COLLAPSIBLE SOILS ASSOCIATED WITH THE EAGLE VALLEY EVAPORITE IN A PORTION OF THE ROARING FORK VALLEY, COLORADO

The Roaring Fork River valley between Glenwood Springs and Basalt, Colorado, is a major transportation route to the Aspen area that has become an urban corridor. Hazardous geologic processes and conditions have affected construction activities in this area including soils with the potential to collapse when wetted. The author conducted field investigations for an engineering geologic map of the corridor and studied these processes and conditions in detail. The purpose of this presentation is to discuss the engineering geologic mapping of deposits that include collapsible soils. The mapping described the surficial geologic deposits, their origin, and provided an indication of the resulting engineering properties of the material. The presentation will also describe the Eagle Valley Evaporite. Deposits derived from this formation are especially prone to volume decreases upon wetting and saturation. Coalescing debris fans at the base of steep outcrops of the evaporite are typically characterized by two conditions favorable to soil collapse. These are the presence of soluble evaporite minerals and a metastable structure. Upon wetting there is dissolution of the evaporite minerals and collapse of the soil structure. These processes are driven by water. Honeycomb structures can slowly form within these deposits as the evaporite minerals dissolve creating deposits prone to sudden collapse upon further wetting. Both of these processes contribute to a decrease in volume of the material. Visual examples within the project area indicate that significant volume decreases of the deposit can occur rapidly or over longer periods of time. The dissolution of evaporite minerals and soil collapse are examples of linked geologic processes that can detrimentally affect engineered works with sometimes catastrophic results. The rate of collapse and dissolution are greatly increased by the application of water through irrigation and surface water infiltration that exceeds annual rainfall. The presentation will highlight typical areas within the project area that are prone to soil collapse and provide visual examples of resulting structural distress.