fitted using rope access; short blockades were needed only to install the concrete slabs and lower the track on each structure. Remote monitoring is now in place to check for any ongoing movement. Results to date show complete stabilisation of the arches.
Close cooperation between Network Rail, Carillion and Cintec engineers has rectified the problems with these two viaducts without disrupting train services. At the same time, their listed status has been respected. With Network Rail reportedly having several other structures suffering from similar defects, the team could be busy for some time yet!
Truly ancient monument
While Cintec is used to strengthening bridges of 18th century vintage, the company’s expertise is also being called upon to stabilise a structure that is 3,000 years old! The voids inside the Egyptian pyramids are very much like the underside of a stone bridge and similar methods can be used to prevent any movement. So the company’s engineers are now out there working their magic, ensuring that these ancient structures will be around for a few more centuries.
