Expansive clay, also called expansive soil, is a

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

prone to large volume changes (swelling and shrinking) directly related to changes in

water content Water content or moisture content is the quantity of water contained in a material, such as soil (called ''soil moisture''), rock, ceramics, crops, or wood. Water content is used in a wide range of scientific and technical areas, and is expressed ...

. Soils with a high content of expansive minerals can form deep cracks in drier seasons or years; such soils are called

vertisol A vertisol is a Soil Order in the USDA soil taxonomy and a Reference Soil Group in the World Reference Base for Soil Resources (WRB). It is also defined in many other soil classification systems. In the Australian Soil Classification it is c ...

s. Soils with

smectite A smectite (; ; ) is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secon ...

clay minerals Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay mineral ...

, including

montmorillonite Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite grou ...

, kaolinite, and illite group minerals, such as montmorillonite-containing

bentonite Bentonite ( ) is an Absorption (chemistry), absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelli ...

, have the most dramatic shrink-swell capacity. The mineral make-up of this type of soil is responsible for the moisture retaining capabilities. All clays consist of mineral sheets packaged into layers, and can be classified as either 1:1 or 2:1. These ratios refer to the proportion of tetrahedral sheets to octahedral sheets. Octahedral sheets are sandwiched between two tetrahedral sheets in 2:1 clays, while 1:1 clays have sheets in matched pairs. Expansive clays have an expanding crystal lattice in a 2:1 ratio; however, there are 2:1 non-expansive clays.

Identification of expansive clay

To confirm the presence of expansive clay soils, geotechnical engineers may perform laboratory testing techniques to assess shrink-swell potential.

Atterberg limits The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit. Depending on its water content, soil may appear in one of four states: solid, semi-solid, plasti ...

measure the plasticity index (PI) and liquid limit (LL) to determine soil behavior, where PI over 35 indicates high expansion potential, and LL over 50 suggests significant swelling characteristics. A free swell test measures the free swell index (FSI) by comparing the volume of dry soil in water to its volume in kerosene. A mineralogical analysis is an X-ray diffraction (XRD) test that can identify specific clay minerals, such as montmorillonite, which would indicate shrink-swell qualities of the soil. Field assessment for expansive soil may also be conducted, including a visual inspection to identify surface characteristics such as gilgai topography (alternating mounds and depressions) and deep shrinkage cracks. Another field assessment is a simple hand test that involves rolling moistened soil into a thread. If the thread can be rolled to a diameter of 3 mm without crumbling, the soil likely contains a measurable amount of clay content, indicating some expansiveness.

Risk assessment

Identification criteria for soil expansion based on oedometer swell potential values:

Managing expansive soils

Managing the effects of expansive clay on structures built in affected areas is an important consideration in

geotechnical Geotechnical engineering, also known as geotechnics, is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics to solve its engineering problems. I ...

and

structural engineering Structural engineering is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and joints' that create the form and shape of human-made Structure#Load-bearing, structures. Structural engineers also ...

. Several techniques are used across the construction sector, depending on environment, cost, and schedule limitations.

Structural engineering

Structural solutions are engineered designs integrated and implemented during the foundation phase of construction. Structural design elements mitigate soils from the upward or adjacent pressures of swelling soils through physical means. One or more techniques may be employed to manage expansive soil effects. These include designs that call for digging deep foundations or installing pile systems extending beyond affected soil depths and reinforced slab-on-grade construction with underlying crossbeams. Void spaces beneath foundations, created with void forms, provide empty spaces for swelling soil to move into, insulating the foundation from the impact caused by this pressure. Void forms come in various materials and formats, including degradable (paper, carton forms) and non-degradable (collapsible plastic, EPS foam), designed to work under structural concrete slabs, piers, and beams.

Chemical stabilization

Chemical stabilizers are specialized compounds that modify soil properties to create a more stable foundation for built-in expansive soil conditions. They work by altering the soil's chemical composition and physical characteristics through biological, physical, and chemical additions. Common chemical expansive clay treatments include lime,

portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar (masonry), mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in th ...

fly ash Coal combustion products (CCPs), also called coal combustion wastes (CCWs) or coal combustion residuals (CCRs), are byproducts of burning coal. They are categorized in four groups, each based on physical and chemical forms derived from coal combust ...

, and polymer-based additives.

Moisture control

Moisture control manages the swelling potential of expansive soils through preventative measures designed to redirect environmental water sources and maintain a consistent moisture equilibrium beneath and around structures. Strategies may include drainage systems and grading for surface water management, using sump pumps, impermeable vertical moisture barriers, and landscaping designs incorporating vegetation, moisture-regulating ground cover, and/or buffer zones around the structure.

References

{{Soil type Types of soil Soil mechanics Soil physics Sediments