UK shrink-swell hazard under climate-change-driven weather extremes

Shrink-swell in clay soils is a major geohazard in the UK and costs the economy over £400 million a year. By 2050 this will rise to over £600 million due to climate-change-driven weather extremes. Research predicts a 50% increase in shrink-swell subsidence across Europe by 2040. Many towns, cities and major infrastructure corridors in the UK are founded on clay-rich soils, especially in the south-east UK, where they underlie most of London. Volume change of clay minerals within these soils is the primary cause of shrink-swell, and is controlled by mineralogy, temperature and rainfall. So, some areas are more susceptible than others depending on their underlying geology and regional climate. Between January and June 2022, the UK experienced the driest weather in over 40 years, culminating in July, with the hottest days on record when temperatures exceeded 40 degrees Celsius for the first time ever. These unprecedented hot, dry conditions will have resulted in levels of clay shrinkage never previously experienced in the UK, and significant rainfall will be necessary for their recovery. Climate change is expected to increase shrink-swell hazard in those areas already susceptible to subsidence and will expand into other areas where subsidence has not occurred before. But the rates of these changes are unknown.

Understanding of shrink-swell processes at present is based on laboratory-based research and theoretical relationships, but these results require validation from field research on in situ ground conditions such as moisture changes with depth. The recent rapid onset heat extremes and drought, therefore, is a unique opportunity-right now-to investigate how the UK subsurface responds to increasingly extreme weather conditions. The overarching science question is: How will shrink-swell hazard change under an increasingly extreme UK climate? To answer this we will determine: a) the impact of the 2022 drought and unprecedented July heatwave on subsidence and soil moisture in shrink-swell-susceptible regions; b) how soil moisture varies with depth during these extremes, and how this evolves with the onset of wetter conditions; and c) whether the extreme changes experienced in 2022 can underpin improved understanding of climate change impact on future shrink-swell hazard?

The shallow subsurface response to the extreme dry and hot conditions just experienced, is short-lived and will evolve with wetter Autumn-Winter weather. Right now, there is a unique opportunity to record in situ shallow subsurface moisture and ground deformation at its lowest ebb for decades, and to record and monitor the ground response to the onset of wetter conditions. For this, urgency funding is necessary to establish three shrink-swell observatories for regular field observations and continuous geophysical measurements of shrink-swell behaviour as weather conditions change. Samples will also be acquired for laboratory analysis that will link measured physical properties to the field experiments.