SLOPE STABILITY HAZARDS IN THE BLACK HILLS, SD

Geological conditions leading to slope instability within the Black Hills exist primarily in Tertiary and Cretaceous formations as well as Precambrian metasediments. Colluvial materials resting on steep slopes periodically fail naturally, however, are more common after the slopes have been mechanically disturbed by cutting of the toe or overloading at the top. Spring precipitation often exacerbates subtle motion promoting failure. In the immediate vicinity of Rapid City, residential subdivisions often are located within geological formations susceptible to slope failure and many structures show signs of stress within a few years. Construction is common on shale slopes that contain obvious signs of instability including stair-step scarp topography, sinkholes and dissolution features, cracked and sunken roadways, rotated tress, and slumps in colluvium. New residential construction on the eastern dip-slope of Cretaceous Sandstones typically is founded on approximately 5 m of inclined colluvium and terrace deposits. A false sense of security is supplied by installation of Keystone block retaining walls ranging from three to twenty feet in height. Success of retaining walls is dependent upon routing surface and ground water away from the site. In the central Black Hills, Precambrian metasediments possess strong foliation that promotes sliding of rocks onto roadways and is mediated by sloping cuts to match the inclination of the rocks or by terracing. Ancient landslides exist throughout the region, a few having been reactivated by recent construction activities.