Shrink-Swell Subsidence

Primary reference(s)

BGS, 2020. Swelling and Shrinking Soils. British Geological Survey (BGS). Accessed 27 September 2020.

Additional scientific description

The properties of expansive soils are attributable to the presence of swelling clay minerals. These clays range in their potential to absorb water according to their different structures. Expansive clay groups with increasing susceptibility to swelling include kandites (e.g., kaolinite, haloysite), illites (e.g., phengite, glauconite), vermiculites and smectites (e.g., montmorillonite, talc). These minerals are a product of weathering, commonly formed on land and then transported to the oceans. Their distribution reflects the underlying source rock geology, its diagenesis and stress history (e.g., stress-induced smectite to illite transformations) and the nature of the weathering, for example, wet climates are associated with kaolinite rich soils and dry environments are characterised by smectite clays (Eberl, 1984).

Metrics and numeric limits

For the most expansive clays, expansions of 10% are common (Nelson and Miller, 1992). In the field, expansive clay soils are recognisable in the dry season by the deep cracks that form in roughly polygonal patterns. The zone of seasonal moisture content fluctuation can extend from one to tens of metres in depth. This creates cyclic shrink/swell behaviour in the upper part of the soil column, and cracks can extend to considerable depths.

Examples of drivers, outcomes and risk management

Beneath the depth of influence of atmospheric change in moisture content, the water demand of vegetation, particularly trees on clay soils dominates the moisture content changes that lead to the soils shrinking (subsidence) and swelling (heave). Where subsidence and heave occur beneath or close to properties and infrastructure this can result in damage (Florida Department of Environmental Protection, 2020). The most obvious way in which expansive soils can damage foundations is by uplift as they swell with moisture increases. Swelling soils lift up and crack lightly-loaded, continuous strip footings, and frequently cause distress in floor slabs. Uplift is commonly differential, reflecting the different resisting forces across the structural foundations.

The extensive distribution of these soils across the world has necessitated characterisation through index testing to inform remedial measures. At its simplest, the plasticity indices are utilised to define inorganic clays with inherent swelling capacity (e.g., BRE, 1993). Expansion of soils can also be measured in the laboratory directly, by immersing a remolded soil sample and measuring its volume change or using LiDAR techniques (Hobbs et al., 2014).

The best way to avoid damage from expansive soils is to extend building foundations beneath the zone of water content fluctuation as modified to reflect the presence of vegetation (Rogers et al., no date).