Mass movement- The down slope movement of earth material under the influence of gravity.
Figure 10.3
Key role in mass movement is gravity
In order for the material to move down slope, it must overcome
1. Friction –surface resistance to relative motion.
2. Inertia – tendency of an object at rest to remain at rest.
Internal Causes of Slope Failure
Inherently weak earth materials
1. Clay – most earth failures in the soil zone are due to clay.
Clay layer in a sedimentary rock beach cliff
2. Pyroclastic materials – Light colored tuff from Santorini produced 3500 year ago in Minoan eruption.
3. Shale – sedimentary rock made of clay and silt particles
Bedding planes, friend or foe?
Internal problems caused by water1. Adds weight to a slope
8.34 lb/gallon 54 gal. = 457.36 lb!
54 gal.barrel
2. Saturation increases the pore-water pressure
Low pore water pressure High pore water pressure
3. Dissolves the cementing material in rock
4. Causes “piping”, the subsurface erosion of earth material
The Vaiont Dam Failure
Vaiont, Italy 1963Setting:Dam on river in a glacially carved
canyon.
Some Factors Leading to Slope Failure1. Valley is a syncline2. Cavernous weathering of limestone rock3. Valley downcutting by river4. Slope saturation - water from reservoir and heavy rains.
Massive landslide and ~3,000 dead.
Falls Occurs when one or more rocks fall down
steep slopes.
Rockfall, South Island, New Zealand
Moonlight Beach, Encinitas
Yosemite 1996. 162,000-ton rock mass fell.
Yosemite rockfall 1996
A common cause of rock fall is frost wedging.
Talus cones
SlideOccur when a more or less coherent mass
moves down slope.
Rotational slide- slump
1. Curved failure plane.
2. Head moves down, toe moves up and out.
Don’t move far or fast
In California, triggered by heavy winter rain.Pacific Palisades, CA
Another cause of slumps – road cuts.Elks Lodge near Lewiston, Idaho
Translational slide – landslide1. Mass moves
down and out.2. Moves far and
fast.3. Primary causes:
heavy rains or earthquakes.
Landslide on Big Sur Coast
Madison River Canyon Landslide 1959
Madison River Canyon from top of slide
View from debris dam and visitors center
FlowMass loses coherence and moves as a viscous fluid.
Earthflow- Downslope movement of a fluid layer of mainly soil and weathered rock material that is saturated with water.
Earthflows often begin as slumps.
Usually, like slumps, they do not move very fast.
Road cut caused slump/earthflow
Debris flow- Downslope movement of a fluid mixture of mud and larger rock debris.
Venezuela 1999
Key differences between earthflows and debris flows:
1. A debris flow moves faster; meters/second.2. A debris flow carries larger rock material.
Debris flows can carry large boulders because their density is 1.5 – 2.0 times that of water.