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Expansive Soil and Rock

Expansive soils are one of the nation’s most prevalent causes of damage to buildings and construction. Annual losses are estimated in the range of $2 billion. The losses include severe structural damage, cracked driveways, sidewalks and basement floors, heaving of roads and highway structures, condemnation of buildings, and disruption of pipelines and sewer lines. The destructive forces may be upward, horizontal, or both.

Soil with a high clay content exhibiting typical desiccation-cracking when drying out after natural or human-caused wetting. Photo credit: US Department of Agriculture.
Soil with a high clay content exhibiting typical desiccation-cracking when drying out after natural or human-caused wetting. Photo credit: US Department of Agriculture.

Innocuous as it sounds, Colorado’s most significant geologic hazard is expansive or swelling soil — that is, soil laced with layers of various clays. These clays cause more property damage than any other natural hazard. Bentonite and montmorillonite (weathered volcanic ash) clays—in the form of soils or soft bedrock—underlie many populated areas of Colorado. They can expand up to 20% by volume when exposed to water and exert a force of up to 30,000 pounds-per-square-foot, more than enough to break up any structure they encounter. One Denver suburb has the dubious distinction of suffering more annual monetary loss from expansive soil than any other in the nation.

Expansive soils are one of the nation’s most prevalent causes of damage to buildings and construction. Annual losses are estimated into the billions of dollars. The losses include severe structural damage, cracked driveways, sidewalks and basement floors, heaving of roads and highway structures, condemnation of buildings, and disruption of pipelines and sewer lines. The destructive forces may be upward, horizontal, or both.