Mass Wasting
Movement of Earth materials
down slope due to gravity.
Translational Landslide
Mass wasting is caused by gravity
Shear force
Normal force
Force = mass x g
Mass wasting is resisted by shear strength.
Shear strength is proportional to:
• Normal force
• Cohesion (internal and external)
• Friction
• Dryness
Normal force
Water is a critical variable.
Dry
Moist
Saturated
Moderate
High
Very Low
Shear strength : resists movement
Types of Mass Wasting
Creep: slow, continuous down-slope movement of soil
and unconsolidated sediments over time.
Caused by freeze / thaw cycles and/or pore
water pressure.
Types of Mass Wasting
Creep: slow, continuous down-slope movement of soil
and unconsolidated sediments over time.
Caused by freeze / thaw cycles and/or pore
water pressure.
Creep produces a
transported soil called
Colluvium.
Colluvium is a jumble of
talus blocks and clay,
similar to glacial till.
Rates of creep can be as
low as inches per year.
Types of Mass Wasting
Landslide: movement of rock or debris down a slope
along one or more distinct failure
surfaces.
Rates: 1m/day to 300 km/hr
Types of Landslides
Rockfall: vertical fall of material through the air.
Types of Landslides
Rockslide: mass of bedrock detaches and slides along a
failure plane.
• Common when sedimentary layers dip parallel to
slope.
• Rockslide may break up and begin tumbling - rock
avalanche.
• Vaiont Dam disaster, Italian Alps, 1963.
Vaiont Dam Disaster
1. Nearly 3,000 died.
2. A landslide broke loose and filled the reservoir behind the dam,
causing a huge wave of water to top the dam, flooding towns
downstream.
3. The dam itself held.
4. High precipitation preceded the disaster.
5. Piezometers in place indicated the increasing hydraulic head in
the slope that broke.
6. However, geologists failed to recognize the impending disaster
until too late.
7. Rates of creep of rock mass were 20 cm / day prior to slide.
8. Engineer in charge committed suicide.
Vaiont Dam Rockslide
Vaiont Dam Rockslide
Vaiont Dam Rockslide
• Failure along saturated layers of clay and gypsum (very weak).
• Rising water level in reservoir increased bouyancy and pore pressure
in rock mass at the base of the slide.
Vaiont Dam Rockslide
Vaiont Dam Rockslide
Gros Ventre Landslide, Wyoming
June 23, 1925
Saturation of weak sedimentary layers dipping toward valley.
The landslide dammed the Gros Ventre River, creating Lower Slide Lake.
Two years later, a portion of the slide dam failed, causing a flood that destroyed the
downstream town of Kelly, killing 6 people.
Thistle Landslide, Utah, 1983 (rate of flow = 2’ per day)
Thistle Landslide, Utah, 1983
Most costly landslide in U.S. history due to the economic impact of closing the main railroad
and two state highways accessing eastern and southeastern Utah.
The direct cost of the landslide was estimated at $200 million but, some estimates of the
total cost reached as high as $400 million (equivalent to $882 million in 2011).
Types of Landslides
Slumping: mass of unconsolidated material detaches and
rotates along a curved failure plane.
• Series of scarps left at head of slump.
• Mass of material flows outward at toe of slump.
• Commonly triggered by saturation of hillside slopes
by rain.
• Also called a Rotational Slide
scarps
toe
Slumping
La Conchita, California, a
small seaside community
along Highway 101 north
of Santa Barbara.
This landslide and debris
flow occurred in the spring
of 1995.
La Conchita, California
January 10, 2005
Heavy rains trigger
landslide.
10 dead
36 houses destroyed
Remobilization of previous
landslide deposit.
La Conchita, CA
Photo taken in 2002
September, 2004
Types of Landslides
Earthflow: mass of unconsolidated material that moves
down-slope as a viscous liquid.
• May break up into slump blocks.
• Commonly triggered by saturation of hillside slopes
by rain or septic systems.
• Can be triggered by undercutting at base of slope.
Original slope
Earthflow
Undercut slope
Earthflow
Earthflow
Earthflow
Earthflow
• Slumgullion earthflow,
near Lake City, Colorado.
• 7 km long.
• 700 years old, yet still
moving.
Earthflow
Mill Creek, CA
Types of Landslides
Mudflows: earthflows supported by water-saturated mud.
• Commonly triggered by saturation of fire-swept
hillside slopes by heavy rains.
• Can be triggered by volcanic eruptions of
stratovolcanoes, which melt glaciers on volcano
summit.
• Volcanic mudflow = Lahar
Lahar - Mt. St. Helens
Lahar - Mt. St. Helens
Armero, Colombia
Lahar - volcanic
mudflow
1985
25,000 dead
50 km
40 mi / hr
Types of Landslides
Debris Avalanche: very rapidly moving mass of rock,
sediment, air, and water.
• Triggered by large avalanche on steep slope.
• Additional debris swept up as mass moves down
mountain, gaining speed and momentum.
• Debris tumbles chaotically, may trap cushion of air at
base.
• Speeds of several hundred mph attained.
• Yungay, Peru, 1970
Yungay, Peru
• Earthquake dislodges
half-mile wide slab of
glacier ice.
• Viscous debris mass ten
stories high.
• Buried two towns on
market day afternoon.
• 66,000 dead
Yungay, Peru
Yungay, Peru
before
Yungay, Peru
after