landslides Flashcards
what is mass wasting?
movement of earth material down slope under influence of gravity
mass wasting can involve unconsolidated material or consolidated material; what are they and give an example of each
unconsolidated: loosely arranged or unstratified, whose particles are not cemented together
ex. sediment / soil
consolidated: materials of sufficient hardness or ability to resist weathering / erosion
unconsolidated
ex. rocks
what type of landslide occurred in Hope, BC on January 9th
rock avalanche
why / how do mass movements occur?
gravity, shear force, and normal force
what is shear force?
one pulling the block down parallel to the slope
what is normal force
pulling the block directly into (i.e., perpendicular) to the slope
how do shear and normal force work together to cause a landslide?
The shear force pulls the block down the slope, but the block doesn’t move unless the shear force overcomes the strength of the bond between the block and the slope.
mass movements are classified based on several characteristics
nature of material
nature of movement
speed of movement (1mm/year to 5m/sec)
what are the common systems for classification of landslides?
cruden and varnes
what are the types of landslides as classified by cruden and varnes?
falls
flows
lateral speeds
topples
slides
complex
what is a fall as stated by cruden and varnes?
rapid, free fall, rolling, bouncing, metres/min to metres/sec
what is a flow as stated by cruden and varnes?
viscous fluid movement, range from slow to fast
what is a lateral speed as stated by cruden and varnes?
lateral movement of the ground, usually accompanied by subsidence
what is a topple as stated by cruden and varnes?
rapid rotation of a mass about a point
what is a slide as stated by cruden and varnes?
movement of large slabs / blocks / consolidated material along one or more distinct surfaces
- rotational
- translational
what is a complex as stated by cruden and varnes?
2+ types
what is a translational landslide?
can contain loose sediments or large slabs of bedrock
what is a rotational (slump) landslide
landslides that occur along a curved or spoon-shaped surface
how are ways we have further weakened the slope to cause a landslide?
- coal mining
- dump garbage into cracks formed by slope failure (adds weight)
- constructing structures at crest (adds weight)
- building storm sewers that discharge water onto the slope
how do rock avalanches form?
triggered by an earthquake, a large quantities of material flow
how fast can rock avalanches move?
velocities of hundreds of km/hr
what happened at Mount Huascan in Peru?
rock and snow / glacial ice avalanche
what is the difference between a rockfall and debris avalanche?
Rockfalls involve the detachment and freefall, bouncing, or rolling of individual blocks of rock from a cliff face (loose boulders roll down due to low gravity)
debris avalanche is a rapid and turbulent flow of a large mass of debris mixed with water and air. Its movement is more fluid-like and affects a larger area (great deal of loose surface material appears to have slid down toward lower elevations)
what is a soil creep?
slowest unconsolidated mass movement (1 - 10mm.yr)
what is regolith (soil creep)?
a very slow deformation of the surface debris where upper layers move down the slope faster
the mechanism of creep can follow two cycles, these are?
freeze-thaw
dry-wet
what are two hallmarks of a soil creep?
- bent tree trunks / rock layers / power poles / fences / gravestones
- building foundations / roads crack
what is solifluction?
cold climates when water in the surface layers of the soil freeze and thaw; when surface thaws, soil becomes saturated and oozes downhill carrying rock and broken debris
at what place do fractures commonly develop?
fold hinges
what is the air cushion model?
compressed air trapped beneath the debris greatly reduces friction between the debris and slip plane. allows large boulder to travel great distances
mega landslides that occur underwater create?
mega tsunamis
how can we prevent landslides?
- reduce slope angle, place support at base, and reduce weight on slope
- build retention structures
- leave stabilizing vegetation
- decrease water / pore pressure of rock / soil
- other stabilizing techniques, like support walls, rock bolts, driving tunnels, chain-link fence over rocks by road
how do water forces on soil affect slopes?
water fills voids and increase weight
exerts pore pressure to decrease stress and strength
