DEVELOPMENT, MASS MOVEMENT AND WEATHERING PROFILES: A SLOW MOVING WEATHERED ROCK SLIDE IN WESTERN NORTH CAROLINA

Two landslide styles shape Western North Carolina crystalline rock landscapes: shallow translational slides and debris flows derived mainly from colluvial hollows and big, slow-moving (BSM) rotational and translational landslides. BSMs generally fail in deep (> 15-m) weathering profiles and move millimeters per year. The Hunters Crossing weathered rock slide began moving in 2003 and soon threatened several structures. Tension cracks accommodated extension of the slide head and a driveway bulged at the slide toe, indicating rotational slide movement; a survey of ground elevations was consistent with these observations. However, whether the slide moved in response to development and a broken water line or due to groundwater recharge and an over-steepened toe slope is unclear. Several issues remain unresolved: (1) the slip surface geometry, (2) the geotechnical properties of the weathering profile, (3) the groundwater recharge rate to the slide, and (4) the uniqueness of geological and/or artificial characteristics that led to movement at Hunters Crossing relative to the adjacent slopes. Understanding the cause of the failure may help local communities plan development.

We report here on an ongoing study related to issues 1-4. Inclinometers installed in the slide have deformed slowly and absolute deformation is on order of the size of the borehole diameter. Maximum deformation is approximately at 13-m depth, although this differs from estimates made from a previous seismic study (maximum depth of 11 m). Depth to bedrock in two inclinometer boreholes is 20.9 and 25.3 meters and cores are sandy to sandy loam textured material derived from completely decomposed bedrock. Density changes midway through the weathering profile as the standard penetration value increases from < 20 blows/foot to > 30 blows/foot. The Hunters Crossing ridgeline is subparallel to the strike of steeply dipping foliation, with variably dipping axial planes of 1- to 10-m scale folds locally. Compositional layering observed in unoriented drill cores of saprolite, and bedrock is slightly dipping (< 20 degrees) and may facilitate the movement of the landslide. Rainfall during the monitoring period has fluctuated but the groundwater level varies by less than a meter.