Another type of ground subsidence that commonly occurs in Colorado is the settlement and ground collapse that occurs in certain types of geologically recent, unconsolidated sediments — usually referred to as soils by engineers and contractors. This group of soils that can rapidly settle or collapse the ground is known as collapsible soils. Ground settlement can damage man-made structures such as foundations, pavements, concrete slabs, utilities, and irrigation works.
Hydrocompactive soil is the most common type of collapsible soil. The term suggests that the introduction or presence of water and the resultant compaction of the soils once they become wet is the driving mechanism.
Hydrocompactive soils form in semi-arid to arid climates in the western US and large parts of Colorado in specific depositional environments. It is characterized by low density and low moisture content. The soil grains in this dry soil are not packed tightly together. Instead, the grains are precariously stacked, like a house of cards. This loose soil skeletal fabric is preserved because the grains are “tack-welded” to each other by clay and silt buttresses, soil suction pressures, and other sensitive binding agents that all have one thing in common; they are water sensitive. While strong in a dry state (commonly referred to as meta-stable state), the introduction of water into these dry soils causes the “tack-welding” binding agents to quickly break, soften, disperse, or dissolve. The larger soil grains then shift and shear against each other to reorient into a denser configuration. This relatively rapid densification of the soil causes a net volume loss of the soil deposit, which manifests at the ground surface as subsidence or settlement.
Ground settlements from the adverse saturation of thick collapse-prone soils have been documented at over six feet (1.8 m). The location shown in the photo above is on an alluvial fan in the town of Meeker where regional settlements have exceeded four feet (1.2 m.).
This hazard was recognized in western Colorado in the 1890s when previously untouched land was first irrigated. It remains one of the primary geologic hazards that damages home foundations in the region.
Collapsible Soils in Colorado, published by the CGS — and the 2009 winner of the John C.
Frye Award in Environmental Geology, awarded by the Geological Society of America and the Association of American State Geologists — is an excellent information source on this important issue.
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.
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.
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.
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.
Dispersive soils subside by actual soil-mass loss through erosion and the formation of subsurface pipes and fissures, which collapse at the surface to create sinkholes and other pseudo-karst landforms. Certain soil and rock contain soluble minerals like gypsum. Dissolution and removal of the soluble mineral constituents of these types of soil or bedrock can also cause ground settlement.