COLLAPSIBLE SOILS IN COLORADO: A STATEWIDE PERSPECTIVE

Colorado has the geomorphic and climatic conditions that are amenable to the creation of low-density, low moisture-content sediments or soils that collapse or compact upon wetting. This phenomenon induces ground settlement and spontaneous ground openings that can damage structures that are founded on them. Problems related to settlement from collapsible soils was first mentioned in Colorado near the turn of the last century when settlers moved into the territory and began to alter natural drainages, impound water, and create irrigation systems. Subsequently, over the last 100 years, settlement damage and economic loss, many times with accompanying litigation, have cost hundreds of millions of dollars. The Colorado Geological Survey (CGS) has completed a study of the occurrence of collapsible soils in Colorado, which included analyses of published data and hundreds of case histories and soil test data compiled from consultant geotechnical engineering reports. Soil collapse and settlement are found in geomorphic and climatic systems that are typical in many areas of Colorado: a dry climate with annual precipitation about 15 inches per year (38 cm), poorly indurated clay and silt-rich bedrock, and ample topographic relief. This study confirms that the processes of soil collapse and settlement in Colorado include mechanical collapse of the soil skeletal fabric, soil-mass loss through dispersion and piping erosion, and soil-mass loss due to chemical dissolution. In certain smectite clay-rich environments, collapsible soil sediments are found proximal to, and even interbedded with swelling soil sediments, or has both swell and collapse properties, dependant on loading and degree of saturation. Based on the compiled data and understanding the geomorphic systems where recent sediments are prone to collapse, the CGS has mapped susceptibility zones for specific regions of the state where they widely occur.