COLLUVIUM IN THE APPALACHIAN PLATEAU PHYSIOGRAPHIC PROVINCE OF PENNSYLVANIA, WEST VIRGINIA AND OHIO

The Appalachian Plateau Physiographic Province is the largest landslide risk area on the United States Landslide Hazard Map. This distinction is the result of bedrock geology and precipitation. Rocks are generally flat lying interbedded Paleozoic age claystone, shale and sandstone with a few limestone and coal seams. Weathering of rocks in valley walls plus downslope creep and sliding of the weathering products, particularly those derived from claystone and shale, produce colluvial masses of various thicknesses and lateral extents which exist in states of marginal equilibrium on many, if not most slopes.

Most of the present slope stability problems in the region involve slump type slides, or slow earthflows of colluvial soil. The precarious equilibrium of these masses is frequently upset by man’s activities, e.g., removal of toe support, loading the slope, or changing surface and subsurface drainage. Abnormally high precipitation also initiates movement of colluvial masses.

The colluvial soil tends to be stiff to hard, with no discernible layering or structure. However, creep and sliding processes create a series of shear surfaces within the soil mass and at the soil-rock interface. Due to slope creep the colluvium exhibits residual strength characterized by negligible cohesion intercepts and friction angles of 8° - 20°. Measured residual friction angles for most claystone derived colluvium are on the order of 11° - 16°.

Geotechnical engineers have understood the formation of colluvium and its resulting low shear strength since the late 1960’s. These specialists have generously shared their knowledge through technical papers, workshops and field trips. Yet numerous slides occur because some geologists and geotechnical engineers do not recognize the unique properties of colluvium.