1. Lecture Outlines Natural Disasters, 6th edition
Patrick L. Abbott

2. Mass Movements Natural Disasters, 6th edition, Chapter 10
Christiane Stidham, Stonybrook University

3. Mass Movements El Cajon, California, 2000
Isolated thunderstorm rained and hailed, eroded soil around 200 ton boulder in hillside so that it rolled free, eight hours later
Crashed into house with noise ‘louder than an earthquake’ while owners were on ski trip, destroying 60% of house
 Gravity induced disasters
Catastrophic mass movements usually triggered by some other event, such as earthquake, volcanic eruption, major rainstorm

4. The Role of Gravity
Power behind agents of erosion: rainfall, water flow, ice gliding, wind blowing, waves breaking
Geologic time: all slopes are inherently unstable
Failures may be catastrophic and sudden or slow and gradual
Can measure pull of gravity, using trigonometry to measure downhill force
Figure 10.3

5. The Role of Gravity Creep
Slowest, most widespread form of slope failure
Almost imperceptible downhill movement of soil and uppermost bedrock layers
Swelling and shrinking of soil in response to:
Freezing and expanding of water in pores
Absorption of water, expansion of clay minerals
Heating by Sun and increase in volume
Figure 10.4

6. The Role of Gravity Creep
Soil expands perpendicular to ground surface, shrinks straight downward in response to gravity
Figure 10.5

7. External Causes of Slope Failures
Typical landslide: mass whose center of gravity has moved downward and outward, with tear-away zone upslope and pile-up zone downslope
External processes that increase likelihood of slope failure:
Adding mass high on slope (sediment deposition)
Steepening slope (fault movements)
Removing support from low on slope (stream or wave erosion)
Water in its External Roles
Rainfall is added mass, rain runoff causes erosion that sets masses moving on slopes and undercuts bases of slopes

8. Internal Causes of Slope Failures
Inherently Weak Materials
Clays (most abundant of sedimentary minerals) form during chemical weathering of rocks
Clay crystals are very small, shaped like books
Chemical composition of clays can change  altering strength, size and water content  altering strength of rock
Figure 10.7

9. Internal Causes of Slope Failures
Quick Clays: Most mobile of all deposits – fine rock flour scoured by glaciers, deposited in seas and later exposed above water
Weak solid – loosely packed, ‘house of cards’ structure held together by salt
When exposed, fresh water dissolves salt and ‘house of cards’ structure can collapse so that ground turns to liquid and flows away
Figure 10.8

10. Internal Causes of Slope Failures
Canadian Quick-Clay Slope Failures
Common in eastern Canada
Recognize problem areas, take preventative actions (move towns)
Ontario, Canada: 3.5 million m3 mass liquefied, flowed into river
Figure 10.9

11. Internal Causes of Slope Failures
Water in Its Internal Roles
Weakens earth materials by
Weight: water is heavier than air that usually fills pore spaces of sedimentary rocks in slopes
Absorption and adsorption: water is absorbed (internally) and adsorbed (externally) by clay minerals, decreasing their strength, because positive side of water molecule attaches easily to negatively charged clay surfaces
Figure 10.10

12. Internal Causes of Slope Failures
Water in Its Internal Roles
Weakens earth materials by
Dissolve Cement: water flowing through rocks can dissolve minerals holding rock together (dissolved gypsum and clay cement of St. Francis dam in California, 1928)
Piping: water flowing through rocks can physically erode away (remove) loose material
Figure 10.11

13. Internal Causes of Slope Failures
Water in Its Different Roles
Weakens earth materials by
Pore-water Pressure: pressure on water in pore spaces of rocks increases with increasing weight of sediment piled on top of rocks, and if pore space water becomes over-pressurized, gives ‘lift’ to overlying sediments making them unstable
Quicksand where sand grains are supersaturated with pressurized water
Pore-water pressure equals weight of sands  no shear strength
Water-pressurized sand on slope flows downhill
Water-pressurized sand in depression is quicksand, high-viscosity liquid
Figure 10.12

14. Internal Causes of Slope Failures
Water in Its Internal Roles
Weakens earth materials by
Water Table: gravity pulls water down to saturate open spaces in subsurface rocks as groundwater; top of groundwater is water table

15. Internal Causes of Slope Failures
Vaiont, Italy, 1963
Fractured rock layers dip toward valley on both sides
Rock layers have old slide surfaces, clay layers, limestone layers with caverns
Water filling reservoir saturated rocks in toes of slopes and elevated pore-water pressures
Figure 10.13

16. Internal Causes of Slope Failures
Vaiont, Italy, 1963
Heavy rains triggered landslide – 1.8 km by 1.6 km mass (240 million m3) slid at up to 30 m/sec into reservoir
Figure 10.14
Block filled part of reservoir and displaced water to crash over dam and into towns at both ends of reservoir

17. Internal Causes of Slope Failures
Decreases in Cohesion
Rocks that are buried compress into smaller volumes
Rocks that are later uplifted to the surface expand in volume, fracture and increase porosity  reduces strength of rock, increases openings for water to further weaken rock

18. Internal Causes of Slope Failures
Adverse Geologic Structures
Ancient slide surfaces: sliding creates a smooth, slick layer of ground-up materials that can easily slide over and over again, especially when wet
Daylighted Bedding (Orientation of layering in hillside)
Layers at flatter angle than hillside  daylighted bedding allows slippage
Layers at steeper angle than hillside  difficult to slip
Structures within Rocks
Not cemented together
Clay layers
Soft rock layer on strong layer
Split apart by joints
Ancient fault  slide surface

19. Internal Causes of Slope Failures
Triggers of Mass Movements
Most failures have complex causes
Slopes lose strength over time through numerous events and near-failures
Underlying causes push slope to brink of failure
Finally immediate cause triggers collapse
Triggers could be heavy rains, earthquakes, thawing of frozen ground, construction projects

20. Classification of Mass Movements
Speed of movement (extremely slow to extremely rapid) and water content (wet or dry)
Figure 10.16

21. Classification of Mass Movements
Downward – falling or subsiding
Downward and outward – sliding and flowing
Figure 10.17

22. Falls
Elevated rock mass separates along joint, bedding or weakness and falls downward through air in free fall until hitting the ground, bouncing and rolling
Yosemite National Park, California, 1996
162,000 ton granite mass slid and launched into air, fell 500 m before hitting valley floor, pulverized into cloud of dust
Blast knocked down 1,000 trees
Magnitude 3+ earthquake
50 acres covered with inch-thick layer of dust
Vertical column of dust 1 km high
One person killed by tree
Figure 10.20

23. Slides Movement of block above failure surface Rotational slides:
Move downward and outward above curved slip surface, with movement rotational about an axis parallel to slope
Head moves downward and rotates backward
Toe moves upward on top of landscape
Move short distances
Figure 10.22

24. Slides Ensenada, Baja California, 1976
Slump preceded by arcuate cracks in hillside
Cracks widened, area slid slowly, residents evacuated
Toe of slide lifted sea floor above sea level
Figure 10.23

25. Slides Translational Slides
Move on planar slip surface such as fault, joint, clay-rich layer
Move as long as on downward-inclined surface, and driving mass exists
Different behaviors:
Remain coherent as block
Deform and disintegrate to form debris slide
Underlying material fails so overlying material slides

26. Slides Point Fermin, California, 1929
Sandstone block on clay layer slid seaward, with no resisting mass
Figure 10.26
Figure 10.25