Lecture Outlines Natural Disasters, 6 th edition

1. Lecture OutlinesNatural 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 layeringin 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