or Mass wasting
Movements occur because downward pull of gravity overcomes friction that resists it.
Shearing Stress: downward component of gravity parallel to slope
that causes mass movement (pulls down).
Shear strength: Is the friction counteracting shearing stress in a solid.
Shearing Stress > Shear Strength => Sliding
Effect of the slope:
Angle of repose: Maximum angle at which the material is stable
slopes (higher angles) (very low angles)
that angular particles rounded particles
support larger particles small particles
solid rock layered sed. rocks
Slope steepness proportional to shearing stress
steepening because of * natural ____________ erosion (streams-coast)
* mountain ___________________ (tilt original bedding)
Effect of fluids: Erosion
Reduce friction between bedding layers
WATER Increase pore water pressure in saturated rocks
Saturation (pores completely filled) reduces friction (cohesion & strength)
In contrast : slight wetting increases adhesion
Slide Increases shearing stress by its mere (added) weight
risk Expansion and contraction from freezing and thawing makes soil loose
enhanced Absorption by clays (MONTMORILLONITE x 20)
Triggering mechanism = heavy rainfall
High risk = intense cloudburst in ______________________ regions
tropical soils very rich in clays high
very weathered rocks slide
tropical & humid weather potential
Effect of vegetation
Reduces soil moisture = transpiration
landslides in hilly terrains
add stress to the one already existing because of gravity
reduce friction by jarring apart unconsolidated materials
Rockflour = clay size sediments formed by_________________grinding (<0.02 mm)
Difference between Quick clays? and Sensitive clays?
Impact of Human Activities:
Increased risk 1 clear away vegetation
(mass movements) 2 artificial oversteepening of slopes
* open pit quarrying
* stepped home-building sites on hillsides
* highway roadcuts
3 removal of rock at bottom ends of dipping layers
4 slopes cut at angle > angle of repose. uncons.
5 build on unstable slopes or artificially steepened slopes => + weight=> + shear stress
6 add water: lawn, septic tank
Types of mass movement
type of material moved
rate of movement = wetter moves faster
small narrow zone
1) Bridge bridge above structure to be protected
need solid foundation
reduce angle of slope
2) Slope reduction & weight reduction support foot
* walls: low thick, at toe of slide
structures * ground covers
subsurface drainage (for permeable soils)
4) Fluid removal bore holes fine grained
hot air materials that drain
5) Vertical piles driven into slide
=> slide needs to be solid & thin, low angle
not effective in unconsolidated
6) Rock bolts: anchored in stable rocks. For thin slides
7) For unconsolidated: "baking" = heat to harden.
Portland cement - Dry
8) Flood control channels to divert mudflows
CHAPTER 8: MASS MOVEMENTS
REVIEW QUESTIONS AND TOPICS
1. What is the name of the main forces that intervene in mass
movements? How do they work?
2. Draw a diagram to compare the effect of steeper and shallower slopes in the component of the weight force parallel to the terrain.
3. Define angle of repose of an unconsolidated material. How do size, roundness and cohesiveness of the rock affect it?
4. Outline at least three ways in which water undermines the stability of a slope. Outline two ways in which plants help to maintain the stability of a slope.
5. Compare and contrast quick clays and sensitive clays.
6. Outline at least three ways in which human activities can undermine the stability of a slope.
7. What type of mass movement affected the area of Portuguese Bend (California)? What measures were taken to mitigate its effects?
8. What materials can be involved in a flow? What is the characteristic common to all flows? Describe different types of flows.
9. What do slides and slumps have in common? In what characteristic do they differ?
10. Mention at least four preventive measures to deal with mass movements. Specify in what particular situation it is appropriate to apply each of them (type of movement, type of material, etc.)