Innocuous as it sounds, Colorado’s most significant geologic hazard is 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 underlie many populated areas of Colorado. They can expand up to 20% by volume when exposed to water and exert up to 30,000 pounds of force per square foot, more than enough to break up any structure they encounter. One zip code in a Denver suburb has the dubious distinction of suffering more annual monetary loss from swelling soil than any other in the nation.
The “roller-coaster road” is the result of uneven swelling and heaving of steeply dipping bedrock layers.
Where the claystone layers turn up on end near the foothills, the effects of swelling are intensified and the phenomenon is called heaving bedrock, which causes heave ridges. These ridges cause roads to ripple, including C-470 near Bowles Avenue and Wadsworth Boulevard, a telling sign that extraordinary precaution is needed to prevent structural catastrophes in the area.
Sound building techniques can prevent swelling-soil damage to homes, but it is crucial that builders follow these techniques faithfully. CGS geologists wrote a booklet about this hazard for homeowners entitled A Guide to Swelling Soils for Colorado Homebuyers and Homeowners that has sold more than 175,000 copies and won several national awards.
Following is a video on swelling soils produced by the Colorado Geological Survey:
Swelling 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.
Case History: Southern Colorado State University Campus
Several structures on the Southern Colorado State University Campus northeast of Pueblo have been damaged because swelling soils were not recognized or compensated for adequately in design, construction and maintenance of buildings, sidewalks, driveways, and water lines. Water percolating into dry soils exposed by construction excavation caused the clays to expand, exerting tremendous upward pressures. Floors, walls, ceilings, sidewalks, water lines, driveways, and other improvements have sustained an estimated $1.5 million in damages.
Case History: Colorado State Prison
In 1976 at the site of the new maximum security facility for the Colorado State Prison in Fremont County, swelling soils and bedrock were shown on geologic maps. Field investigations and soils tests resulted in a remedial plan by the geologic and soils engineers, architect, builder and others on foundation design, drainage and landscaping. Millions of dollars in potential damages were avoided.
Case History: Fairway Vista
Surface view of a near-vertical bentonite layer in the Upper Cretaceous Pierre Shale in Jefferson County, Colorado. The layer heaved with a differential displacement of 3 inches within 24 hours after a rainstorm at this construction site. Note the hump in the fence aligned with the trend of the bentonite layer. Heaving bedrock damage is occurring in the subdivision in the background. Photo credit: Dave Noe.