Geologic hazards in North Carolina — Landslides

This section provides background information on landslide types, events, causes and triggers.

Landslide contents

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

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 the Internet for broader distribution. This page is on "Background on landslide types, events, causes and triggers." 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 1 - Title slide.

 

Slide 3 - Schematic of typical hillslope setting for debris flows. ‘Debris’ is earth material generally greater than coarse sand size. A debris flow occurs when the water content of the soil is sufficient for the material to flow like a viscous fluid. Debris flows usually travel down existing stream channels. Over geologic time debris can accumulate to form a fan-shaped deposit at the toe of the slope.

 

Slide 4 - A 60,000 cubic yard debris slide related to mountain development. Fortunately no injuries occurred, but reconstruction to mitigate sedimentation into a trout stream, and stabilize the slope in the golf course was expensive. In a debris slide, the water content of the transported material is not great enough for it to liquefy and flow. Debris slides can mobilize into debris flows if enough water is available.

 

Slide 5 - Photograph of scar left by the July 1997 rockslide in Pigeon River Gorge. Planes of weakness along bedding, foliation and fracture planes outline the scar, with the bedding plane dipping toward I-40. These planes formed in the rock millions of years ago during deposition, metamorphism, folding and uplift of the mountains. The orientation and occurrence of water along these planes are major factors in the stability of rock slopes. Track-hoe excavator on the slope for scale. Direct costs alone to reopen I-40 and stabilize the slope were on the order of $10 million.

 

Slide 6 - Although most landslides occur in the mountains, certain rock types in the Piedmont can be unstable when modified as in a cut slope that is too steep or too high. The Triassic sedimentary rock deposits of the Deep River Basin are an example of a rock that produces clay (good for bricks) when it weathers (bad for roads). Bottom left: Small cut slope failure along I-540 during construction in 1999 that progressed into a larger failure (top right) after Hurricanes Dennis and Floyd. The cost to repair this slide in weathered rock was about $3 million. Part of the reason for the expense was that the slope failure progressed out of the highway right-of-way into private property.

 

Slide 7 - Landslide causes and triggers. Causes are those conditions that make a slope unstable or marginally stable, and set the stage for future slope movements. Triggers are those events or conditions that actually initiate movement of the slope – the condition or event that tips the scales from ‘stable’ to ‘unstable.’ Causes can be thought of as long term processes, whereas triggers can be thought of as short term processes.

 

Slide 8 - Top Left: Paths of some hurricanes that triggered major debris flow events in the Southern Appalachians. Bottom Right: Graph showing rainfall amounts from hurricanes and storms that triggered debris flows and other slope movements in western N.C. Back-to-back storms caused major debris flow events in 1916, 1940, and 2004 (Hurricanes Frances and Ivan). As a general rule, the 24-hour threshold line indicates the amount of rainfall necessary (approx. 5 inches) within a 24 hour period to increase the landslide threat on an isolated to scattered basis (Eschner and Patric, 1982). Rainfall intensity is also important. Four inches of rainfall over a 1-hour period triggered scores of debris flows near Mt. LeConte in the Great Smoky National Park in 1951 (indicated by red arrow). Long range planning should take into account that storm sequences similar to Frances and Ivan have occurred in the past and will likely occur again.

 

Slide 9 - April 2005 tally of 130 slope movements triggered by heavy rainfall from the remnants of Hurricanes Frances and Ivan (as studies continue the total will probably increase). Landslides triggered by Hurricanes Frances and Ivan killed five people (Peeks Creek) and destroyed at least 27 homes. Right: Maps showing counties reporting landslides triggered by Frances and Ivan. Top Left: Locations of 90 of the 130 reported landslides triggered by the remnants of Hurricanes Frances and Ivan in western N.C.

 

Slide 10 - Map showing general areas affected by slope movements (landslides) and flooding from historical hurricanes. Information from Geology of Gorges State Park, N.C. Geological Survey Information Circular 31.

 

Slide 11 - Bottom Left: The NCGS began development of a slope movement – slope movement deposit data base in 2004. Slope movement (landslide) entries are for known, specific slope movements and, slope movement deposits include debris fans which record prehistoric slope movement activity. Most database entries are for locations in mountainous western N.C. As of September 2005 the database contains over 780 entries. Top Right: Portion of the slope movement-slope movement deposit database plotted on a digital shaded relief map showing the area around Asheville, Hendersonville, Franklin and Bryson City.

 

Slide 12 - Relative ages of pre-historic debris fan deposits near Roan Mountain N.C. Debris fans are often composite deposits built up over tens of thousands of years by multiple slope movement and flood events. Mapping debris fan deposits is important because it provides information on past landslide activity as well as locations of deep unconsolidated deposits that may be unstable in steep, high excavations.

 

Slide 13 - Infinite slope equation (a simplified slope stability equation) and schematic showing major factors that contribute to the stability of a mountain slope with relatively thin soil (<6 ft) overlying steeply sloping bedrock typical of debris flow initiation zones. Infiltrating precipitation increases the pore water pressure at the typically abrupt soil-rock contact, and overcomes the shear strength of the soil-vegetation layer. Vegetation generally increases the stability of a slope by adding root strength and evapotranspiration. Theoretically, if the factor of safety (FSs) is greater than 1 then the slope is stable, and conversely if the factor of safety is less than 1, then the slope is unstable. Bottom Left: Inset photo shows data collection at a debris flow initiation zone characterized by shallow colluvial soil in sharp contact with a steeply sloping bedrock surface.

 

Slide 14 - Breaks in the ground surface called tension cracks and scarps (photo left), and curved trees (photo right) indicate slope movement. The drain field for a mountain cabin resort is located in the grassy area in the photo left, and is likely a contributing factor to this slow-moving landslide. Hammer in lower center of photo left for scale.

 

Slide 15 - Earthquake shaking > Intensity VII can trigger landslides. Note: Intensity is different than magnitude. Isoseismal maps of the Charleston, S.C (bottom right), and Skyland, N.C. (top left) earthquakes modified from Stover and Coffman, 1993.

 

Slide 16 - Repeated freeze-thaw cycles and ice wedging along bedrock discontinuities can trigger slope movements, especially rockslides.

 

Slide 17 - Slope stability analyses from the planning through construction and inspection phases of developments can help mitigate the potential for landslide damage. The analyses are best done by the cooperative efforts of qualified geotechnical engineers, geologists and soil scientists.

 

Slide 18 - Some things that can be done to help reduce losses from landslides.

 

Slide 19 - General considerations for steep slope development.

 

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.