Soil Moisture Deficit on the railway

Soil Moisture Deficit on the railway

Published 1 September 2022 | Average read time
3 min read
Stories Delays explained Environment

Recent extreme temperatures have presented many challenges for the railway – the latest addition: Soil Moisture Deficit (SMD)

Summer on the railway can bring extremely hot temperatures. These extreme temperatures pose a problem for the clay soil that our railway is built on – particularly in the Southern Region on the West of England Main Line. Because of this, we have introduced a limited timetable on the West of England Main Line from Friday 2 September until October.

July’s heatwave dried out soil across the region through heat radiation. Trees and vegetation then soaked up any remaining water from the ground.  

As the soil dries out it shrinks. This is a problem called Soil Moisture Deficit (SMD). Watch below to find out more.

What does Soil Moisture Deficit do to the railway?  

When the ground beneath the railway moves or shrinks, it can cause the track to move with it.  

It isn’t safe to run trains at full speed if this happens – just like it wouldn’t be safe to driver over potholes in a road vehicle. We have to introduce temporary or emergency speed restrictions when this happens.  

In the summer of 2022 Britain experienced its driest July since 1935. This was the the driest July since 1836 in the Southern Region. These drought conditions have led to the highest levels of SMD since we began keeping records in 2009.

Raised track due to sunken sleeper on dry ground

The problem with clay 

NR engineer holding dry clay

SMD is a particular problem with embankments that contain clay soil. Many of our embankments in the Southern Region are made of clay soil. 

Clay absorbs a lot of water and severely shrinks when it is dry. This means that we see more track movement due to the pronounced ground shrinkage.  

Little was known about the makeup of the soil that was being built upon when our railway was built in the 1800s.

Engineers then didn’t know how the railway would react in the temperatures we’re experiencing, which could not have been predicted back then.

To re-engineer all our clay embankments would cost tens of billions of pounds and take decades to complete. We’ve decided to continue to research the problem rather than cause massive disruption to your journeys.  

Close up of rail in Southern Region during heatwave

What we do to prepare 

NR engineers realigning track with help of specialised tamper train

We usually know when to expect SMD, but in extreme cases like what we’re seeing in 2022, we’re having to deal with it earlier and for longer. 

The best defence against SMD is preparation. We monitor sites where SMD might be of concern and may remove trees and vegetation that are absorbing water and drying out embankments.  

We are starting to plan for the effects of SMD at the same time as our hot weather preparation. We ensure that the track is at the best possible quality before the hot weather sets in.  

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