ENGINEERING PROPERTIES AND SITE CHARACTERIZATION OF THE HELEN'S STORE LANDSLIDE, DURANGO, COLORADO

The Helen’s Store Landslide, located 14 miles northeast of Durango, Colorado (N37º 21’ 42”, W107º 40’ 18”), has caused considerable damage to La Plata County Road 240. The ~290-foot long, 20 to 30-foot-wide active, landslide occupies a 25% slope. The landslide primarily occurs along the contact between the Dakota Sandstone and the Mancos Shale in steeply-dipping, landslide-prone, mountainous terrain. The oversteepened toe of the landslide is unvegetated, and consists of pervasively cracked, visually dilated, low-permeability clays and sandy clays; localized depressions perched on top of the toe, are occupied by cattails (Typha), indicating zones of elevated soil moisture. The highly disrupted material along the toe of the landslide also contains weathered, iron-rich clasts and anomalous soil and/or paleosol components. A preliminary field investigation of the landslide was conducted to determine the engineering soil properties and to estimate slope stability. Longitudinal cross-sections were measured in the field and XSTABL was used to estimate slope and soil mass stability. Field shear vane and soil compressibility tests were performed to determine in-situ soil strength. In the laboratory, soil from the toe of the landslide was tested for uniaxial shear strength, soil consistency (Atterberg limits), and the grade and classification of the soil (sieve analysis). To aid in determining the origin of the anomalous material incorporated in the toe of the slope, soil and clasts were collected from the toe of the landslide and surrounding outcrops for comparative XRD analyses of the clay fraction.