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 contents. 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 re-orient 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.
Warped sidewalk due to collapsing soils near Meeker, Colorado.
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.
There are other, less common, types of collapsible soils in Colorado. Click this link to learn more.
The Colorado Geological Survey recently produced an educational video on hazardous soils in the state:
This photo illustrates over four feet (1.2 m.) of displacement of a residential sidewalk after a water main broke beneath it. Note the settlement and downward deflection of the sidewalk and use of a boulder as an additional step. Photo by Jon White.
On-line map viewer of statewide and regional maps of collapsible-soil susceptibility in Colorado.