Mass Wasting

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Mass Wasting

• A/Prof John Worden
• DEC
• University of Sth Queensland

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Mass Wasting

• Mass Wasting refers to the down slope Movement
  of soil, rock and unconsolidated materials in
  response to Gravity.
• Not a response to normal erosive agents of water,
  wind and ice
• Strength of gravity gt the strength of slope
  materials (i.e., resistance to deformation)
• Material or mass moves down slope at variable
  velocities ranging from very slow to catastrophic
  and is generically referred to as a Landslide
• Mass moves as either falls, slides, flows and
  creep
• Triggered by heavy rains, floods, earthquakes,
  etc
• In USA alone, between 25-50 deaths and from
  US1-2 billion in property damage each year
• An engineers role to minimise these losses during
  construction and land development.

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Mass Wasting

• A natural consequence of weathering and rock
  fragmentation
• Very important in hilly to mountainous terrains
• Can scar mountainsides and produce debris on
  valley floors that may dam streams and obliterate
  highways, etc.
• The recent Threbo, NSW disaster is an prime
  example.
• What factors influence mass wasting? There are
  three
• Nature of slope materials
• Amount of contained water and
• The steepness of slopes or their instability.
• The last two factors are impacted by human
  activity such as excavations for buildings and
  highways. All reduce resistance to movement.

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Mass Wasting

• Nature of Slope Materials
• Highly variable and dependent on local geology
• Massive granite slopes most stable , more so than
  foliated metamorphic schist slopes,
  unconsolidated sediment slopes are least stable.
• Loose dry fine sand has a repose angle of 35?,
  while for angular pebbles this is 45. Any
  steeper angles will collapse to the repose angle.
• Thus the angle of repose varies with size and
  shape of particles
• The amount of moisture between particles controls
  surface tension which acts to bind particles
  together and increase the angle of repose.
• Consolidated materials form steeper and less
  regular slopes due to cohesion. Over-steepening
  denuding slopes of vegetation lessens cohesion
  ? instability.

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Mass Wasting

• Water Content
• When ground becomes saturated with water, it is
  lubricated and internal friction (cohesion) is
  lowered so that particles can easily slide past
  one another.
• Should consolidated materials absorb large
  amounts of water, pore water pressures may rise
  sufficiently to separate grains producing fluid
  flow.
• If slope soils are stripped of vegetation and no
  longer bound by root systems, they are subject to
  water invasion and may become unstable.
• The net outcome is an unstable slope that will
  tend toself correct back to a stable repose
  angle by mass wasting.
• Frequent culprit is poorly designed drainage from
  septic tanks in hillside home developments after
  soils become waterlogged.

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Mass Wasting

• Steepness of Slopes
• Slope stability depends on weathering and
  fragmentation of rocks.
• Shales in arid areas of Australia tend to weather
  and fragment into small pieces that mantle the
  bedrock. The resultant slope angle of the bedrock
  closely resembles that of loose coarse sand (?
  40). With time these slopes become unstable and
  experience slides.
• Hard cemented Sandstones in contrast, break into
  large blocks with steep bare bedrock slopes above
  mantled broken rock slopes beneath.
• Structures in the bedrock influence slope
  stability and the bedrocks capacity to absorb
  water.
• Typical examples include bedding and fractures.

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Mass Wasting

• Triggers for Mass Movement
• If the right combination of materials, moisture
  content and steepened slopes exists, a slide or
  flow is inevitable. Only the trigger is missing!
• Heavy rainstorms may trigger the unstable slope,
  or badly designed runoff water disposal systems
  can have the same effect (i.e. Threbo).
• Frequently vibrations like those produced by
  earthquakes, can convert water-saturated sandy
  layers in clay to slurries by liquefaction. Large
  blocks are then free to slide downslope.
• Often slopes are gradually oversteepened either
  by natural causes or by human intervention.
  These eventually suffer sudden collapse.

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Mass Wasting

• Classification of Mass Wasting
• Geologists classify mass movements using three
  parameters
• Nature of Material- (Rock or Unconsolidated
  Material).
• Type of Movement- (falling, sliding or flowing).
• Velocity- ( from cm/yr to many kilometers/ hr).
• Falling denotes freefall
• Sliding occurs when the bulk of the material
  moves as a single unit
• Flowing refers to material that moves as a
  fluid
• Movements must be reconstructed from the
  deposited debris after an event.

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Mass Wasting

• Rockslides
• Masses of bedrock moving as a single unit, and
• where rocks slide freely down slope along bedding
  planes or joint planes.
• Generate Talus slopes at the base of the
  rockslide.
• Rock Avalanche
• Large masses of rocky material (many ?500,000 m3)
  that flow down hill at velocities of tens to
  hundreds of kilometers/hour.
• Often triggered by earthquakes.
• Among the most destructive mass movements.
• Resemble snow avalanches.

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Mass Wasting

• Rockfalls
• Newly detached individual blocks and masses are
  released into free fall from cliffs, overhangs or
  very steep mountainsides.
• Cohesion is weakened by weathering along joints,
  or even the water freeze/thaw cycle.
• Fallen blocks accumulate at the base of the slope
  as Talus deposits.
• Talus deposits may be matched to rock units
  above them and build up over long periods of
  time.

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Mass Wasting

• Unconsolidated Mass Movements
• Often termed Debris and includes soil, bedrock
  fragments, trees, and human-created objects (i.e.
  houses, cars, fences,etc).
• Most unconsolidated mass movements slower than
  rock movements.
• Many flow like very viscous fluids (i.e. honey).
• Generally slower velocities result from lower
  slope angles over which they move.
• Slowest movement is Creep.

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Mass Wasting

• CREEP
• Downhill movement of soil or debris at 1-10
  mm/yr.
• Controlled by soil type climate steepness of
  slope and density of vegetation..
• Very slow deformation of Regolith
• Upper layers move faster than lower layers
• Causes trees, telephone poles and fences to lean
  or move downslope
• Mass of creeping soil may break poorly-supported
  retaining walls, structures and foundations.

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Mass Wasting

• Solifluction
• Restricted to cold regions where water in surface
  layers alternatively freezes and thaws
• When water thaws in surface layers they become
  saturated waterlogged
• As there is no deep drainage (due to frozen
  deeper soil layers), soil oozes downslope
  carrying every thing with it.
• Earthflow
• Fluid movements of relatively fine-grained
  materials such as soils, and clay.

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Mass Wasting

• Debris Flows
• Fluid mass movements of rock fragments supported
  by a muddy matrix.
• Tend to move more quickly than earthflows
• Occasionally may exceed 100km/hr.
• Slumps
• A slow slide of unconsolidated material that
  moves as a single unit.
• Usually slumps slip along a basal plane that has
  the shape of a concave upwards surface.
• May occur with multiple consecutive slip
  surfaces.
• A common natural feature of the Range escarpment
  which can be readily induced by
  poorly-conceived human activities.

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Mass Wasting

• Mudflow
• Flowing masses of material mostly finer than sand
  that contain large amounts of water.
• Dry cracked mud absorbs water, internal friction
  decreases and mass of mud loses its resistance to
  movement and flows.
• Flows with velocities of several km/hr.
• Most common in hilly and semiarid regions
• Tend to travel down upper valley slopes and merge
  on valley floors
• Occur after infrequent prolonged heavy rainfall
• Can carry large boulders, trees, etc.
• Cause heavy losses of human lives.

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Mass Wasting

• Debris Slide
• Have higher velocities than slumps
• Rock and soil materials move largely as a single
  unit along planes of weakness such as a
  waterlogged clay zone towards the base of the
  debris.
• During the slide event, sections of the slide may
  behave as a chaotic flow
• As it moves downslope, it may transform into
  mostly a flow traveling in a fluid manner.

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Mass Wasting

• Debris Avalanche
• Fast downhill movements of soil and rock in humid
  mountainous regions.
• Velocities reflect high water content and steep
  slopes
• Recorded velocities of 200-435km/hr known..
• Where unstable slopes exist and frequent
  earthquakes occur, mass movements need to be
  predicted.
• These flows remove everything in their paths
• A close association with volcano slopes in humid
  regions
• Mt St Helens in USA generated a debris avalanche
  that moved down the north flank of the volcano
  at 200km/hr.
• Great loss of human lives, property and
  infrastructure result.

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Mass Wasting

• Summary
• The vast bulk of mass wasting is natural.
• Human activities may trigger landslides in
  vulnerable areas, such as when we change natural
  slopes.
• At Vaiont, Italy, Engineers constructed a
  concrete dam (265 m high) in a steep-walled
  valley of limestone and shale.
• On October 9, 1963, a debris slide of 240 million
  m3 (2km x1.6 km x 150 m thick) plunged into the
  deep impounded waters behind the dam.creating a
  huge spillover. A 70 m high flood wave killed
  3000 people down stream..
• Mass movement danger had been flagged by
• A small rock slide in 1960
• Ancient slide scar on the valley walls above the
  dam
• Inherent weakness of cracked and deformed
  outcropping limestone and shale reservoir walls.
• While the landslide was natural, consequences
  could have been much less severe.

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Mass Wasting

• We cannot prevent most natural mass movements,
  but we can minimize our losses through careful
  control of construction and land development.
• Careful engineering can keep water from making
  material more unstable.
• In areas that are extremely prone to mass
  movements, development may have to be restricted.