Landslides and Avalanches

Landslides in the U.S. annually cost over 1.5 billion dollars in losses and 25-50 deaths.

Avalanche

Landslide Occurrences

A landslide consists of earth materials that move by gravitational force.The motion involves down slope movement as well as lateral spread or scatter materials.Landslides can even involve upslope movement, when it sends debris up the opposite valley wall.“landslide” applies to rapid movement of earth materials, although landslide movements often begin with slow creep.An avalanche is a similar mass movement of snow and ice. Takes 20 lives each year.

Avalanche

Landslide – No people were killed.

Landslide Occurrences

Down slope movement is inevitable.Down slope movement is a hazard usually when it is rapid enough to threaten man-made structures.The largest landslide was 9 miles wide and 12 miles wide. (pre-historically in Iran).

El Salvador Landslide

Dangers of Landslides

Result in an annual cost to society at a cost of 1.5 billion dollars.Highways – 1 billion, buildings – half a billion.Very few occur in areas where they cannot be anticipated.25 people die annually from slides in the U.S.600 die worldwide from slides.Rapid slope failures occur as a result of the presence of specific types of earth materials and particular conditions and processes.

Mudslide from Heavy Rains in Brazil

Indications of Natural Instability

1. History of past landslides in the nearby area in the same stratigraphic units as the site in question. Ex: Libby dam where shales are known for slope failure.

2. Soil types that are rich in silt-to-clay sized material, particularly soils that are rich in swelling clays. Movement occurs in these soils in spring when the soil begins to thaw and snow melt. In China, dry loess soils went into fluid motion during an earthquake. A mudflow destroyed the Roman city of Herculaneum.

Sinkhole Caused by Landslide

Indications of Natural Instability

3. Down slope orientation of planes of weakness in bedrock.

• Translational movements occur where bedding planes point in a down slope direction.

• Orientation of schistosity (alignment of platy and rod-shaped minerals in metamorphic rocks) in a down slope direction.

• Ex: Gros Ventre landslide of Wyoming, slides in the Cretaceous rocks near Rapid City, South Dakota.

• Ex: Vaiont Reservoir in Italy

Gros Ventre landslide of Wyoming

Indications of Natural Instability

4. Slope undercutting. • Landslides are particularly common along

stream banks, reservoir shorelines, and large lakes and seacoasts.

• Ex: Soft glacial sediments along the shores of the Great lakes.

5. Earthquake tremors. • A tremor can provide the coup de grace to a

slope with marginal instability.• Ex: In 1959, Madison Canyon of Montana and

the Hope Mountain slide in British Columbia in 1965.

Madison Canyon of Montana

Indications of Natural Instability

6. Sensitive slopes subject to intense rainfall.

• Periods of intense rainfall can trigger the movement of unstable slopes.

• Ex: Appalachian Mts. have frequent slides during thunderstorms. Hurricanes can also trigger landslides.

Ash flow in Knoxville, TN

Human Causes of Rapid Slope Failures

All loose materials such as sand or soil have a natural angle of repose, which is the maximum angle that can be measured on a slope when the material is stable and at rest.When earth material is maneuvered into large piles that have side slopes that are steeper than the angle of repose of the material, the situation invites a landslide.Mine wastes are stacked in huge piles with steep side slopes. When the wastes weaken, the huge piles can fall in a landslide.

German landslide from old mine