Rock Slope Stability
The following images were included in a MS PowerPoint presentation used by North Carolina Geological Survey geologists from the Asheville Regional Office at many public landslide outreach meetings. The presentation has been adapted to the Internet for broader distribution.
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| Left: View looking upslope along the track of the Occoneechee Mountain rockslide. The rock slope exposed in an old quarry failed during the night of Feb. 18-19, 2001 during a rainfall event (bottom right) preceded by numerous freeze-thaw cycles (top right). Meteorological data courtesy of the National Weather Service, Raleigh, N.C. |
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| Past rockslide area at MM 401 along the Blue Ridge Parkway (BRP) south of Asheville. Rock discontinuities (dip vectors) that plot within the shaded area of the stereonet (bottom right) indicate possible failure surfaces in a rock slope. The NCGS is mapping bedrock and collecting rock slope stability data along the North Carolina segment of the BRP as part of a geologic inventory of the BRP funded by the National Park Service. |
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| Freeze-thaw and ice-wedging may have been a contributing factor in this previous rockslide, reactivated on Dec. 11, 2004. Sliding took place along the steeply dipping fracture and foliation surfaces (photo upper left). Large photograph, center right, shows rock boulders blocking the Parkway. Remaining photos show scarps and tension crack developed along planes of weakness in the bedrock. |
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| B1: This weathered-rock slide developed in a cut slope in completely weathered schist and gneiss. The cut slope failed in the late June to early July, 2005 time frame, and affected the home at the top of the photo. B2: Some of the recent slip surfaces coincide with ancient iron and manganese coated fault planes with slickenlines. |
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A. View from Caney Fork Overlook toward a past wedge failure in a cut slope in migmatitic metagraywacke of the Ashe Metamorphic Suite/Tallulah Falls Formation. Numbers inRED indicate locations where discontinuity measurements were made to use in stability analyses.
B. Dip vector stereonet plot of the data. Intersection of the joint (1) and S2 foliation plane (2) great circles indicates the critical orientation of the line of intersection of the two planes along which sliding occurred. C. Wedge failure factor of safety analysis. Calculated factor of safety = 0.4. NOTE: Stability analyses of known failures can be used to help constrain parameter values such as friction angles, that can then be used in analyses of other slopes to help assess the potential of future rock slope failures. Kinematic (stereonet) and factor of safety analyses performed using Rockpack III software (Watts and others, 2003) Watts, C.F., Gillam, D.R., Hrovatic, M.D., and Hong, H., 2003, User¿s Manual Rockpack III for Windows (ROCK Slope Stability Computerized Analysis PACKage), C.F. Watts and Associates, 48p. |
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A. View from Caney Fork overlook toward a past plane failure in a cut slope in migmatitic metagraywacke of the Ashe Metamorphic Suite/Tallulah Falls Formation. Circled numbers in photograph A (Top Left) indicate locations where discontinuity measurements were made to use in stability analyses. These are explained below:
B. Dip vector stereonet plot of structural features. Projection is equal angle, lower hemisphere. C. View of brittle and ductile features on right flank release surface (Data point 12 in A). Folds are outlined in red. D. Subhorizontal slickenlines shown in C. Arrows indicate right lateral movement. Scribe is 15 cm (~6 in) long. E. Schematic geologic cross section through the plane failure. F. View of the partly to completely decomposed biotite, muscovite schist exposed along the right flank release surface. G. Plane failure factor of safety analysis of slope failure. Calculated factor of safety = 0.87. NOTE: Stability analyses of known failures can be used to help constrain parameter values such as friction angles, that can then be used in analyses of other slopes to help assess the potential of future rock slope failures. Kinematic (stereonet) and factor of safety analyses performed using Rockpack III software (Watts and others, 2003) Watts, C.F., Gillam, D.R., Hrovatic, M.D., and Hong, H., 2003, User¿s Manual Rockpack III for Windows (ROCK Slope Stability Computerized Analysis PACKage), C.F. Watts and Associates, 48p. |
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| Top Right: 50 ton boulder in a rockslide/fall that occurred during Hurricane Cindy (July 6-7, 2005) and caused severe damage to the 4-year old Broad River Fire Station, Buncombe County, North Carolina. Bottom Left: Schematic geologic cross section through the weathered rock cut slope that failed showing orientations of fracture and foliation surfaces that form planes of weakness in the rock. Water infiltrating along these planes probably triggered the rockslide. |
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| View of the rockslide damage from inside the Broad River Fire Department. |
Contact Information
For additional information about landslide hazards in North Carolina, please contact Dr. David Korte with our Asheville Regional Office:
2090 U. S. Highway 70,
Swannanoa, North Carolina 28778.
828-296-4540
David.Korte@ncdenr.gov