Sep 2009
M7.6
Landslides
and
other
forms
of
mass
wasting
Particles create an angle of repose
based on their size and angularity
Angle of
repose
35°
Fine sand
40°
Coarse sand
45°
Angular pebbles
More cohesive
Damp sand
Surface tension
binds particles
Less cohesive
Dry sand
Dry particles are
bound only by
their size and
friction.
Water-saturated sand
Saturated particles
are separated by
water, which acts
as a lubricant,
allowing them to
flow.
Water can fill pores in soil
• Cohesion: force holding soil grains together
• Loose soils have 10-45% pore space
– Small amount of water increases cohesion
– Too much water pushes grains apart, reducing
cohesion
Frictional Resistance Prevents Sliding
• Gravitational Force pressing down on slope
• Friction is the ‘Roughness’ of slippage surface
• Area of contact does not affect friction coefficient
• Slide occurs when gravitational force exceeds frictional
resistance
Slope Material influences sliding
• Loose materials slide easier:
– Soil
– Loose sediment
– Soft sedimentary rocks such as clay or shale
Clays can increase chance of landslide
• Clays absorb water and expand to weaken rock
• Kaolinite: soaks up water
• Smectite: forms from volcanic ash, with open
structure between layers that fills with water 
swelling soils
Mud in bays, lakes is likely to fail
“Quick-clays”
Landslide associated with 1964 Alaska Earthquake
Sand and
gravel
Clay
Clay
Water-saturated
sandy layer
Before earthquake
After earthquake
Landslide Triggers
(1)
(2)
(3)
(4)
Oversteepening
Earthquakes
Rainfall
Volcanic eruptions
1. Oversteepening decreases stability
• Steeper slopes are less stable
• Slope angle is increased when
– Fill is added above
• Construction of homes with magnificent views
– Slopes are undercut below
• Erosion at base of slope, by waves at coast
• Excavation of road at base of slope
Oversteepening decreases stability
2.Earthquake
loosens large masses
of rock
Earthquake
Denali Earthquake 2002
3. Adding Water
• Water reduces strength of slope
• Heavy or prolonged rainfall saturates soil,
• Human actions add water to slopes
– Lawn-watering, crop irrigation
– Leaking water/sewer pipes, swimming pools
– Filling reservoir behind dam
Rain has soaked
fine-grained
permeable soils,…
…which quickly loosen…
Waterpermeable
soil
Waterimpermeable
rocks
…and flow downhill.
Rain soaks muds and rubble…
Clear-cut slopes
Shale
Jointed
bedrock
…resulting in
a flow of mixed
mud, rock, and
surface debris.
Irrigation caused this slide in So. Cal.
Venezuela 1999
Venezuela 1999
Venezuela 1999
Venezuela 1999
Venezuela 1999
4. Eruption causes landslides
Snow and ice
Water-permeable
volcanic ash
Waterimpermeable
lava
A volcanic eruption has
melted snow and ice that
soaks volcanic ash over
impermeable lavas.
The resulting mud
moves quickly
downhill.
Types of Landslides
Landslides are classified by:
-Material type
-Movement type
-Movement velocity
Velocity can range from
<1 mm/yr to 100 m/sec
Rockfall: Extremely rapid
Ice wedging prepares
rocks to loosen and
fall away.
Individual blocks
free-fall down slope.
Rockfall creates talus slopes of loose rock
Debris Avalanches: extremely rapid
• Triggered by Peru
earthquake
• Can begin as rockfall, but
become larger and run
further.
• Avalanche went 14 km to
with average speed of 270
km/hr
Debris Avalanche
An earthquake has
loosened large masses
of rock…
…that flow downhill
at high velocity on a
cushion of air.
Earthquake
Debrisflows, mudflows, and LAHARS: Rapid
Snow and ice
Water-permeable
volcanic ash
Waterimpermeable
lava
A volcanic eruption has
melted snow and ice that
soaks volcanic ash over
impermeable lavas.
The resulting mud
moves quickly
downhill.
Rockslide: moderately rapid
Frost wedging has
loosened jointed
bedrock layers…
…that move
downhill as
a unit.
Translational Slide: goes along existing weakness
This one cost taxpayers $400 million
Slump: Slow to moderate
Scar
Unconsolidated material
slowly slides as a unit.
Scar
Building
foundations
shear and crack
Gravestones and
fence posts lean
Trees grow with
curved trunks
Road cracks
Power poles
lean
Creep: extremely slow
Soil Creep
• Slow, downslope
movement of soil and
weak rock
• Involves near-surface
movement by alternate
expansion and shrinkage
of soil
Snow Avalanches: usually rapid
• Trigger for avalanche could be
–
–
–
–
–
Weight of skier crossing slope
Vibrations of snowmobile
Movement of glacier
Changes in temperature
Earthquake
Failure of Landslide Dams
• Any moderately fast-moving landslide can block a river or
stream to create a dam and temporary lake before
eventually failing
• Time before failure and size of flood depends on
–
–
–
–
Size, height and geometry of dam
Material making up dam
Rate of stream flow, how fast lake rises
Use of engineering controls (artificial breaches, spillways or
tunnels)
• Dams from mudflows, debris flows and earth flows are
noncohesive and erode quickly
Failure of Landslide Dams
• Most landslide dams fail when water overflows and erodes
spillway that drains lake
• If dam-failure flood incorporates significant sediment, can
turn into debris flow – much more dangerous
• Useful dams can be constructed on top of landslide dams
– Rockfalls or rock slides are most stable
– 1928 St. Francis high-arch concrete dam failed – built on toe of old
landslide
Mitigation of Damages from Landslides
• Damages can be extremely costly
• Not covered by most insurance policies
• In U.S., landslides cost more than $2 billion
and cause 25-50 deaths per year
• Globally, cost more than $20 billion, cause
about 7500 deaths per year
• Major landslide disasters increase with
growth in population in dangerous areas
Mitigation of Damages from Landslides