Managing railway earthworks in a changing climate
The fatal derailment at Stonehaven has put the condition of railway earthworks into sharp focus. GARETH DENNIS looks at how the extremes of weather as a result of climate change can impact on earthwork resilience.
A version of this article also appeared in Issue 912 (26 August 2020) of RAIL magazine.
I originally wrote this piece whilst Coronavirus was rightly holding the attention of news cycles, but just prior to its publication, a fatal derailment occurred near Stonehaven in Aberdeenshire following very high rainfall and a spate of landslides. Suddenly the impact on our railway infrastructure of a rapidly changing climate and its impact on weather extremes became headline news.
These extremes are taking an increasing toll on all aspects of railway operation, but earthworks — hidden away out of sight in most cases, and having had little attention since being cut or laid down over a century ago (nearly two centuries ago in some cases) — are at particularly risk of failure. Whilst we shouldn’t speculate over the details of the derailment, it is vital to understand the wider impacts of climate change on the resilience of our infrastructure.
February 2020 was the wettest February on record for the UK, and the fifth wettest calendar month since 1862 — it is worth noting that two of other record-holding months were in 2009 and 2015. Storms Ciara, Dennis and Jorge wreaked havoc across the country, with parts of the rail network being severely impacted by flooding and washouts.
Storm Ciara was Britain’s worst storm in seven years, with a combination of 100mph winds and intense rainfall resulting in widespread closures. Only a few days later, Storm Dennis brought even more flooding, landslides, damaged overhead line equipment (OLE) and vegetation being blown into the path of passing trains. Not much after that, Wales was particularly badly hit by Storm Jorge, with lines in the Cardiff Valleys and Aberdare being partially closed, and — having only recently required significant repairs from previous flooding — the Conwy Valley Line was severed entirely.
This has been followed by the sunniest May on record for the UK, and the driest May on record for England. This may sound less impactful — other than for those of us locked indoors with nowhere to enjoy the sun — but it actually represents another dangerous extreme that stretches the capability of our rail infrastructure.
Whilst temperatures over this period have been warmer than usual, we haven’t seen exceptional extremes across the country in the same way we have for rainfall and sunshine — though heat in the Scottish Central Belt was enough to result in cancellations to manage the effects of high rail temperatures (see my Analysis piece in RAIL856).
This notwithstanding, the first half of 2020 was characterised by weather extremes. And throughout this year, we’ve also had a string of earthworks failures (I work as part of a team who have had to design the remedy for several of them), though without severe consequences. But to what extent is this year’s extreme weather related to climate change as the planet warms?
To understand the current trajectory of global warming, there is no better visualisation than Ed Hawkins’ “warming stripes” which shows the rise of average global temperatures since 1850. At a climatic level, things are getting a lot warmer.
How, then, does that manifest itself in terms of weather? Alex Priestley, a meteorologist and geophysicist, explains: “As global temperatures increase, the UK will see more weather extremes. Heatwaves and summer droughts are getting much more likely, as is heavy rainfall and high winds. This means more inland flooding. As sea levels rise, we’ll see more coastal flooding too.”
This sounds like the perfect storm for stretching infrastructure to its limits, and as I hope to explain, earthworks are particularly vulnerable.
But what precisely are earthworks, how do they work, and how many are there on our railway?
The specifics of the difference between an “at-grade” railway and one that is supported by an embankment or carried within a cutting depends on who is managing the asset. A common classification is where the railway is 3m above or below the normal ground level, but some bits of Network Rail widen this classification to 2m.
So what of the earthwork itself?
At its simplest, soil consists of three components: solids (usually consisting of particles or grains of rock), water and air. Just as with a block on a table moving when submitted to a force, it is the friction between the solid components of a soil that stops it failing. Similarly, as you angle your table over, at some point the force of gravity will overcome the friction between the block and the table.
In our soil, this angle is the maximum stable slope angle, and as you increase the percentage of a soil that is formed of water, you reduce the friction between the soil particles… Too steep an angle or too much water in the soil, and our block falls off the table.
So a soil’s water content — specifically, the pressure of water pushing the solid bits of a soil apart from each other, known as pore water pressure — is directly tied to its strength. This means that healthy earthworks rely on functioning drainage.
You might wonder why motorways don’t suffer from the same challenges: essentially, by the middle of the last century, civil engineering consultants and contractors had an immensely better understanding of geotechnical engineering than their predecessors. Look to older roads and you’ll see regular landslips, just as the railway suffers from.
Network Rail manage around 16000 route kilometres, of which 60% utilises earthworks — that’s around 10000 kilometres of embankments and cuttings to keep intact. Lots of this was built with steep sides and — in the case of embankments — with poor quality fill consisting of whatever the railway company could cheaply get its hands on.
The Victorian approach to material procurement and construction gives us a range of serious geotechnical problems, most of them hidden (at least to the naked eye). These issues are further compounded by more recent vegetation growth and the poor condition (or lack of existence) of suitable drainage.
On average, there are around 100 earthworks failures every year that result in an impact on GB train services, though this average has fluctuated over the last few years to be as high as 140 per year… Ten years ago, this figure was only at 65 failures.
A substantially busier railway means that even if trains are only indirectly impacted through delays, these quickly ripple around the network causing major problems such as displaced staff and overcrowding. Even more critically, greater numbers of services means an increased risk of a train being derailed by an earthworks failure — and the consequences of this can be severe, not least if more than one train is involved.
This is precisely what happened at Watford Tunnel in 2016, where a catastrophic head-on collision between trains with a closing speed of 150mph was only barely averted.
So what can be done?
Getting a grip on vegetation management is probably the easiest of the various responses required.
Grass and low-level foliage are a good thing as they restrict the amount of water that trickles down into the earthwork. Roots can act as binders to protect topsoil (and perhaps the earthwork below). However, tall, mature trees that have a high water demand cause earthworks to swell and shrink cyclically, making a mess of track geometry and increasing the risk of failure.
Controversial though the subject may be, safety must take precedence. There should be no high water-demand trees within a distance from the track of around 1.5 times their height.
Getting out and renewing what we already have is clearly also a necessity.
This doesn’t just mean shoring up, digging out, reinforcing or otherwise augmenting the earthworks themselves, but means cleaning out, replacing and providing new drainage of both tracks and earthworks.
HMRI already knew that the condition of the railway’s earthworks was of critical importance to the integrity of the network, hence their desire for a focus on renewals and maintenance in Control Period 6 (Network Rail’s funding settlement from 2019 to 2024). We’ll be able to look back on whether this translated to a leap in renewals in five years, but the preceding Control Period (CP5 from 2014–2019) had already seen a jump in volumes.
A detailed breakdown is only available from 2014/15 onwards, but the figures show that there was a doubling in the volume of earthworks receiving attention between that year and the succeeding four years (there is often a year offset in relation to the kick-off of physical work at the start of a Control Period as contracts are put in place and designs are completed). More impressively is the near-quadrupling in the volume of renewal, refurbishment, maintenance and newly-built track and earthworks drainage over that period.
The task gets harder year on year, though. Network Rail is dealing with an array of challenges: decades of bare-minimum patch-and-mend, the tremendous leap in traffic that the railway is carrying, and the increases in extremes of wet and dry.
How, then, can new ideas help to reduce the likelihood and impacts of earthworks failures?
Remote monitoring of earthworks is one way in which we have been better predicting the condition of earthworks, preventing sudden failures and avoiding the danger and disruption they cause. However, Network Rail need to be supported to fund such initiatives on an ongoing basis, and to store, manage and make the most of the resulting data. Would more long-term involvement from universities enhance this capability?
Equally as important is that improved weather and climate forecasting supports clearer operational and strategic thinking. Whilst this is very much the domain of the Met Office, could there be a closer working partnership with Network Rail?
There are no perfect answers to these questions (a well-funded asset owner is clearly vital), but unless we make the most of the data revolution, when it comes to the impact of a changing climate on our earthwork assets, the challenge might be insurmountable. Stonehaven has realised the industry’s worst fears: that our infrastructure is being pushed harder than we can keep pace with.
