Swelling

• Many types of clay particles are capable of absorbing large volumes of water, causing them to increase substantially in size compared with their dry state. Repeated cycles of dry and wet conditions cause such soils to form networks of fissures, resulting in the familiar cracked, polygonal landscape typical of non-sandy desert areas. Individual clay particles may shrink and swell up to 1,000 percent. While overall soil swelling is not nearly this extreme, volumetric changes of even 5 percent can damage human structures.
  
  

Hydrocompaction

• Hydrocompaction is essentially the opposite of swelling. In dry areas with a mix of sand, silt and clay, dry clay or silt particles act as a sort of cement, holding the sand particles in an open matrix with air spaces in between. This open soil structure is vulnerable to water infiltration in the form of soaking rain, irrigation or broken pipes. The sudden influx of water dissolves the silt and clay, causing the air spaces between the sand articles to collapse and the affected area to sink, often dramatically.
  
  

Impact on Structures

• Whenever soil shrinks or expands, it can have drastic effects on adjacent structures like foundations, retaining walls, sidewalks -- even canals and dams. A foundation wall built in dry, compacted soil faces huge forces pushing against it if that soil swells after a heavy rainfall or broken water pipe soaks it. A hydroelectric dam built on soil prone to hydrocompaction may fail catastrophically in the event of a large subsidence under one edge. Even subsidence in an agricultural field may cause crop loss.
  
  

Managing Soil Problems

• Soil surveys and soil engineering tests before construction begins are a vital way to reduce the risk of later structural damage from soil swelling or hydrocompaction. If only a small area is affected, it may be cost-effective to remove and replace the problem soil with stable soil. In larger areas, applying hydrated lime to shrinking and expanding soils helps to stabilize them. Soil prone to hydrocompaction can be mechanically compacted, and drainage can be improved to prevent future soil saturation.