What are landslides, and what causes them?
The term “landslide” describes many types of downhill earth movements ranging from rapidly moving catastrophic rock avalanches and debris flows in mountainous regions to more slowly moving earth slides. Some landslides move slowly and cause damage, gradually, whereas others move so rapidly that they can destroy property and take lives suddenly and unexpectedly. Gravity is generally the force driving landslide movement. Factors that trigger landslide movement include heavy rainfall, erosion, poor construction practices, freezing and thawing, earthquake shaking, and volcanic eruptions. Landslides are typically associated with periods of heavy rainfall or rapid snowmelt and tend to worsen the effects of flooding. Areas burned by forest and brush fires are particularly susceptible to landslides.
Landslides — Slope failures — Mass wasting
Every landslide, or slope movement, is unique, and is best judged on a case-by-case basis. The specific behavior of individual landslides is most often unpredictable, even when studied in detail. When they will move, and by how much, is often speculative. Cycles of wet weather with above average rainfall, particularly when followed by storms with intense rainfall, trigger landslides.
Three common landslide types (or slope movements) in North Carolina include:
● debris flows,
● debris and earth slides, and
● rockslides.
Types of Landslides
1)-Debris flows — Debris flows are rapidly flowing mixtures of soil, rock particles and water. The media, when referring to debris flows, often uses the term "mudslide". The soil mass that initially moves is usually a rocky, silt-sand mixture that is not highly plastic or cohesive (i.e., one that contains a lot of clay). The lower cohesion allows the soil to "liquefy" during heavy rains and move rapidly, up to 30+ mph down slope. Once movement begins, there is often insufficient time to reach safety. Debris flows often originate in hollows on steep mountain slopes where thin (<6ft) soil overlies hard bedrock. The failed soil mass usually (though not always) travels down an existing drainage and accumulates into a lobe-shaped mass near the toe of the slope. Steeper slopes (>25-30 degrees) combined with a long run-out distance between the point of origin and the flatter toe-slopes create the potential for fast movement.
Debris flows often originate in hillslope depressions, or hollows, near the headwaters of mountain streams. Often triggered by intense rainfall, debris flows typically follow mountain stream channels. Building homes or other structures at the base of steep hillslopes (>30 degrees), especially near stream channels, increases their vulnerability to damaging debris flows. Land-disturbing activity on steep slopes can also increase the likelihood of debris flows.
Indicators That Further Movement is Likely in the Upslope Area
● Unstable (i.e., loose, wet soil) material remaining in the scar area — Exposed bedrock in the scar and along the track indicates the bulk of the material may have already been taken downslope and the situation is relatively more stable,
● Tension cracks, scarps, or leaning trees above the scar area,
● Seepage or flowing water exiting the slope within or above a mass of soil remaining on the slope; water exiting the slope at the soil-rock contact, and
● Culverts or other road drainage patterns directing runoff toward the unstable area.
2)- Debris or Earth Slides — Debris (soil-rock mixture) or earth (clay-silt soil) slides usually move at a slower rate than debris flows because the water content is too low for the mass to "liquefy". This can be due to the higher clay content of the soil that requires more moisture to liquefy. Movement rates are typically on the order of inches per day to feet per day, particularly during wet periods. During wet weather cycles, slides can be self-perpetuating. Initial movement opens tension cracks and scarps providing pathways for water to infiltrate deeper into the slide mass, thereby further decreasing the stability of the slope. Further movement widens existing tension cracks and scarps allowing more infiltration pathways, and so on. Shearing along the slide planes usually decreases the strength of the soil, a further destabilizing factor.
Movement Indicators
Ground Slopes
● Enlarging tension cracks and scarps, especially those forming upslope of the main scarp,
● Leaning, tilting or dying trees, tree roots stretched across scarps and tension cracks.
● Bulging or hummocky ground surface,
● Changes in normal drainage patterns,
● Surface water entering the slide mass, and
● Culverts or other road drainage patterns directing runoff toward the unstable area.
Structures
● Cracked masonry, or pavement,
● Walls, posts, rooflines not plumb or level, or buckling,
● Tilted or cracked chimneys,
● Windows and doors stick, broken glass,
● Plumbing or gas line leaks,
● Leaning utility poles, street signs, and
● Employ the services of a qualified building inspector
Notes on Stabilizing Slopes to Salvage Houses
Each case is unique, and involves evaluating a number of alternatives. Stabilizing a slope can easily exceed the monetary value of a house. Covering a slope with plastic or placing rock on the slope is a short-term tactic, at best. It is very easy to waste money on attempts to stabilize a moving slope.
1. Can the structure, such as a mobile home, be moved to a nearby stable or safe location?
2. In most cases, the successful long-term stabilization of a slope above or below a home requires the services of a geotechnical engineer experienced in the soil and rock types in the affected area.
3. Knowledge of the extent, geometry and properties of the unstable material relative to the structure must be known, or conservatively assumed, to help ensure successful, cost effective stabilization. For example, if a home is built on an embankment, what is the extent of the embankment and where is pre-existing natural ground surface?
4. Are the unstable slopes above or below the structure - or both?
5. Can the unstable material be removed or regarded to leave a stable slope?
6. If the unstable cannot be removed and preserve the structure, can the slide mass be stabilized in place?
7. Does the design life of the stabilization equal or exceed that of the structure to be saved?
8. What is the allowable differential settlement of the structure to be versus the degree of settlement allowed by the stabilization effort?
9. Walls over 4-6 feet high should be designed by an experienced geotechnical or structural engineer familiar with the materials used in the wall and the material being supported by the wall. Common types of successful, engineered retaining walls used to stabilize slopes above and below structure:
● Gravity walls - usually made of rock, or concrete or concrete block,
● Driven H-piles with treated timber lagging (with or without tie backs),
● Reinforced earth walls - geo-textile layers within a compacted earth fill, and
● Wire rock baskets (e.g., gabions)
10. Other stabilization methods:
● Pressure grouting,
● Soil nailing, and
● Drainage (e.g., horizontal drains)
11. Walls typically used for landscaping, (e.g., railroad ties, or stacked treated lumber) have severe limitations when used to stabilize a slope to protect a home. Timber walls typically have a design life of less than 15 years.
12. Proper surface and subsurface drainage is a key component of successful slope stabilization. Gutters and septic systems can add unwanted water to unstable areas.
3)-Rock Slides and Rock Fall - Rock slides and rock fall usually occur along roadways, but can occur on any modified or natural rock slope. They can occur in conjunction with heavy rainfall, but often occur at other times, usually without notice. Freeze-thaw cycles and wedging by tree roots can loosen blocks of rock from a slope. Rock can easily weigh 165 pounds per cubic foot, so not much moving rock is needed to cause damage or injury. A rolling and bounding basketball-sized rock can easily go through a roof. Rocks slide or fall because of pre-existing planes of weakness within the rock mass. Where these planes of weakness are inclined toward and intersect an excavated slope, the odds of a rockslide increase.
Indicators of Potential Rock Movement
● Apertures between rock blocks, open or dilated;
● Surfaces of the planes of weakness are degraded or weathered, particularly with red-brown to yellow-brown staining;
● The location has a past history of rock slides; and
● Visible, flowing water along planes of weakness.
Stabilizing Rock Slopes
Stabilizing a rock slope is usually very expensive and requires specialized equipment along with engineering experience and expertise. Examples include:
● Drilled-in rock bolts, and
● Wire mesh blankets, usually only effective for containing small blocks of a rock on a slope.
References
North Carolina Geological Survey, 2005, When the Earth Moves, Randy Bechtel, editor: Information Circular 32: North Carolina Geological Survey, Raleigh, North Carolina, 24 p.
Varnes, D. J., 1978, Slope movement types and processes: In: Landslide Analysis and Control: In Schuster, R. L., Krizak, eds. Transportation Research Board Special Report No. 176, National Academy of Sciences, Washington, D. C., P. 11-33.
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