GSC 1620 Chapter 8
Mass Movements (Wasting)
Mass Wasting
Mass Wasting – downhill movement of Earth materials under the influence of gravity
Annually in the U.S. average mass wasting property damages exceed 4 billion dollars and typically 50 or more people are killed
Landslide is a colloquial term used by most people to describe all types of mass wasting (see slides)
Venezuela, 1999~30,000 deaths fromrain induced debris flows
Venezuela, 1999Note the largeboulders moved by the debris flow
Mass Wasting
The adjacent photos show the effects of mass wasting in Laguna Beach, CA (6/1/05)
The average price of homes in this community is 1.75 million dollars!
Mass Wasting
Gravity – force of attraction between two bodies; the magnitude of this force is dependent only on the masses of the bodies and their separation distance (see slide)
The Earth’s gravity pulls objects toward its center
BA
Distance
Gravity Force = Constant x (Mass A x Mass B) Distance2
Background
On sloped surfaces, the force of gravity can be conceptually subdivided into the shear force (stress) and normal force (see slides)
Gravity
Shear stress Normal force
Shear strength (friction)
If the shear stress actingon the slope exceeds theslope’s normal force plusshear strength, whatshould happen to the slope materials? Mass waste or not?
Gravity
Normal force
Shear stress
Shear strength (friction)
Notice how the shearstress increases and thenormal force decreasesas the slope angle increases
Angle of Repose
Angle of repose – maximum slope angle at which particles remain stable; for loose materials this angle typically varies from 25 – 40 degrees
If the slope material’s angle of repose is exceeded by natural or human forces, portions of the slope should mass waste (see slides)
Solid rock
Vertical cliff
Gravel slope Sand Clayey soil
In unconsolidated (loose) slope materials, larger and moreangular particles have a higher angle of repose
Slopes
Natural slopes should be viewed as landforms that have attained equilibrium with their surrounding conditions; if that equilibrium is disturbed by natural or human forces the slope may mass waste to reestablish its equilibrium
Other Mass Wasting Factors
The amount of water in the slope materials and the type and distribution of slope vegetation also affect slope stability (see slides)
Role of Water in Slope Stability
The more completelythe surface is coveredby vegetation and themore interlocking and denser the root system, the more stable a slope of unconsolidatedmaterials.
The removal of vegetation during land development or fire sets the stage for mass wasting.
Mass Wasting Triggers
Most mass wasting events are “triggered”Human and natural triggers existCommon human triggers:Slope oversteepeningVegetation removal Addition of excessive weight to slopeAddition of excessive water to slope (see
slide)
Mass Wasting Triggers
Four typical natural triggers:
1) Prolonged and/or intense rainfall, or rapid snow melting, that saturates the slope materials
Mass Wasting Triggers
2) Earthquakes – violent ground shaking induces ground failure
(5/17/2008)
FOX News
Mass Wasting Triggers
3) Volcanic eruptions – remember lahars?
Mass Wasting Triggers
4) Stream or wave undercutting – water erosion removes support for the overlying slope materials
Mass Wasting Classification
Rate: potentially quite fast; perhaps100 miles per hour
Rate: generally slow; a few feet toperhaps tens of feet per day
Rate: potentially quite fast; perhaps100 miles per hour
Rate: generally slow; a few feet toperhaps tens of feet per day
Californian slumps and flows
1995
2005 – renewed mass wasting claims ten lives
Rate: quitefast; up toabout 125miles per hour
Wave undercutting of slope
RockfallBlocks
Waveundercutting
Sediment Fall
Many subcategories: debris, mud, earth, lahar, snow avalanche,creep
Chaotic, turbulentinternal motion
Rate: vast range; extremely slow (inchesper year) toperhaps 600miles per hour
All fast-moving, extensive travel distance(tens of miles) flows possess ahigh degree of lubrication fromwater and/or air
California Mudflow
In 1970, a verypowerful earthquakein Peru triggered a debrisavalanche that buriedmost of the towns ofYungay and Ranrachircaand killed about 20,000 people. Estimatedmaximum speed offlow: 600 miles perhour!
SnowAvalanche
Why have deathsfrom snow avalanchesin North America been increasing?
Mass Wasting Types
Mass wasting movement rates vary from exceedingly slow to exceedingly fast
Don’t be fooled – even the very slow mass wasting processes can threaten human development over long time periods (see slides)
Rate: maybe12 inchesper year
Submarine Slumps
Mass Wasting Also Occurs Beneath Water
Recognition of Mass Wasting Hazards
The steeper the slope the higher the riskThe less vegetated the slope the higher
the riskKnowledge of the subsurface geology and
local/regional geologic and climatic conditions helpful
Geologically recent mass wasting events are often recognized by slope scars – sections of oversteepened slopes mostly devoid of vegetation (see slides)
1925
Recognition of Mass Wasting Hazards
1925, Wyoming
CNN.com; 5/26/08
Mass wasting slope scar
Slope scars
Human Intervention
Humans have used various methods to attempt to reduce the risk of mass wasting including: slope retention structures; mass wasting diversion structures, slope drainage; slope reduction; and slope stitching (see slides)
None of these methods is failsafe!
Concreteblock retentionstructure
Retention structuresmade of rock rubbleencased in wire mesh
Mass wasting diversion structure builtto divert snow avalanches, Canadian Rockies
(SlopeDrainage)
Drain Pipes
Slope Reduction
Figure 8.30A
Slope Stitching
U.S. Geological Survey (USGS) Landslide Warning System
Constructed in response to 1982 rain-triggered landslides that killed 25 in the San Francisco, CA area and caused $66 million damage
The USGS method is a mathematical prediction of slope stability based on: rainfall intensity and rainfall total, storm duration, slope angle and slope stability characteristics (e.g., soil/rock type, slope fluid content)
U.S. Geological Survey (USGS) Landslide Warning System
The mathematical relationships between the variables are determined from observations of previous landslides
First test of model: February, 1986; of ten landslides with known occurrence times, eight took place when forecast
Other Work
In addition, ground tilt meter and GPS measurements can be used to assess precursor changes to slope stability
Slope pore water pressures can also be measured – remember, the higher the slope material water pore pressure the greater the risk of mass wasting (see slides)
Slopestabilitymonitoringsite
Woodway, WA
Other Work
Attempts to improve these methods continue – see picture describing the USGS and US National Forest Service experimental flume and the use of “smart” rocks
Then review U.S. landslide potential map
U.S. Landslide Potential
United States Geological Survey