Harringworth Viaduct is a major structure on the line between Manton Junction and Corby, set in glorious Northamptonshire countryside. It is a brick arch structure, 1,182 metres long with 82 spans, three of which cross roads and one of which crosses the River Welland, and was built between 1876 and 1878 using bricks made in three brick kilns on the site.

Rail Engineer has reported upon previous work on the structure, which needs regular attention. Brickwork repairs and other works have been required almost annually for a number of reasons, of which more later.

The current works are being undertaken for Network Rail by AMCO, which also undertook some repair work to the viaduct in 2012, following previous repairs by May Gurney (now Kier) in 2009. On a beautiful sunny day, Rail Engineer was shown around the site by Ian Watson, AMCO’s regional manager for the East Midlands, project manager Shaun Trickett, and members of their site team.

Twin goals

Ian and Shaun described how they are, in effect, working on two agendas. The first is a continuation of the repairs that have been made regularly in the past. The second is the more complex one of strengthening the structure to allow heavy freight trains to pass over it at 60mph, their normal maximum on Network Rail’s infrastructure. Currently they are restricted to 20mph, and this is inhibiting the infrastructure operator from using the route as a diversionary one for freight trains serving the port of Felixstowe.

The first task is, on the surface, a straightforward one, something which might seem to require simple repetition of what has been done before. However, this time there are some changes in the remit, the biggest of which is in the scope. Instead of being asked to repair the seriously damaged areas on the viaduct, leaving less urgent damage to be dealt with at some future date, AMCO has been asked to repair everything.

That makes a lot of sense both economically and in engineering terms, but the most important issue must be the economic one. The costs of mobilising a team on such a site are high, so it makes sense to do as much work as possible. Also, the land under the viaduct is agricultural and a flood plain, so there are costs in obtaining access over the farmland as well as providing environmental protection, setting-up site facilities and creating access tracks under and adjacent to the structure. All that, and more, before any work can even be started.

The other change is a technical one – Network Rail’s requirement to avoid the use of pattress plates on this structure in the way that they have been in the past. Harringworth Viaduct is Grade 2 listed, and a great deal of effort is being given to maintaining its appearance unchanged.

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Deteriorated brickwork

The repairs to be undertaken were determined by employing Donaldson Associates. A team from its Derby office, under project manager and senior engineer Manesha Pieris together with civil engineering director Peter Harris, undertook the necessary investigations and prepared reports detailing the defects and recommending how to deal with them.

The piers, arches, spandrels and parapets all required significant work to ensure that they could continue to function safely under current loadings, before any consideration could be given to increasing the speed of freight traffic.

The piers were consistently cracked vertically, as well as having some areas of spalled or drummy brickwork and others which needed repointing.

Spalling, and the occasional complete loss of a brick, is prevalent on the arches too. Drummy areas also feature, but the cracking is very much the significant issue affecting the arches. Some have transverse cracks at the third points suggestive of hinge formation, a serious concern. Most arches have longitudinal cracks, usually a short distance in from each face and apparently directly below the inner faces of the spandrel walls. Many also have a similar crack beneath the centre of the six-foot of the tracks above.

The spandrels have significant areas that are pushing outwards and/or separating from the arch rings, horizontal cracks and often water seepage.

Parapets are of substandard height above the ballast shoulder, many lengths are leaning outwards significantly, and there are a number of horizontal and vertical cracks.

Dealing with the cracks

AMCO and Donaldson worked out appropriate repair methods based upon Donaldson’s reports and agreed solutions with the client, Network Rail. These range from conventional repairs, for example repointing, through relatively familiar methods such as stitching and grouting, to some more innovative approaches.

Of particular interest are the parapet repairs. The vertical cracking is being dealt with by replacing bricks that have cracked through, and by installing 6mm stitch bars within the brickwork across the cracks. It is also intended to tie the bases of the parapets more securely to the rest of the structure by means of brackets or stitch bars.

None of that is too unusual, but the approach to the horizontal cracks is more innovative. The concept of inserting vertical stitch bars is not new but the means of doing so is. Readers of Rail Engineer will have heard of Foulstone Forge before, in the context of bespoke pieces of equipment. In this instance, it involved a special drilling rig to mechanise and speed up the process of drilling the holes vertically down into the parapets.

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The drilling rig is designed to be self-stable once it has been lowered onto the top of the parapet by an RRV. It consists of a frame that holds three drills at the correct centres along the wall and, once aligned, it can drill three holes down into the parapet within three minutes. 20mm bars are inserted and grouted into the holes, and failed bed joints are raked out and repointed to complete the repair.

All of these works are scheduled to be completed by March 2017, at a cost of some £7.6 million.

Piers and arches

Donaldson Associates also addressed the question of strengthening for heavy freight. Under the railway system of categorising track and structures by load capability, the viaduct is currently rated as RA0, the lowest possible category, meaning that 25 tonne axle-load freight is restricted to 20mph. This will have to be increased to the maximum (RA10) if freight trains are to be permitted to run at 60mph.

Using arch-assessment software known as Archie-M, provided by Obvis, Donaldson assumed in the analysis that all of the identified structural defects in the viaduct would be rectified. Running the analysis confirmed the current RA0 rating and indicated that, even in a repaired state, the structure would remain at risk of collapse if RA10 loading was applied.

The modelling indicated that the load path in the piers at certain spans would lie outside the pier structure, suggesting collapse would be likely. A Category 3 check of this work, a third-party design check by an independent organisation, was undertaken by Archie-M developer Bill Harvey Associates and confirmed these findings.

So Donaldson Associates prepared a report suggesting options for the required strengthening to achieve RA10. These were threefold:

  • Over-arch reinforced concrete saddles (see above);
  • Under-arch strengthening by reinforced concrete arches supported on steel needlesthrough the piers;
  • Increased ‘backing’ to the arches, carried out by removing the random rubble fill either side of each arch over the piers and replacing it with mass concrete up to the level of the arch tops.

The first option would add the least dead weight to the structure and have practical advantages including the simple incorporation of new waterproofing and of works to tie in the parapets. However, it would be extremely expensive, require lengthy blockades and, because it would add depSaddlesth over the tops of the arches, it would cause problems with vertical track alignment.

The second option would be even more expensive and would add three to four times the extra weight of the first, whilst also significantly altering the appearance of the viaduct. Practical difficulties would also result, such as the need to construct scaffolding over the river and to deal separately with the waterproofing and parapet works. These last would mean that the need for possessions would probably still be there, negating the advantage that under-arch strengthening might otherwise have offered.

Option three would add less dead weight than option two, but still almost three times that of option one. Like that option, it would allow the waterproofing and parapet works to be dealt with simultaneously, but would require similarly major possessions.

Both the second and third options might well require underpinning to strengthen the pier foundations to handle the extra dead weight and live loading combination.

The complexity of this is such that further work is required for option development and selection to proceed. As a result, Network Rail has deferred the strengthening works to CP6. Meanwhile, a fibre-optic monitoring system will be installed beneath the arches of the viaduct using technology developed by the University of Cambridge Centre for Smart Infrastructure, led by Matthew DeJong. The outputs from this system will be validated by conventional deflection monitoring of a sample number of arches.

The results of this monitoring will be used by the project team and Network Rail to develop a preferred strengthening option, which will then be submitted for approval.

In the meantime, Network Rail’s Infrastructure Projects East Midlands civils renewals team based in Derby, led by project manager Chris Chatfield and scheme project manager Jon Batsford, is keeping a close eye on the situation. By maintaining the current speed restrictions, and by responsible asset management, loading on the structure is minimised and well within safe limits. “The structure is still safely carrying daily passenger traffic,” Chris commented.

And a great view those passengers will have too as they cross this spectacular viaduct.