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

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Presentation on theme: "Mass Wasting A/Prof John Worden DEC University of Sth Queensland."— Presentation transcript:

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

2 Mass Wasting lMass Wasting refers to the down slope Movement of soil, rock and unconsolidated materials in response to Gravity. vNot a response to normal erosive agents of water, wind and ice; vStrength of gravity > the strength of slope materials (i.e., resistance to deformation); vMaterial or mass moves down slope at variable velocities ranging from very slow to catastrophic and is generically referred to as a Landslide ; vMass moves as either falls, slides, flows and creep; vTriggered by heavy rains, floods, earthquakes, etc; vIn USA alone, between 25-50 deaths and from US$1-2 billion in property damage each year; vAn engineers role to minimise these losses during construction and land development.

3 Mass Wasting lA natural consequence of weathering and rock fragmentation; vVery important in hilly to mountainous terrains; vCan scar mountainsides and produce debris on valley floors that may dam streams and obliterate highways, etc. vThe recent Threbo, NSW disaster is an prime example. lWhat factors influence mass wasting? There are three: vNature of slope materials; vAmount of contained water; and vThe steepness of slopes or their instability. lThe last two factors are impacted by human activity such as excavations for buildings and highways. All reduce resistance to movement.

4 Mass Wasting lNature of Slope Materials: vHighly variable and dependent on local geology; vMassive granite slopes most stable, more so than foliated metamorphic schist slopes, & unconsolidated sediment slopes are least stable. vLoose & 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. vThus the angle of repose varies with size and shape of particles; vThe amount of moisture between particles controls surface tension which acts to bind particles together and increase the angle of repose. vConsolidated materials form steeper and less regular slopes due to cohesion. Over-steepening & denuding slopes of vegetation lessens cohesion  instability.

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

6 Mass Wasting lSteepness of Slopes: vSlope stability depends on weathering and fragmentation of rocks. vShales 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. vHard cemented Sandstones in contrast, break into large blocks with steep bare bedrock slopes above mantled broken rock slopes beneath. vStructures in the bedrock influence slope stability and the bedrock’s capacity to absorb water. v Typical examples include bedding and fractures.

7 Mass Wasting lTriggers for Mass Movement: vIf the right combination of materials, moisture content and steepened slopes exists, a slide or flow is inevitable. Only the trigger is missing! vHeavy rainstorms may trigger the unstable slope, or badly designed runoff water disposal systems can have the same effect (i.e. Threbo). vFrequently 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. vOften slopes are gradually oversteepened either by natural causes or by human intervention. These eventually suffer sudden collapse.

8 Mass Wasting lClassification of Mass Wasting: vGeologists classify mass movements using three parameters: Nature of Material- (Rock or Unconsolidated Material).Nature of Material- (Rock or Unconsolidated Material). Type of Movement- (falling, sliding or flowing).Type of Movement- (falling, sliding or flowing). Velocity- ( from cm/yr to many kilometers/ hr).Velocity- ( from cm/yr to many kilometers/ hr). vFalling denotes freefall; vSliding occurs when the bulk of the material moves as a single unit; vFlowing refers to material that moves as a fluid; vMovements must be reconstructed from the deposited debris after an event.

9 Velocity Dominant Material Nature of motion Moderate  1km/hr Fast  5km/hr RockFlow Rock Avalanche Rock Slide or Fall RockslideRockfall

10 Mass Wasting lRockslides: vMasses of bedrock moving as a single unit, and vwhere rocks slide freely down slope along bedding planes or joint planes. vGenerate Talus slopes at the base of the rockslide. lRock Avalanche: vLarge masses of rocky material (many  500,000 m 3 ) that flow down hill at velocities of tens to hundreds of kilometers/hour. vOften triggered by earthquakes. vAmong the most destructive mass movements. vResemble snow avalanches.

11 Mass Wasting lRockfalls: vNewly detached individual blocks and masses are released into free fall from cliffs, overhangs or very steep mountainsides. vCohesion is weakened by weathering along joints, or even the water freeze/thaw cycle. vFallen blocks accumulate at the base of the slope as Talus deposits. vTalus deposits may be matched to rock units above them and build up over long periods of time.

12 Velocity Dominant Material Nature of Motion Slow  1cm/yr Moderate  1km/hr Fast  5km/hr Unconsoli-datedflowCreepSoli-fluctionEarthflowDebrisflowMudflowDebrisAvalanche Unconsoli-dated slide or fall SlumpDebrisslide

13 Mass Wasting lUnconsolidated Mass Movements: vOften termed Debris and includes soil, bedrock fragments, trees, and human-created objects (i.e. houses, cars, fences,etc). vMost unconsolidated mass movements slower than rock movements. vMany flow like very viscous fluids (i.e. honey). vGenerally slower velocities result from lower slope angles over which they move. vSlowest movement is Creep.

14 Mass Wasting lCREEP: vDownhill movement of soil or debris at 1-10 mm/yr. vControlled by soil type; climate; steepness of slope; and density of vegetation.. vVery slow deformation of Regolith; vUpper layers move faster than lower layers; vCauses trees, telephone poles and fences to lean or move downslope; vMass of creeping soil may break poorly- supported retaining walls, structures and foundations.

15 Mass Wasting lSolifluction: vRestricted to cold regions where water in surface layers alternatively freezes and thaws; vWhen water thaws in surface layers they become saturated & waterlogged; vAs there is no deep drainage (due to frozen deeper soil layers), soil oozes downslope carrying every thing with it. lEarthflow: vFluid movements of relatively fine-grained materials such as soils, and clay.

16 Mass Wasting lDebris Flows: vFluid mass movements of rock fragments supported by a muddy matrix. vTend to move more quickly than earthflows; vOccasionally may exceed 100km/hr. lSlumps: vA slow slide of unconsolidated material that moves as a single unit. vUsually slumps slip along a basal plane that has the shape of a concave upwards surface. vMay occur with multiple consecutive slip surfaces. vA common natural feature of the Range escarpment which can be readily induced by poorly-conceived human activities.

17 Velocity Dominant Material Nature of Motion Slow  1cm/yr Moderate  1km/hr Fast  5km/hr Unconsoli-datedflowCreepSoli-fluctionEarthflowDebrisflowMudflowDebrisAvalanche Unconsoli-dated slide or fall SlumpDebrisslide

18 Mass Wasting lMudflow: vFlowing masses of material mostly finer than sand that contain large amounts of water. vDry cracked mud absorbs water, internal friction decreases and mass of mud loses its resistance to movement and flows. vFlows with velocities of several km/hr. vMost common in hilly and semiarid regions; vTend to travel down upper valley slopes and merge on valley floors; vOccur after infrequent prolonged heavy rainfall; vCan carry large boulders, trees, etc. vCause heavy losses of human lives.

19 Mass Wasting lDebris Slide: vHave higher velocities than slumps; vRock 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. vDuring the slide event, sections of the slide may behave as a chaotic flow; vAs it moves downslope, it may transform into mostly a flow traveling in a fluid manner.

20 Mass Wasting lDebris Avalanche: vFast downhill movements of soil and rock in humid mountainous regions. vVelocities reflect high water content and steep slopes; vRecorded velocities of 200-435km/hr known.. vWhere unstable slopes exist and frequent earthquakes occur, mass movements need to be predicted. vThese flows remove everything in their paths; vA close association with volcano slopes in humid regions; vMt St Helens in USA generated a debris avalanche that moved down the north flank of the volcano at 200km/hr. vGreat loss of human lives, property and infrastructure result.

21 Mass Wasting lSummary: vThe vast bulk of mass wasting is natural. vHuman activities may trigger landslides in vulnerable areas, such as when we change natural slopes. vAt Vaiont, Italy, Engineers constructed a concrete dam (265 m high) in a steep-walled valley of limestone and shale. vOn October 9, 1963, a debris slide of 240 million m 3 (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.. vMass movement danger had been flagged by: A small rock slide in 1960;A small rock slide in 1960; Ancient slide scar on the valley walls above the dam;Ancient slide scar on the valley walls above the dam; Inherent weakness of cracked and deformed outcropping limestone and shale reservoir walls.Inherent weakness of cracked and deformed outcropping limestone and shale reservoir walls. vWhile the landslide was natural, consequences could have been much less severe.

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

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