A Definition

Collapsing and settling soils are relatively low density materials that shrink in volume when they become wet, and/or are subjected to great weight such as from a building or road fill. The process of collapse with the addition of water is also known as hydrocompaction.

Characteristics

Collapsing and settling soils have considerable strength when dry and generally are not a problem to structures and improvements. When they become wet, they are subject to rapid collapse and can be reduced in volume as much as 10 to 15 percent. Surface ground displacement of several feet or more can result. Similar processes frequently affect old landfills or poorly placed earth fills.

Consequences

The large ground displacements caused by collapsing soils can totally destroy roads and structures and alter surface drainage. Minor cracking and distress may result as the improvements respond to small adjustments in the ground beneath them.

Aggravating Circumstances

Man’s activities are definitely the cause of most soils collapsing. These activities include watering grass and shrubs, failing to repair leaking water lines in utility trenches, impounding water, blocking drainages by highways, loading excessive weight upon collapsible soils, and any activity which increases subsurface moisture in soils prone to collapse.

Man-made and/or man-placed materials frequently are subject to collapse and settlement. The filling of mined out areas, natural depressions and swamps with trash and debris is a common practice. Eventually the site is put to another use. Decomposition and compaction at landfill dumps also can result in generation of explosive methane and poisonous hydrogen sulfide gases, as well as pollution of subsurface water with carbolic acid or other chemicals. These problems, in addition to settling, occur despite compaction during the landfill operation.

Damage to structures erected on landfills is common if proper construction methods are not used to counteract settling and other problems. Dangerous methane can seep into basements and crawl spaces and explode, demolishing the structure.