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