page_title: Shrink–swell capacity
section_title: Damage
hierarchical_section_title: Shrink–swell capacity / Damage
caption_attribution_description: English: Parched/cracked earth in Bourne Woods. These cracks were noticed near the Ponds - after a few weeks without rain.
context_page_description: The shrink–swell index of clay refers to the extent certain clay minerals will expand when wet and retract when dry. Soil with a high shrink–swell capacity is problematic and is known as shrink–swell soil, or expansive soil. The amount of certain clay minerals that are present, such as montmorillonite and smectite, directly affects the shrink-swell capacity of soil. This ability to drastically change volume can cause damage to existing structures, such as cracks in foundations or the walls of swimming pools.
context_section_description: Clay groups with a high shrink–swell capacity tend to damage crops during dry spells, as the soil contracts, by pulling roots apart. Soils with shrink-swell capacity can cause engineering problems, or damage to existing structures. The swelling can cause structures to heave or lift, and the shrinking can uneven settling of sediment underneath foundations, potentially causing the structure to fail. Some common structures that sustain soil damage are foundations, walls, driveways, swimming pools, roads, pipelines, and basement floors. Roughly half of the houses in the United States are built on soils that are considered unstable, and half of those will sustain damage from soil. This damage includes large cracks in walls and foundations, buckling of driveways and roads, and jamming of doors and windows. 
  Shallow pipes, which are buried in the zone of seasonal moisture fluctuation, are put under stress by shrinking soils, which can cause breakage of water or sewage pipes. Swimming pool shells can crack due to this pressure as well, and leaky pools can also introduce a lot of water into the surrounding soils over time, which could end up lifting pool decks and nearby foundations. Annually, there is an estimated $7 billion in damage caused by clay shrink-swell soils. All this damage is caused by the force exerted by expanding soil, or ground heave.
  Expansive soils are the most problematic in regions with very defined wet and dry periods, as opposed to areas that maintain a certain level of moisture throughout the year, as this annual cycle causes the soils to expand and swell every year. Water can also be introduced into the soil through people, or their infrastructure. Damage is quite often caused by differential swelling which is caused by pockets of wet soil situated right next to dry soil. Examples of localized water sources include sprinkler systems, cesspools, leaky pipes, and swimming pools.
caption: Parched earth in Bourne Woods
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