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Mass Wasting

Mass Wasting. A/Prof John Worden DEC University of Sth Queensland. 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;

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Mass Wasting

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  1. Mass Wasting A/Prof John Worden DEC University of Sth Queensland

  2. Mass Wasting • Mass Wasting refers to the down slope Movementof soil, rock and unconsolidated materials in response to Gravity. • Not a response to normal erosive agents of water, wind and ice; • Strength of gravity > 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 US$1-2 billion in property damage each year; • An engineers role to minimise these losses during construction and land development.

  3. 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.

  4. 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 tocohesion. Over-steepening & denuding slopes of vegetation lessens cohesion  instability.

  5. 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.

  6. 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 bedrock’s capacity to absorb water. • Typical examples include bedding and fractures.

  7. 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! • Heavyrainstormsmay 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

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