4.2 Mass movements Flashcards
What causes mass movements?
Downslope movement of rock and weathered debris by gravity.
Gravity moves material downslope, and sticks particles to the slope.
Downslope movement proportional to weight of the particle and slope angle
Water lubricates particles and fills spaces between them, forcing them apart under pressure.
As pore pressure rises it greatly increases the amount material moves
May also be caused by weathering
What factors increase shear stress or reduce shear strength?
Stress: Removal of lateral support, removal of underlying support, loading of slopes from weight, vegetation and accumulation of debris, lateral pressure from water in cracks, freeze/thaw, pressure release, transient stresses - earthquakes movement of wind
Strength - weathering - disntegration of granular rocks, hydration of clay, solution of minerals, changes in pore water from saturation, softening of material pressure, changes of structure from fissures, remoulding of sands and clays, organic effects such as burrowing of animals and decay of roots.
What are the features of slides and flows
Move downward and outward, outward ones more hazardous as a single solid mass moves along a single plane or zone - often fast movements of dry material. Hazardous flows often occur when rock is mixed with water to form earth flows, mudflows and lahars
Occur in solid rocks and weathered material. The plane along movement takes place is inclined and material moves in a mass, breaking up where it comes to rest.
In a slip/slump the movement is rotational around a curved slip plane, resulting in a sloped slip face and unbroken toe. They can occur in areas with rock layers of different permeability.
What human activity may increase the risk of slides?
- Increased slope angle,
- Extra weight
- Removing vegetation
- Exposing rock joints
- Bedding planes,
- restraining structures
- Excavating
- Filling steep slopes,
- Draining slopes
- Watershed management
What are the impacts of landslides?
- Sudden unexpected downhill movement
- Collapses cliffs and coasts an mountainsides
- Destroys buildings, land and kills people
- Debris may also fall with little warning and rock falls destroy roads and kill people
- Mudslides level out large areas and destroy houses
- Water level rise - flooding
- Rockslides
- Mud and debris flows
- Avalanches
- Earthquake triggered MM
- Volcanic triggered MM
- Landslide dams from dam breaks
- Dam failures
What is plastic flow?
Clay rich material is saturated and put under lots of stress, starts to flow.
Solilfluction occurs as saturated soil flows down a slope, often happening in permafrost areas so impermeable to water and overlying soil is more saturated and slides down
Scree slopes collect in cones or fans from about
What factors cause mass movement?
- Loading
- Thermal expansion
- Wetting/drying
- Solifluction
- Exceeding liquid limit
- Faults and fault lines
- bedding planes exploited
- Steeper angles
How may landslides be predicted and monitored?
- Warning signs indicated by cracks, tilted structures, bulging walls, steep slopes, arcuate shaped cracks
- large movements can be monitored by GPS and laser surveys which measure distance and direction accurately of a movement
- This can be used to calculate amount of water the slope can retain before moving
- Rainfall and changes in soil moisture monitored as indicate groundwater content and pressure exerted
- Tiltmeters and strain meters record changes in crustal strain
- Debris flow warning systems also monitored
What is landslide hazard mapping?
Knowledge of slope stability and previous movements
- Areas classified with high, medium or low susceptibility to landslides and degree to which affected is classified.
- Evaluates magnitudes and frequencies
- Testing soil and rock properties
What are some hard methods?
- Drainage pipes
- Retaining devices
- Grading slopes
- Diversion walls
- concrete blocks and gabions strengthen slopes and absorb energy
- Drained creeeps
- Regrading
- Limited human activity
- Early warning systems
- Pinning
- Netting
- Afforestation
- Grouting and shotcrete
What are avalanches?
Mass movements of snow, soil, ice debris and boulders at high speeds
- Small scale, only occur in low population densities and controllable
- Frequently 20 degree north facing slopes where sun inhibits stable snow - up to 60km/h
What causes an avalanche?
Snow is strengthened by interlocking snow crystals and cohesion from bonding of snow crystals. Remains in place is strength exceeds stress.
Changes in pressure, snow, temperature and movement of meltwater cause structure to change, unstable, move downslope.
Loose avalanches occur after snowfall, slab avalanches occur later, when the snow has developed and loses cohesion. Slab avalanches are much larger and cause more destruction - often caused by a sudden rise in temperature causing melting which lubricates the slab and makes it unstable.
What are the features of an avalanche?
Starting zone: most volatile area, where unstable snow fractured from the surrounding snow and starts to slide, usually high up.
Track: path the avalanche follows - usually sees paths of missing trees and known tracks
Runout zone: where the snow and debris come to a stop and is deposited - like a scree zone
What are slab avalanches?
Build up of material reaches a critical point where mass> friction, like a slide. High speed and dangerous from fresh, damp snow or old brittle snow.
Hard slab avalanches are stiff, cohesive slabs over a weak layer deposited on hardened layers. The addition of load causes the weak layer to collapse the hardened slab slides down the cliff.
- Usually occurs in spring where there is partial melting
- Slabs fracture above and move over the smooth bed surfaces, allowing the slab to move quickly
- Slab avalanche has appearance of a block cut out
Features:
- Crown fracture at the top (scar)
- Flank fractures at sides
- Bottom fracture at bottom
What are powder avalanches?
- Turbulent suspension powder snow
- Accelerate rapidly and grow in mass volume, causing fine grained snow to mix with air above behaving like a gas - creates dangerous blast of air after snow
- Forms from all type of snow but usually fresh dry powder snowfall
- Move rapidly and have high masses over long distances - destructive
