Managing the impact of extreme weather and climate change on embankments in Southern Region
Changing weather conditions
According to the Met Office, all of the UK's ten warmest years on record have occurred since 2002. Heatwaves, like that of summer 2018, are now 30 times more likely to happen due to climate change. Heavy rainfall is also more likely. Since 1998, the UK has seen seven of the ten wettest years on record. The winter storms in 2015 were at least 40% more likely because of climate change.
Heatwaves impact our railway by drying out the soil it is built on, both through heat radiation and through trees and vegetation soaking up all the water. This makes it harder to keep track in good condition as the land dries out unevenly. In addition, as trees become water stressed, they are less able to withstand high winds and storms and can fall onto the railway.
Heavy rain has an even worse impact
Two recent landslips in Kent, at Newington and High Brooms, occurred after the months of December 2020 and January 2021 saw double the expected rainfall. At the same time, a vulnerable embankment at Salfords, near Redhill, Surrey, needed emergency repairs ahead of already-planned work as sensors detected it moving.
This followed previous incidents on an even bigger scale the previous year. November 2019 had three times the expected rainfall, and a single week that December saw a whole month of rain and the soil became so saturated it didn’t dry out completely until May.
The result was a series of landslips of varying severity in early 2020, the most notable of which were at Epsom and at Edenbridge, which remained closed until late March that year. We also had line closures from Horsham to Dorking for a land slip to be repaired at Ockley. There were other less serious slips:
- On the Brighton Main Line at Wivelsfield, near Burgess Hill.
- On the East Grinstead line at Cookspond near Dormans.
- On the Uckfield line at Hever.
So far in 2021 we have ongoing landslips across the region including:
- At High Brooms on the Hastings line between Tonbridge and Robertsbridge in Kent
- At Newington between Gillingham and Sittingbourne in Kent
- In the Salfords area on the Brighton Main Line in Sussex
- Ockley landslip in December 2020.
Southern Region looks after earthworks that if joined together would stretch between Lands End and John O Groats. Our budget for those earthworks is over £133m, over five years.
Climate change is projected to increase the severity and frequency of extreme weather and we are planning now in order to provide a safe, reliable network in the future.
Network Rail has a Weather Resilience and Climate Change Adaptation Strategy which aims to embed management of climate change within everything that we do and Southern Region has Weather and Climate Change Action Plans detailing the work we are doing to increase the resilience of the railway.
We are expecting to spend £130m in the next five years in the Southern region, improving our railway’s resilience by tackling vulnerable earthworks and rolling out remote sensors on 6 miles of line, to alert us ahead of any problems.
How we manage the impact of extreme weather impacts on embankments in Southern Region
Our railway was the among the first in the world and some of our busiest lines were built as early as the 1830s. This includes the Redhill-Tonbridge line, where the Edenbridge landslip happened on 22 December 2019. Work began here in 1836.
To save money, railway builders kept their land purchases to a minimum, which meant embankments and cuttings were very narrow and steep-sided. This makes them vulnerable to extreme weather. In addition they employed contractors of varying quality. Notably, the tunnels on the Tonbridge to Hastings line were built without enough brick layers and had to be strengthened later on, making the tunnels narrow and single track.
The cutting at High Brooms was recently “regraded” or reduced in steepness, from a gradient of 40%, to 32%. In comparison, modern railways such as HS1, and motorways, have cuttings of just 18%. To get the existing railways to that level would be impossible without buying huge amounts of land and indeed, many people’s houses too.
In addition, knowledge of “soil mechanics” was not advanced in those days and embankments were made of whatever material was extracted as spoil from cuttings. In some cases, such as Edenbridge and previous landslips in places such as Stonegate, different layers of material were dumped on top of each other and the railway built on top. This is a particular problem when chalk and clay are mixed – which is often the case in the south. Water permeates the chalk and then hits the clay and runs off it. As the different materials absorb water at different rates, eventually one layer will slide off the other and a landslip develops.
Clay is a continuing challenge for Network Rail as it absorbs water and expands during the winter and then dries off and contracts as trees and vegetation soak up the water. This leads to poor track quality and at worst, landslips.
Landslips traced directly to clay construction include: High Brooms, Newington, Wivelsfield, Edenbridge, Ockley and East Grinstead. In fact, all our major landslips recently have been on railways built on clay soil and one of our challenges on the Hastings line is that it cuts through large deposits of Wadhurst clay.
It is a myth that trees hold embankments and cuttings together and landslips happen when we remove them. Trees are quite useful at the toe of embankments, as their weight acts as an anchor, but anywhere else on embankments and cuttings, their weight can pull them down and during summer they soak up water and cause the earthworks to dry out. In addition, in a wet winter their root balls weaken and they can fall on the railway in high winds.
We manage our infrastructure under these challenging conditions by:
- Monitoring track quality and sites where we have concerns;
- Taking proactive action, such as at Barnehurst and Wivelsfield in 2020 and Wadhurst in 2021, to rebuild and regrade weak earthworks (effectively making them less steep);
- Rebuilding drainage, such as on the Brighton Main Line upgrade project, maintain it and keep it free from debris; and
- Removing trees and vegetation from cuttings and embankments.
How are we going to adapt the railway to climate change?
We have commissioned a major report into climate change and our railway infrastructure in the South and what we need to do to adapt it to changing weather patterns. We know we can’t rebuild every mile of railway, but we can continue to work hard to protect it and also find out if there are new ways of working or technology that can help us.