1. MASS MOVEMENT / MASS WASTING
Lesson 9

2. What?
• Downslope movement of material under the direct
influence of gravity
• Most type of mass movements are aided by weathering
and erosion
• Major landslides have natural causes
• Minor landslides are result of human activities
• Slope stability depends on the relationship between
shear strength and force of gravity

3. How are mass movements classified?
Mass movements are classified by their dominant
behaviour 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

6. i. Rate of movement
• Slow
– moving at an imperceptible
rate about 0.01cm per year
- detectable only by the rate
effects of their movement
(e.g. tilted trees, cracked
foundation)
- responsible for downslope
transport of great volume of
weathered material

7. ii. Types of movement
•Falling
•Sliding
•Flow
•Heave

8. Falling
• Rapid
• Rocks of any
size fall through
the air
• E.g. Rockfall

11. Sliding
• Involve movement of material along the
slideplane
• Do not suffer internal disruption as material
moves in
• One block at the same rate at every level
• Slow to rapid movement
• E.g. landslide, slumps, rotational slip

12. Landslide

13. Landslide

14. Rotational Slip

18. Flow
• Material flows as a viscous fluid
• Involves internal disruption of material
• The surface layer contains smaller rounded particles
move fastest while the rate of movement decreases
with depth as particles become larger and more
angular
• E.g. Earthflow, mudflow, solifluction

21. Earthflow

25. Solifluction
•

27. Heave
• A slow form of mass
movement
• Involve the rising of
particles at right
angle to the slope
followed by vertical
setting
• E.g soil creep

32. Factors influencing slope failure?
• Slope angle
• Weathering and climate
• Water content
• Vegetation
• Slope dip
• Trigger mechanisms

33. Slope angle
• The steeper the slope , the less stable it is
i. Natural processes:
• Undercutting by stream or wave action
• This process removes the slope’s base, increases
the slope angle  increases the gravitational force
acting parallel to the slope
• Wave action, especially during storms, often
results in mass movement along the shores or
large lake

36. ii. Human processes
• Grading the slope too steeply or cutting into its side
• Cause stress in the rock or soil until it is no longer
strong enough to remain at the steeper angle and
mass movement.
• Excavation for road cuts and hillside building sites
are major cause of slope failure

38. Weathering and climate
• Occur in loose or poorly consolidated slope material
• As soon as rock is exposed at Earth’s
surface, weathering begin to disintegrate and
decompose and reducing its shear strength and
increasing its susceptibility to mass wasting
• Common in areas of deep weathering

39. 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 helps
decrease friction between grains  loss of
cohesion

40. Vegetation
• Absorbing the rain
decreasing water saturation
of slope’s material leads to
loss of shear strength
• Root system also helps
stabilise a slope by binding
soil particles together

41. Overloading
• Result of human activities: dumping, filling or piling up
of materials
• A material’s load is carried by its grain to grain contact
• With the friction between the grains maintaining a
slope
• The additional weight create by overloading increases
the water pressure within the material  decreases its
shear strength  weakening the slope material

42. Slope dip
• Rock dip in the same direction as the slope
– Water can percolates along the various planes and
decrease the cohesiveness and friction.
– E.g. clay becomes slippery when wet.

43. Slope dip

44. Horizontal dip with joints that are cut
parallel to the slope
• If the rocks are horizontal or dip in a direction
opposite that of the slope, joints may dip in the
same direction as the slope.
• Water migrating through them weathers the rocks
and expands these opening until the weight of the
overlying rock causes collapse and mass wasting
occur

45. Trigger Mechanisms
• Rapid mass movements – triggered by a force
that temporary slope equilibrium
• For examples:
– strong vibrations from earthquakes,
– Excessive amounts of water from snowmelt
– Heavy rainstorm