London soil moisture deficit exceeds surge threshold

Greater London has experienced a significant soil moisture deficit, exceeding a surge threshold of 320mm in July. This condition, driven by prolonged hot and dry weather, means that shrinkable clay soils have contracted, reducing the support available beneath buildings and infrastructure. The Met Office rainfall and evaporation calculation system (Morecs) data for 2026 highlighted the exceptional nature of this year's conditions. By 1 September, these levels remained comparable to those associated with major subsidence events in previous years, such as 2018 and 2022.

Understanding the impact of dry weather

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Prolonged periods of hot weather can draw significant moisture from shrinkable clay soils. As the clay loses moisture, it contracts, which can lead to movement and damage above ground. This effect is exacerbated by factors such as trees drawing water from the soil, intensifying competition for moisture during droughts. The consequences of this uneven drying can affect foundations differently, potentially causing structural issues. The impact of this drying process can accumulate over weeks, and the ground may not immediately return to its previous state even after rainfall.

The risk of ground movement can persist even after temperatures have fallen and rainfall has returned. This is because the soil requires time to absorb and redistribute moisture. The current situation suggests that sustained stress within shrinkable clay soils is a continuing concern, even as summer draws to a close.

Implications for the built environment

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The implications of changing ground conditions extend across the built environment, affecting commercial buildings, industrial facilities, transport networks, and public infrastructure. For commercial properties, ground movement can impact machinery, operations, safety, and business continuity. For infrastructure, consequences can include damage to roads, railways, and utilities. Maintaining assets over decades becomes increasingly challenging when the ground conditions supporting them are not static.

Traditional remediation methods, such as excavation and underpinning, can cause significant disruption and lost revenue for businesses, or closures and restricted access for infrastructure. Less invasive techniques, like geopolymer resin injection, offer an alternative. This method uses small injection points to improve weak ground and restore support without extensive excavation.

Adapting to a changing climate

As hotter, drier periods become more prevalent, ground conditions are becoming a key consideration for climate resilience. For assets with long design lives, there is a need to move beyond planning for past ground conditions and consider those likely to be encountered in the future. This may necessitate changes in how sites are assessed, existing assets are managed, and maintenance is planned throughout their lifespans. Ground conditions may need to be viewed less as a fixed characteristic and more as a dynamic element responsive to a changing climate.

Jon Chevin, Geobear commercial and infrastructure director, stated that while weather forecasts provide detailed atmospheric information, there is less visibility regarding underground conditions. He suggested that as we become better at predicting extreme weather, the next step should be understanding its implications for the ground. This involves considering how weather and ground conditions interact, prompting property owners and infrastructure operators to identify vulnerable assets.