Thursday, October 27th, 2011
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 hydro – implies the introduction or presence of water, and compaction – the resultant compaction of the soils once they become wet. Hydrocompactive soil forms 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 is manifested at the ground surface as subsidence or settlement (see diagram at upper right).
Ground settlements from the adverse saturation of thick collapse-prone soils have been documented at over 6 feet. The location shown in the photo at right is on an alluvial fan in the town of Meeker where regional settlements have exceeded 4 feet.
This hazard was recognized in western Colorado in the 1890s when raw land was first irrigated. It remains one of the primary geologic hazards that damage home foundations in the region.
For more information on collapsible soils, see the CGS publication, “Collapsible Soils in Colorado,” the 2009 winner of the John C. Frye Award in Environmental Geology from the Geological Society of America and the Association of American State Geologists.
There are other, less common, types of collapsible soils in Colorado.