CHAPTER 8: Mass Movements

Mass movements: Definition? Cause?

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
                                       rapid melting

High risk = intense cloudburst in ______________________  regions
    tropical soils                       very rich in clays                                      high
                                                very weathered rocks                                slide
                                                tropical & humid weather                         potential

Effect of vegetation
     Stabilizes (roots)
     Reduces soil moisture =  transpiration

Earthquakes         trigger landslides in hilly terrains
                            add stress to the one already existing because of gravity
                            reduce friction by jarring apart unconsolidated materials

Quick clays
Rockflour = clay size sediments formed by_________________grinding (<0.02 mm)

Characteristics of this structure?

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
                                                                  swimming pool
                                                                 artificial reservoirs

Types of mass movement
                                                                                consolidated (solid)
                                     type of material moved
classification                                                           unconsolidated
                                     rate of movement = wetter moves faster

                  Rock falls more common              coastal cliffs
          Talus: coarse rubble that accumulates at the foot of a slope.

Preventive Measures
                    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
                                                                    remove weight

3) Retention                            * 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


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.)