Failure of Colluvial Slope

Interstate Route 279 crosses the Ohio River 9 miles northwest of Pittsburgh, PA, and continues northward along Kilbuck Run tributary valley. One section of the highway passes through a zone of colluvium on the wall of the tributary valley. When construction of this section began in 1968, several slides were initiated along ancient landslide surfaces in the colluvium. The relationship among the landslides, a weak claystone zone in the stratigraphic sequence (the Pittsburgh Redbeds), and the valley wall topography are analyzed. One typical landslide which was studied in detail is described. Friction angles of 12.5° to 15.5° were calculated for limiting equilibrium of the failure mass and residual friction angles of 13.5° to 16° were determined from repeated direct shear tests on failure surface materials. It is concluded that the average shear strength mobilized along the failure surface material. This residual strength condition is attributed to the shearing displacement of the ancient landslide.