It must be a heart-in-the-mouth moment – that instant transition from mundanity to disbelief as you drive your train around a curve to find the line blocked by fallen trees. Where’s Jenny Agutter when you need her? So on goes the emergency brake, off comes the power; then all you can do is hope.
Those were the events that unfolded early on the morning of 7 January as an out-of-service DMU headed west through a cutting on the Newcastle-Carlisle line at Farnley Haugh near Corbridge. It struck the trees and came to a stand.
The driver’s report described water cascading down the cutting slope, turning to slurry at the bottom. When examining engineers attended, minor cracking was observed in the field above but, after 24 hours, these cracks had opened up considerably to expose a fractured main pumping raw sewage onto the hillside at a rate of 400 litres per minute. Coupled with this, similar volumes of water were pouring over the field towards the railway, the surface drainage having been overwhelmed. Things were not helped by the deliberate pushing-over of a dry-stone wall which had previously been acting as a dam and causing the nearby road to flood. With that, a Highways problem suddenly became Network Rail’s.
An initial conclusion was reached that the toppled trees were basically a function of a large washout flowing down the slope. As we all know, the North had suffered from record rainfall over the preceding weeks, with the December average exceeded by 300%.
Deep cut
To better understand the reality – which proved rather more complex – we need some historical context. Engineered by Francis Giles, the affected section of line was built by the Newcastle and Carlisle Railway, opening to goods and mineral trains between Blaydon and Hexham in November 1834 – a distance of 17 miles, most of which was single track.
Back then, there was no cutting at Farnley Haugh; instead the railway passed through a tunnel 170 yards in length. To handle an expected increase in traffic, this was enlarged after ten operational years to accommodate two tracks, the work being delivered with only one brief interruption to services when a failure of the original lining blocked the passageway with running sands.
The tunnel suffered from considerable water ingress and some distortion due to ground movement. Repair work was undertaken in 1871 at a cost of £1,000, and the arch was subsequently strengthened throughout with iron ribs and laggings. Concerns over gauge clearance and possible formation scour from the adjacent River Tyne led the Chief Civil Engineer of BR’s North Eastern Region to order the construction of a deviation. This resulted in the cutting being excavated between November 1959 and June 1962.
Photographs taken during those works show an area of rock pitching (riprap) mid-slope, used to stop weathering by emerging springs and coinciding exactly with the location of the landslip. Drawings were also extracted from the archives showing an unusually extensive drainage provision at this location. It was therefore reasonable to deduce that the engineers had been confronted by considerable groundwater issues.
Over the intervening 50-plus years, the cutting has not proven problematic except for a couple of minor washouts around 300 yards east of where the slip occurred. These were resolved by a refurbishment of the drainage in 2010.
Safety first
The operation to remediate January’s landslide was mobilised immediately, the works being undertaken by Construction Marine, Network Rail’s earthworks framework contractor for the London North Eastern and East Midlands route. AECOM was responsible for the design.
A compound was secured at the entrance to a field rented from the local farmer and traffic control measures introduced on the adjacent A695 to account for the number of plant movements. Despite the inconvenience caused, locals continued to support the on-site team with hotdogs and boxes of cookies. Around 30 of them accepted an invitation to visit the site over the first weekend to gain some understanding of what was going on. Relations have remained excellent.
Over the first four days, the project team progressed the urgent task of making the slope safe for workforce and machinery. With the inundation of water continuing, trees could still be heard cracking and falling to the ground. Northumbrian Water attended to turn off the sewer and divert it back 50 metres from the crest; meanwhile a field drain was intercepted and taken eastwards to a cascade at the top of the cutting where it discharged into a different drainage system. This allowed the slope to dry out…or at least that was the hope.
Eye in the sky
Initially, the intended methodology for disposing of the spoil involved moving it to the bottom of the cutting and taking it out by rail. However, large amounts of water were issuing from springs mid-slope – as had been the case during the cutting’s excavation – and liquefying the material. From a safety perspective, this instability rendered unsustainable the idea of bringing trains onto the site.
An unmanned aerial vehicle (or drone if you prefer), operated by Central Alliance, was brought in to perform a full photogrammetric survey of the landslide. This was repeated twice during the remediation. From thousands of geo-referenced digital images, a topographical 3D model was produced to an accuracy of a few millimetres. On the first pass, considerable interpretation was needed to account for the fallen trees – numbering around 100 – and reveal the ground profile. Without getting boots dirty, valuable information was derived on the extent of the slip, its shape and the volume of material involved – crucial in estimating how long the line might be closed.
Ground investigations, fulfilled by Geotechnical Engineering, found that the top part of the slope – to a depth of around 15 metres – consistently comprised glacial till: sands and gravels, with boulders of perhaps a metre across. Below this was a variable layer of clay, impermeable to water. At the surface, self-seeded silver birches covered the slope, together with some scotch pines. The former’s shallow root system acted to stabilise the ground to a depth of around 0.5 metres but, with the failure plane much lower, each one also acted as an approximately 20-tonne point load which tended to open the ground up as the wind acted on it. By saturating the sands and gravel with floodwater and sewage – increasing the weight of the ground above the clay layer by 40-50% – all the elements had come together for a significant slip. The back scar reached the line of the sewer where a trench must have been excavated at some point in the past 60 years. Could this have created a weakness which acted as a tipping point? We’ll never know.
What had become clear from observation, investigation and technical insight was that the landslide was not just a washout as originally thought. Something more significant was happening: a rotational failure in the hillside. This changed the complexion of the recovery operation.
The earth moves
The field above the slip hosts the buried remains of a Roman fort, discovered in the 1950s and comprising three camps. The nearest one to the crest – about 15 metres away – dates from just ten years after the Roman invasion. Archaeologists from Historic England were on-site for the first three weeks, supervising the removal of topsoil and recording any finds. Geophysics surveys were carried out to map the structures and ensure all works took place clear of them. Their presence did constrain the location of entry ramps into the cutting.
Tractor-mounted winches, sitting at the crest, were used to pull the fallen trees up the slope where they were fed into chippers. Some of what came out was used to form the surface for a 300-yard walking route from the compound to the site. Thereafter it was possible to get in heavy earth-moving machinery to start the regrading, taking weight off the top of the landslide – which otherwise would have continued to drive it – before removing material at the toe in order to clear the railway.
Throughout, it was regarded as an active landslide with established slip planes, so a constant balance had to be maintained as the slope was battered back from 1:1.6 to 1:2.5. At its peak, more than a dozen large items of plant were involved in the operation, from track dumpers to 40-tonne excavators. Around 35,000 tonnes of spoil was shifted and stockpiled in the field until a final resting place for it was agreed.
As a temporary measure, two deep counterfort drains were provided on the lower of two benches to deal with the water flows from mid-slope. A permanent, future-proof system will replace them as part of the full remediation works – now underway – which will also involve placing rock pitching on the lower slope to control shallow surface movement. Higher up, top soil and seed will see vegetation re-establish itself. There’ll also be inclinometers for monitoring purposes.
Whilst the p-way sustained no actual damage, the Down (Carlisle-bound) line did slew over towards the Up by around 100mm. The correct alignment was restored during tamping. Network Rail’s works delivery unit supplied a rail vac which, over two days, removed fouled ballast from a 70-yard section on each road, as well as a shallow cess drain. Around 600 tonnes of fresh ballast was put back. The vac proved significantly more efficient than the traditional approach, obviating the need to cut the track and lift it out, excavate, replace, weld and stress.
More to come?
Trains started running again through Farnley Haugh on Monday 8 February, barely a month after the line was blocked by trees and slurry. That’s no mean feat given the scale of the landslide and the difficulties it presented. The site will remain active well into the spring to deliver the final design, deal with the spoil and demobilise.
Thankfully, fire-fighting is something the railway is very good at. Just as well – there’s been lots of it to do since the storms ravaged Britain. If the climate change experts are to be believed, severe wet weather is set to become more commonplace during our winters. Whilst there is no emergency brake for that, we can at least see it coming. The challenge then is to make the infrastructure more resilient to its impact. That will have both financial and engineering implications, but it’s something Network Rail is committed to tackling into CP6 and beyond.
