1. Lecture 25
Mass wasting and landslide mechanics

2. Mass Wasting
● Mass wasting includes all processes by which masses of rock and soil move downslope.
● Mass movement occurs when the force of gravity exceeds the strength of the material and it moves downslope.

3. ● Three primary factors ● nature of slope materials (angle of repose)
Mass Wasting
● Three primary factors
● nature of slope materials
(angle of repose)
● amount of water
● steepness and stability

4. Factors that Influence Mass Wasting

5. ● Nature of slope materials ● unconsolidated materials ● sand and silt
Mass Wasting
● Nature of slope materials
● unconsolidated materials
● sand and silt
● rock fragments, sand,
silt, and clay

6. The Behavior of Dry and Wet Sand
Mass Wasting:
The Behavior of Dry and Wet Sand

9. The
Behavior
of Wet
Sand

10. Once you Get Mass Movements…
Material moves downslope due to the pull of gravity
Can happen almost anywhere
Commonly associated with other events (heavy rainfall or earthquakes, for example) and are therefore under-reported
Movements can either be catastrophic (landslide) or slow and steady (creep)
The rate of the mass movement can be increased by water

12. Mass wasting
We are interested in how materials on slopes begin motion, i.e., why and when does failure occur?
Simple case: the sliding block α
Weight (W = mg)
W sin α
W cos α

13. Internal Friction
=> Angle of repose

14. Cohesion
Sand
Or dirt with clay!

19. Factors in Slope Stability
Stability of a slope expresses the relationship between resisting forces and driving forces
Driving forces (DF) – forces which move earth materials downslope
Downslope component of weight of material including vegetation, fill material, or buildings
Resisting forces (RF) – forces which oppose movement
Strength of material
The ratio of resisting forces to driving forces is the safety factor (SF):
SF = RF/DF
SF > 1 Slope is stable (safe)
SF < 1 Slope is unstable (unsafe)
Movement occurs when the shear stress exceeds the shear strength of the material.

20. Causes of landslides Internal Causes External Causes
Steeper slope are prone to slippage. Most of the materials are stable up to a certain angle- “Critical angle” or “angle of repose” – it varies from 300 for unconsolidated sediments to 900 for massive rocks and for partially jointed rocks.
Water (weight & interaction with clay minerals). Main factor responsible for slippage.
Decrease rock cohesion + pore spaces filled with water  higher weight of the material
Water is the most powerful solvent, which not only causes decomposition of minerals but also leaches out the soluble matter of the rock and reduces the strength.
External Causes
Triggering: Earthquake, Heavy rains, Volcanic eruptions, Floods, Droughts, Oversteepening (cutting at foot of slope, piling on head of slope), Deforesting / Devegetating.

21. How to cause a landslide: 1/steepen slope: add/subtract mass in the wrong place
Common when
building near
slopes
Common when
building roads

22. How to cause a landslide: 2/Add water

23. Landslides, Sichuan earthquake: May 2008
How to cause a landslide: 2/External trigger
after (2008) before (2006)
Landslides, Sichuan earthquake: May 2008
Images: NASA Earth Observatory

24. Lake formed by landslide dam, Sichuan, May 2008
At least 20 lakes were formed behind seismically-triggered landslide dams in the area affected by the Sichuan earthquake. The inhabitants of towns downstream of the dams (e.g. Beichuan) were evacuated because of the risk of debris flows and floods if the dams failed catastrophically.
Images: NASA Earth Observatory

25. Creating slope instability
Increase in shear stress / Reduction of material strength
Topography
Steeper slope = increased driving forces.
Removal of support (erosion, previous slides, road cuts and quarries)/ Increase load (weight of rain/snow, fills, vegetation)
Removal of vegetation
Decreases slope stability by increasing erosion, remove root systems (decrease cohesion)
Lithology
rock which are rich in clay, mica, calcite, gypsum etc are prone to weathering.
Water
Increases the pore pressure (decrease the material strength), can erode the base of a slope
Slumps or translational slides can develop months or years after slope is saturated
Climate: influences water and vegetation
Time: Physical and chemical weathering can weaken slope materials decreasing resisting forces
Geological structures: Regional tilting (geological movements), joints & fault create planes of weakness
Human influence: undercutting along the hill slopes, building, deforesting...
Transitory stresses: earthquakes, vibrations of trucks, machinery...
Worldwide, landslide activity is increasing, due to:
Increased urbanization and development in landslide-prone areas.
Continued deforestation
Increased regional precipitation caused by changing climate patterns

26. ● Nature of slope materials ● consolidated materials ● rock
Mass Wasting
● Nature of slope materials
● consolidated materials
● rock
● compacted (cohesive)
sediments and soils

27. ● Steepness and stability ● angle of slope ● accumulation of rubble
Mass Wasting
● Steepness and stability
● angle of slope
● accumulation of rubble
● breakage into large blocks

28. The Accumulation of Rubble on a Slope
Mass Wasting:
The Accumulation of Rubble on a Slope

29. Mass Wasting
● Water content
● lubrication
● liquefaction

30. ● Triggers of mass movements ● earthquake vibrations
Mass Wasting
● Triggers of mass movements
● earthquake vibrations
● rainfall and water infiltration
● overloading

31. Soil liquefaction refers to the process by which water-saturated, unconsolidated sediments are transformed into a substance that acts like a liquid

32. 6. Mass Wasting

33. Mass
Wasting:
Triggered by
Earthquake