Definitions
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The official U.S. Government definition of a landslide is:
The downward and outward movement of slope-forming materials composed of natural rock, soils, artificial fill, or a combination of these.
Classification
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Why classify?
– Lump events, so that we only have to deal with a few types, rather than many individual events
– To help us understand relationships that could be obscured by a mass of data on individuals
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Landslide classification
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Based on type of movement
– Based on type of material
– Fine distinctions based on moisture content, speed of movement, and nature of the movement surface
Landslide Classification
Type of Movement
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Fall—Material loses contact with the ground and moves through the air
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Slide—Material slides or glides along a surface. It may remain as a single unit or break up into several units.
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Flow—Material loses internal cohesion and moves down slope as a thick fluid
Types of Material
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Bedrock—Solid rock
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Soil—Basically the stuff plants grow in
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Unconsolidated material
– rock fragments
– sand or silt
– mud
– a mixture of the above

Finer Distinctions
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Moisture content ranging from wet to dry
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Nature of the movement surface
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Curved
• Induces rotation
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Movements are called slumps
– Planar
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No rotation
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Movements are block glides or slides
Finer Distinctions
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Velocity
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Extremely Rapid— > 10 feet/second
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Very Rapid—0.01 ft/sec to 10 ft/sec
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Rapid—5 ft/day to 0.01 ft/sec
– Moderate—5 ft/month to 5 ft/day
– Slow—5 ft/year to 5 ft/month
– Very Slow—1 ft/5 years to 5 ft/year
– Extremely Slow— < 1 foot/5 years
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Gravity driven
Principles of Landslides
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Nature of the material
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Consolidated versus unconsolidated
– Grain size and differing angles of repose
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Structure of the material
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Round versus angular grains
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Dip of layering
– Presence of fractures
Principles (continued)
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Slope of ground surface
– Greater than 15% (10°) are less stable
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Presence of ice, water, compressed air, or steam
– Wetter, icier, etc. are less stable
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Effects of water
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Decreases resistance to sliding by lubricating the surface
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Decreases cohesive force binding clays together
• Adds weight
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Reacts with some clay minerals to produce volume changes (bentonite clay produced from weathered volcanic ash)

Principles (continued)
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Type of vegetation
– Vegetation holds land surface in place with roots
– Vegetation takes up water keeping the surface material drier
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Proximity to areas of active erosion
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Earthquake frequency—Earthquakes trigger landslides
Recognizing
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Need to recognize ongoing slow motion and/or the potential for rapid motion
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Old landslide deposits
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Isolated small ponds or lakes
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Natural springs
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Abrupt, irregular changes in slope or drainage pattern
– Hummocky, or irregular ground surface
– Back-tilted blocks
– Steep, arcuate scarps
Recognizing
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More old landslide deposits
– Irregular soil and vegetation patterns
– Disturbed vegetation or cultural features
Hebgen Lake Earthquake
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August 17, 1959
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Time of occurrence—11:37 p.m.
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Magnitude—7.1
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Effects
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Fault scarps
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Damaged buildings, notably Old Faithful Lodge in Yellowstone
– Altered eruption of geysers
– Set up seiches (sloshing of water in Hebgen Lake
– Triggered a landslide
The Earthquake
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Two blocks in the Hebgen Lake area dropped and tilted toward the north
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Scarps face south in Hebgen Lake area
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Motion on Red Canyon and Hebgen faults

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Duration unknown—Estimates range from 5 seconds to 2 minutes
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Hebgen Lake seiche
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Lasted for 11 1/2 hours
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Period of 17 minutes
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First two surges went over the dam
The Scarp
View Along the Scarp
Red Canyon Scarp
Red Canyon Scarp
Red Canyon Scarp
Hebgen Lake Dam
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721 feet long
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Rises 87 feet above the river floor
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Dam and the ground on which it rests were dropped 10 feet in the quake
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The dam held
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Concrete core was cracked in at least 4 places
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Concrete spillway was cracked
– Uneven settlement of earth fill around core
Hebgen Lake Dam
Hebgen Lake Dam
Hebgen Lake Dam Old Spillway
Heben Lake Dam New Spillway
Madison Canyon Slide
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Dimensions
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1 mile long
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3/4 mile wide
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38 million cubic yards of rock
– 225 feet thick when emplaced
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Movement
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Top of slide was 1,300 feet above valley floor

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Moving over 100 miles per hour when it hit valley floor
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Ran 400 feet up the other (north) valley wall
Madison Canyon Landslide
Madison Canyon Landslide
Madison Canyon Landslide
Downstream from the Slide
Earthquake Lake in 1959
Landslide Toe in 1959
Madison Canyon Slide
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Effects
– Buried people camped along the river
– Air blast blew people away
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Dammed Madison River creating Earthquake Lake
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26 deaths—7 certain, 19 more likely
Monument Rock
Damaged Cabins on Hebgen Lake
Road Covered by Earthquake Lake
Minimizing Effects
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Identify areas with a high risk of slope failure
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Zoning to prevent building on unstable slopes
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Add drainage
– Divert surface waters in ditches, gutters or culverts
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Subsurface drainage
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Reduces weight
– Increases shear strength
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Cut-and-fill
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Remove steep material at top of slope
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Deposit that material at base to stabilize slope
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Benching
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Cut steps in slope
Minimizing Effects

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Add surface drains
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Retaining Walls
– Anchored in bedrock
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Drains to prevent water pressure from building up
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Rock bolts to fasten unstable rock