slope systems Flashcards
slopes and mass wasting
All slopes are inherently unstable
Therefore all slopes will experience slope failure
This is known as Mass Wasting
mass wasting
The transfer of rock and regolith (soil, sediment, debris) downslope by GRAVITY
With minor help from water, sometimes
Main types
Creep
Slump
Debris flow
Landslide
Rockslide/Rockfall
Subsidence
case studies: Huascaran Landslide (Peru)
May 31, 1970
Great Peruvian Earthquake (7.9M)
North side of Mt. Huascaran collapsed
80 million cubic feet of ice, mud, and rock
11 miles from start to finish
Average speed: Up to 200 mph
Killed up to 30,000 people at the base of the mountain
Town of Yungay (5 miles from Huascaran Peak) almost completely wiped out
case studies: Bingham Canyon Copper Mine Landslide
April 10, 2013
“Largest non-volcanic landslide…in North America in modern times”
144 million tons of dirt and rock
Equivalent to more than 90 acres
Enough to bury NYC Central Park 66’ deep
Costliest landslide in Earth’s history
No fatalities or injuries
Successful forecasting
“successful failure”
February 2013
Movement on the NE wall first detected
TARP (Trigger Action Response Plan) initiated
March 2013
Relocation of employees, equipment, and facilities
Including the visitors’ center
Notification of local community
Access to lower pit limited
April 2013
April 9
Acceleration of movement on slope detected
Haul road closure
Access to pit restricted to special permission only
Sentries posted
April 10
9:30am
TARP hit highest threshold
Full pit closure
No access permitted
9:30pm
Slide occurs along weak layer at the bottom of the Manefay geological unit
11:00pm
Second slide in same place
case study: vajont landslide, Italy
October 9, 1963
A dam was built in a deep canyon
270 million cubic meters slid into the lake
Within 45 seconds
68 mph
Displaced 50 million cubic meters of water
250 meter (820 ft!!) high wave
~2000 people killed
Why do slopes fail
Gravity + …
Gravity is a constant force
So something else must change as well
Slope angle change
Overloading
Increasing the weight
Decreasing the resistance
Lowering friction
Slope stability
Downslope force = gravity
Resisting force = friction, weight, retention measures
Failure = when downslope force is greater than resisting force Fd>Fr
Mass Movement Triggers
cause resistance to decrease or downslope force to increase
Water
Oversaturation
Adds weight
Reduces friction
Vajont Landslide, Italy
Oversteepened Slopes
Angle of repose
Steepest angle at which the material is stable
Changes to the slope angle introduce instability
Bingham Canyon Mine landslide
Mass movement triggers: oversteepened slopes
Slope is at angle of repose
Toe of the slope is cut away (freeway, railroad, stream erosion, etc). Part of the slope is much steeper than the angle of repose.
Slope regains equilibrium (of a sort)
see screenshot
oversteepened slopes: undercutting
see screen shot:
weathering by water below, wedge piece above falls as rockfall
other Mass movement triggers
Removal of Vegetation
Root systems bind soil and regolith together
Deforestation
Range fires
Insect infestation
plants disperse and use up water so it doesn’t all settle in soil
Earthquakes
Temporary lessening of friction
Huascaran Disaster
Planes of weakness
Saturated sand or clay layers
Joints parallel to the land surface
Weak sedimentary bedding
Shale, evaporites
Metamorphic foliation
mass wasting
The transfer of rock and regolith (soil, sediment, debris) downslope by GRAVITY
With minor help from water, sometimes
Main types
Creep
Slump
Debris flow
Landslide
Rockslide/Rockfall
Subsidence
types of mass movement
Creep
Very slow (slowest)
Alternating expansion and contraction of loose rocks
Temperature fluctuations
Solifluction
ground movement
j trees
Landslides/Slumps
Debris Flows
Rockslides/Rockfalls
Land subsidence
Creep: solifluction
Polar regions
Top part of soil thaws
Deep soil is permanently frozen
Rainwater collects in top part of soil
Becomes heavy and slick
Mass Movement: landslide/slump
Mass movement along well-defined failure surface
Sliding of regolith as coherent blocks
Variable speed
“Slump” when slow
“Landslide” when fast
see screen shot
wet year April 1983
Thistle, Utah
1000 ft in width
200 ft thick
1 mile length
Baldy below timpanogos
mass movement: debris flow
Debris Flows
“Mudslides”
Rock fragments, soil, mud, water
Happen during intense rainfall or sudden thaw
(kind of just a muddy flood)
Speed
Fast when high volumes of water/steep slopes
Slower when less water/gentler slopes
Two main types
Mudflows/Debris flows
Common in tropical/subtropical settings
Deep regolith
Abundant water
Grain sizes range from pebbles to large boulders
Cars, trees, cabins, bulldozers
Lahars
la conchita debris flow
Built on narrow flat land between steep slopes and beach
200 homes
1995
Heavy rain year
Slow moving slump
Buried nine houses
No loss of life
2005
Heavy rain year
Slumping turned into mudflow
Buried 13 houses
Killed 10 people
debris flows: lahars
Special kind of debris flow
Common in loose pyroclastic material
Can be very fast flowing
Up to 150 km/hr
(~90 mph)
Warm or cold
mass movement: rockslides/rockfalls (debris falls)
Rockslides/Rockfalls (Debris Falls)
Free-fall of rocks from steep cliffs; OR
Avalanche of rocks down a slope
Frank, Alberta rock fall
1903
Giant wedge of limestone
400 m high
1200 m wide
160 m thick
Seven million tons of rock
Buried the town in 30 m of rock
(~90 feet!)
Over in 100 seconds
Nearly 100 people died
submarine landslides
Deposits
Slurry of mud with larger clasts
=“Olistostrome”
Turbidity currents
“Graded beds”
Can be very large
Hawaiian Islands
***any one that ends in water
can be catastrophic causing water displacement tsunamis, you can’t usually feel landslides so they have to get the word of warning out
mass movement: land subsidence
Land subsidence
Sinkholes are one example
More details in groundwater chapter
Landslide potential mapping
Assesses multiple factors
Slope steepness, strength of bedrock, water saturation, bedding, vegetation cover, earthquake probability…
Uses computer modeling
Can be updated with drastic changes
Wildfire, past landslides, etc
help city planners, contractors, etc.
mass movement prevention strategies
Revegetation
Regrading
Engineering Structures
Drainage
Riprap
Retaining walls
Joints and bolts
test review picture order
g,z,p,a(because the tiny rocks within a rock are older than the rock they are in, and there is a baked mragin where t baked it so it had to exist before),t,d,y,m,e,c,x,b,l,s
q,o,n,l,m,p,h,i,j,k
tr: half life q
17,190
picture
synclines, anticlines, oxidation: chemical weathering, angular unconformity, convergence creates folds, erogenies,
review question: rainbow arches which is older middle or outside
middle, lower numbers are older
