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Mass Movements…read more

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Slope failure is caused by:
a reduction in the internal resistance or shear strength of the slope, and/or
an increase in shear stress that is the forces attempting to pull a mass down slope
Causes of Mass movements are more common during periods of heavy precipitation which leads to increased levels of
saturation in soils, or after cold conditions (solifluction) which leads to increased freeze-thaw activity and
more rock falls
mass Shear stress= the force acting on a body that causes movement of the body down slope
Factors increasing shear stress:
Climate- wet, lots of weathering, extremes of temperature
Slope angle- steepness of slope
and factors
Drainage- wet areas are lubricated
Rock type- geology e.g. clay, structure, beds, porosity and tilt of rocks
Vegetation- type and percentage cover
Animals- burrowing animals, walking on slopes
Removal of underlying support- undercutting by rivers or waves
to shear stress
Lateral pressure- water in cracks; swelling, pressure release
Short term stresses- earthquakes, movement of trees in wind
and reduced Shear strength =The internal resistance of a body to movement
Factors contributing to reduced shear strength:
shear strength
Weathering effects e.g. disintegration of rocks; hydration of clay minerals; solution of minerals in rock or soil
Changes in soil or ground water pressure- saturation; softening of material
Changes in structure- creation of fissures in clay; remoulding of sands and clays
Biological effects- Burrowing of animals, growth and decay of roots
Adding weight to slopes e.g. building, walking…read more

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Creep/Heaves - slow and low water content results in terracettes
therefore pose little threat
About 75% of the soil creep movement is induced by moisture changes
Avalanche-Rapid and can be wet or dry
Flow- rapid highly fluid, saturated soil so are therefore more mobile
Slide-sliding material retains shape and cohesion, clearly defined slide
Types of mass e.g. Southern Leyte landslide in the Philippines in 2006 which killed 1126
movement Slips - on a slide plane, medium water content
Slump - usually rotates along a slip plane, medium water content
e.g. Holbeck Hall Hotel, Scarborough
Falls - occur on steep slopes, especially bare rock faces where joints are
Mass movements become hazardous when they have a damaging effect
on economy and society. If damage to property and/or loss of life is
particularly high, a hazard becomes a disaster…read more

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Mass movements occur on slopes where the driving force is gravity
Two sets of forces operate on slopes: downslope forces and upslope forces
Gravity is main downslope force and increases with slope angle
Upslope forces resist mass movement- includes shear strength of slope materials, the frictional resistance between
slope materials, binding effects of vegetation
Usually related to an external trigger that may be physical or human:
Steepening of slopes by erosion or human activity
Increased loading on slopes due to heavy and prolonged rainfall, building, tipping of waste materials
Physical and Undercutting the foot of a slope
Heavy rainfall that lubricates slope materials and reduces frictional resistance
human factors Heavy rainfall that increases pore water pressure and reduces the coherence of slope materials
triggering Deforestation that reduces the binding effect or tree roots and increases the amount of water absorbed by
slope materials
mass Earthquakes or the vibrations caused by heavy lorry traffic or explosions
Deforestation either by overgrazing or by deliberate logging for timber/fuelwood or new farmland, disrupts the
movement balance of forces on slopes
Torrential rainfall can then trigger mass movements
The mass movement disasters in Honduras (1998), Northern Venezuela (1999) and Guinsaugon (Philippines
2006) were all related to deforestation of steep slopes and extreme rainfall events
Mass movements caused by torrential rainfall from Hurricane Mitch, killed thousands of people in Nicaragua in
Some mass movement hazards are caused entirely by natural processes
In 1985, the eruption of Nevado del Ruiz in Columbia produced mudflows that killed 23,000 people
Similar mudflows occured around Mount Pinatubo in the Philippines…read more

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Environmental impacts:
Relief - reduces slope angles, fills in valleys/hollows, adds steps or bulges to
Drainage - may dam or divert rivers, wetter at foot of slope
Vegetation- trees lean or fall
Soil- collects at base of slope (catena effect)
Impacts of Rock Strata- may bend the ends of beds (cambering)
Social impacts:
mass Buildings/walls - collapse, lean or have soil collect up side of slow
movements Disasters e.g. Aberfan 1966, 147 killed
Economic impacts:
Transport- road and rail distorted or broken, leading to disruption and cost of
Poles lean or fall leading to disruption in services/supply
Loss of farmland, damage to structures e.g. bridges, buildings, pipes
Quarrying and mining disasters…read more

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