COLLAPSIBLE SOILS IN THE UNCOMPAHGRE RIVER VALLEY

The Uncompahgre River Valley in western Colorado, is a major tributary basin to the Gunnison River south of Grand Junction near Montrose. This broad valley contains soils derived from the Cretaceous Mancos Shale along with a thin veneer of recent river alluvium at the existing river channel that is bordered by terraces formed by glacial outwash. The shale underlies the valley and outcrops along the eastern side of the valley in areas known as the “Adobes.” Some of the most common soils in the valley are the transported fine sands, silts and clays that erode from the shale. These deep, fine-grained soils are productive for agricultural uses when irrigated and are frequently used for homesites due to their location on gentle slopes and pervasiveness throughout the valley. However, we have found these weak soils to be problematic for foundations and roadways due to their potential for being collapsible which can cause significant differential settlement. They appear very hard when dry, but lose strength rapidly when wetted and loads are applied. When performing geotechnical investigations for building sites on these soils, a common soil structure observed is vesicular pores, which are visible, discontinuous voids or macro-pores. Deep cracking, piping, and severe erosion are also commonly observed in areas with these shale-derived soils. When tested in the laboratory, these transported soils are dominated by silt and they exhibit initial swelling when wetted, but their long-term behavior under loads is that they are highly compressible and even collapsible, when they fail abruptly. Soil consolidation of 10 to 20% under moderate loads is common. Pedogenic pore enlargement affects the character and behavior of these soils and flood irrigation plays a role in the development of vesicular pores. The landforms where these soils are found, the features observed in the field, the character and behavior of these soils, the differences between flood irrigated and native soil conditions, and the implications for engineering design on such soils will be presented.