2. What? Downslope movement of material under the
directinfluence of gravity Most type of mass movements are aided by
weatheringand erosion Major landslides have natural causes Minor
landslides are result of human activities Slope stability depends
on the relationship betweenshear strength and force of gravity 3.
How are mass movements classified?Mass movements are classified by
their dominantbehaviour according to:i. The type of motion rapid or
slow?ii. The rate of motion (flowing, sliding, falling, heave)iii.
The type of material involve rock, soil debris 4. i. Rate of
movement Rapid- visible movement- sudden movement- involve rock,
soil, debris- dangerous 5. i. Rate of movement Slow moving at an
imperceptible rate about 0.01cm per year- detectable only by the
rateeffects of their movement(e.g. tilted trees,
crackedfoundation)- responsible for downslopetransport of great
volume ofweathered material 6. ii. Types of
movementFallingSlidingFlowHeave 7. Falling Rapid Rocks of anysize
fall throughthe air E.g. Rockfall 8. Sliding Involve movement of
material along theslideplane Do not suffer internal disruption as
materialmoves in One block at the same rate at every level Slow to
rapid movement E.g. landslide, slumps, rotational slip 9. Landslide
10. Landslide 11. Rotational Slip 12. Flow Material flows as a
viscous fluid Involves internal disruption of material The surface
layer contains smaller rounded particlesmove fastest while the rate
of movement decreaseswith depth as particles become larger and
moreangular E.g. Earthflow, mudflow, solifluction 13. Earthflow 14.
Solifluction 15. Heave A slow form of massmovement Involve the
rising ofparticles at rightangle to the slopefollowed by
verticalsetting E.g soil creep 16. Factors influencing slope
failure? Slope angle Weathering and climate Water content
Vegetation Slope dip Trigger mechanisms 17. Slope angle The steeper
the slope , the less stable it isi. Natural processes: Undercutting
by stream or wave action This process removes the slopes base,
increasesthe slope angle increases the gravitational forceacting
parallel to the slope Wave action, especially during storms,
oftenresults in mass movement along the shores orlarge lake 18. ii.
Human processes Grading the slope too steeply or cutting into its
side Cause stress in the rock or soil until it is no longerstrong
enough to remain at the steeper angle andmass movement. Excavation
for road cuts and hillside building sitesare major cause of slope
failure 19. Weathering and climate Occur in loose or poorly
consolidated slope material As soon as rock is exposed at
Earthssurface, weathering begin to disintegrate anddecompose and
reducing its shear strength andincreasing its susceptibility to
mass wasting Common in areas of deep weathering 20. Water content
Two ways:1. Large amount of water from melting snow or heavy
rainfall greatly increase the likelihood of slope failure.2. Water
percolating in a slope material helpsdecrease friction between
grains loss ofcohesion 21. Vegetation Absorbing the raindecreasing
water saturationof slopes material leads toloss of shear strength
Root system also helpsstabilise a slope by bindingsoil particles
together 22. Overloading Result of human activities: dumping,
filling or piling upof materials A materials load is carried by its
grain to grain contact With the friction between the grains
maintaining aslope The additional weight create by overloading
increasesthe water pressure within the material decreases itsshear
strength weakening the slope material 23. Slope dip Rock dip in the
same direction as the slope Water can percolates along the various
planes anddecrease the cohesiveness and friction. E.g. clay becomes
slippery when wet. 24. Slope dip 25. Horizontal dip with joints
that are cutparallel to the slope If the rocks are horizontal or
dip in a directionopposite that of the slope, joints may dip in
thesame direction as the slope. Water migrating through them
weathers the rocksand expands these opening until the weight of
theoverlying rock causes collapse and mass wastingoccur 26. Trigger
Mechanisms Rapid mass movements triggered by a forcethat temporary
slope equilibrium For examples: strong vibrations from earthquakes,
Excessive amounts of water from snowmelt Heavy rainstorm
