Mass movement is dependent on the development of instability on a hill slope when equilibrium has been disturbed. Safety Factor is a measured by ratio of resistance which the slope offers against movement (sheer STRENGHT) to the force which is trying to enact that movement (sheer STRESS)
Factors affecting Sheer STRESS
1) Angle /Steepness of Slope: Inclination of slope will affect the magnitude of gravitational force enacting on the regolith. a. the larger the angle of inclination, the greater the magnitude of gravitational force b. mass movement when magnitude of gravitational force is able to overcome the frictional force opposing the motion of the regolith 2) Weight of regolith: the greater the weight of regolith the greater the magnitude of gravitational force Note: Because gravitational force is stronger on steeper slopes, regoliths are pulled down faster, consequently, regoliths on steep slopes is likely to be thinner.
Factors affecting Sheer STRENGHT
1) Nature of slope –Shape and size of particles:
a. A slope made up of large angular rocks is more stable than a slope made up of small circular rocks This is because larger rocks have a greater ANGLE OF REPOSE 2) Nature of slope-type of particles: clay and slit particle, sand v.s. snow (unstable) b. Slopes made up of sand and gravels has more friction and hence higher sheer strength c. Clay and slit are cohesive
i. Mayback fire since retention of water increases weight d. Dependent on moisture – too much or too little decrease sheer strength e. Greatest internal strength due to effect of solidification and crystalisation which creates extremely strong chemical and physical bonds 3) Nature of slope – Ground cover
f. Roots of vegetation hold loose material together reduces transportation.
Encourages mass movement by increasing Sheer Stress and decreasing Sheer Strength Period of High Rainfall slope failure
1) Saturation of soil (reduction of strength) * saturation is key as a bit of water can increase sheer strength eg. sand Reduction of Cohesion in clay/slit slopes + lubricates contact b/w soil particles * Pore pressure as water exerts force on particles as they fill the pore spaces 2) Increased weight of sediment ( Increasing sheer Stress)
Role of Triggering Mechanisms (Ground Stability) Earthquakes, etc Vibration applied to slope materials can reduce the internal strength of the regolith by shaking material loose from a supported position and reduce cohesion or friction b/w them Undercutting by streams increases the sheer stress as in elevates the angle of inclination Human activities creation of mountain roads undermines slopes increase sheer stress 1) Landslides common in Himalaya esp during monsoon
a. Slopes becomes saturated many faults that can act as shear planes i. Increase number of settlement along foot of slopes b. Undercutting cause a landslide near Kulu in Himachal Pradesh, India (Killed 65) Buildings on slopes: Holbeck Hall Hotel in Scarborough rotational slump due to particularly wet spring (regolith: boulder clay) + undercutting of cliff by sea + weight of hotel Deforestation: Rio de Janerio, lack of cover Landslides, avalanches, mud flows and debris flows (rainfall) Classification of Types of Mass Movement
Carson and Kirby’s Classification of Mass Movement
1) Slowest but widespread Highly Effective type of mass movement, a. Due to expansion and contraction of soil from heating and cooling / wetting and drying i. Heating and cooling
1. Expands by day, contracts by night
2. In case of Freeze thaw action:
a. Presence of ice crystal increases volume by 9% ii. Wet-dry peroids: significant for clay minerals which expands when wet 3. heavy rainfall increases moisture increase volume and...
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