This section provides rock slope stability information about landslides.

Landslide contents

The landslide section consists of several pages to facilitate on-line user viewing. This page is the second of five that are based on a recent staff geologist presentations at numerous public meetings. The pages are:

• Selected examples -- an overview
• How to recognize landslides and how to deal with them
• Landslide presentation - Background on landslide types, events, causes and triggers
• Landslide presentation - Rock slope stability
• Landslide presentation - Examples of historical landslide events in North Carolina
• Landslide presentation - Hurricanes Frances and Ivan, September 2004
• Landslide presentation - Peeks Creek debris flow - Macon Co., Hurricane Ivan

Landslide presentation

The following images were included in a MS PowerPoint presentation used by North Carolina Geological Survey geologists from the Swananoa office at many public landslide outreach meetings. The presentation has been adapted to theInternet for broader distribution. This page is on "Rock Slope Stabilty." Links to other topics appear in the contents shown above.

Slide numbers correspond to those of the original MS PowerPoint presentation. Slide numbers "missing" are slides that were turned into text. Captions are from the original presentation.

Slide 21 - 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.

Slide 22 - 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.

Slide 23 - 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.

Slide 24 - 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.

Slide 25 - 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 indicate locations where discontinuity measurements were made to use in stability analyses. 1. Joint - 158/75. 2. S2 - foliation 230/70. 3. Upper slope exfoliation joint - 190/25. 4. Cut Slope face orientation – 190/75. 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.

Slide 26 - 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. Numbers indicate locations where discontinuity measurements were made to use in stability analyses. Structural details for top left photograph. 1. S2 foliation – 216/67. 2. Exfoliation joint – 202/36. 3. Exfoliation joint – 205/35. 4. Exfoliation joint – 214/59. 5. S2 foliation – 216/63. 6. S2 foliation in migmatitic metagraywacke – 230/70. S3 foliation in schistose layer – 205/88. 7. Just off the photograph, exfoliation joint – 220/57. 8. Weathered biotite, muscovite schist layer (See F). 9. Joint – 122/62. 10. Fault subparallel to foliation – 235/87. Offset is approximately 15-23 cm (6-9 in). 11. Joint – 100/70. 12. Slip surface with subhorizontal slickenlines on surface – 72/65. Movement is right lateral (See C and D). 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.

Slide 27 - 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. 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.

Slide 28 - View of the rockslide damage from the inside the Broad River Fire Department.

For additional information

The contact for additional information about geologic hazards in North Carolina is Mr. Richard Wooten, P.G.; his e-mail is Rick.Wooten@ncmail.net. He is located in the Swannanoa, North Carolina office (western North Carolina) and can be reached by telephone at 828.296.4500. His mailing address is: 2090 U. S. Highway 70, Swannanoa, North Carolina 28778. An alternate North Carolina Survey staff geologist contact is Dr. Jeff Reid, P.G., 512 North Salisbury Street, Raleigh, North Carolina, 27699-1612. His telephone number is 919.733.2423 x403. His e-mail is Jeff.Reid@ncmail.net.