Innocuous as it sounds, Colorado's most significant geologic hazard is swelling soil-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. The can expand up to 20 percent in 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.
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 that has sold more than 175,000 copies and won several national awards.