Natural Hazards and Disasters Chapter 8 Landslides and Other Downslope Movements

1. Natural Hazards and Disasters Chapter 8 Landslides and Other Downslope Movements

2. Lindsborg, Kansas

3. Falling Mountains La Conchita sits below 100 m terrace on California coast near Santa Barbara After extraordinarily heavy rainfall in January 1995, residents noticed cracking of house walls and movement in hillside above homes Slide moved downhill slowly at first, then faster until burying several houses (while residents ran out of way) Few people moved away

4. Falling Mountains In 2005, without warning, part of 1995 landslide remobilized as debris flow, rapidly burying 15 houses and killing 16 people Unstable slopes of loose sand and clay are topped by avocado orchard (irrigation permeates and further weakens slope) and cut by road across face of slope Many residents still choose to stay, although future landslides are inevitable

5. Falling Mountains

6. Slope Processes • Downslope ground movement is natural part of landscape evolution • Gravity pulls rock on slope vertically downward • Rock may move slowly in gradual process of creep • Can slide or roll down hill • Catastrophic when large volume of material moves downslope quickly

7. Slope Processes • Ability of slope to resist sliding depends on • Total driving force (gravity) pulling it down vs. • Resisting force holding it up – strength of material and friction holding it in place • Slopes maintain dynamic equilibrium to keep forces in balance • Near-equilibrium values reflect local environment: • Slope material • Climate • Water content of soil • Ongoing landslides – movement stops when forces are again in balance

8. Slope and Load • Angle of repose: steepest angle at which any loose material is stable • Depends on angularity and size of grains and moisture content • Relationship between slope angle (steepness of slope) and load (weight of material on slope) determines slope failure • Steeper slope  greater downslope force  greater likelihood of slope failure

9. Slope and Load

10. Frictional Resistance • Friction: resistance to sliding • Force pressing down on slope • ‘Roughness’ of slippage surface • Area of contact does not affect friction coefficient • Small mass will slide on same slope as large mass, given same material • Mass will slide (or slope will fail) when force exceeds frictional resistance • If friction is high enough, mass will stay in place • Anything that reduces friction increases likelihood that rock will move

11. By the Numbers Slope Failure Mass will slide if: Frictional resistance + Cohesion < Driving Force f + C < F (N – p) x tan a + C < F resisting force < driving force where • N = force perpendicular to the slope • (N – p) x tan a is (force against slope minus pore pressure of water) times tangent of slope angle • p = pore pressure • a = slope angle • C = cohesion (soil cohesion is soil strength + root strength) Addition of water to slope is particularly important

12. Cohesion and Water • Cohesion: important force holding soil grains together • From surface tension of water between loose grains or cement between grains • Loose soils have 10-45% pore space • Small amount of water in pore space increases cohesion • Too much water fills pore spaces under pressure and pushes grains apart, reducing cohesion • Water pressure at base of slope is under load of water above it

13. Cohesion and Water

14. Slope Material • Material and topography of slope play role in slides • Inherently loose materials include • Loose aggregates such as soil • Loose sedimentary deposits not yet cemented into rock • Soft sedimentary materials such as clay or shale

15. Internal Surfaces • Rocks commonly contain planar internal surfaces of weakness, at random angles • Layers in sedimentary rock • Fractures in any kind of rock • Contacts between rocks of different strength • Faults • Slip surfaces of old landslides • Any such surface that is oriented nearly parallel to slope is likely to become a slip surface • Rocks is especially likely to slide if zones of weakness are angled downslope • Daylighted layers: no resisting mass to hold them back • Layers that dip at gentler angle than slope of hill

16. Clays and Clay Behavior Clays that absorb water and expand can weaken rock, even lift it Feldspars, most abundant minerals in rocks, weather to form clay minerals, with structures that can lead to landslides Kaolinite: weak positive and negative charges, soft and weak structure, soaks up water Smectite: forms from volcanic ash, with open structure between layers that fills with water  swelling soils

17. Clays and Clay Behavior • Quick clays: water-saturated muds in marine bays, estuaries, old saline lakebeds that are especially prone to collapse and flow when disturbed • Mixture of fine silt, clay grains and water in tiny pore spaces • Flakes deposited in random orientation give mass total pore space of 50% or more • If loose arrangement is disturbed, by earthquake or by heavy load on top, quick clay liquefies and flows almost like water for few minutes until water escapes, then mass becomes stable and will not flow again • Common along northern coasts of Canada, Alaska, Europe

18. Clays and Clay Behavior

19. Causes of Landslides • Landslides can be caused by • Moisture conditions • Instability of slope material • Jarring by earthquakes • Changes in slope imposed by external factors, such as • Undercutting of slope by stream or road-building • Loading of upper slope by construction, addition of water or removal of vegetation

20. Oversteepening • Steeper slopes are less stable  oversteepening slope increases likelihood of slope failure • Slope angle is increased when • Fill is added above • Construction of homes with magnificent views • Slopes are undercut below • Erosion at base of slope, by waves at coast • Excavation of road at base of slope

21. Overloading • Balance between forces acting on slope can be upset by • Adding material or load at top • Removing material from toe

22. Adding Water • Additional water reduces strength of slope  more likely to slide • Heavy or prolonged rainfall saturates soil, increases pore water pressure and causes slides • Human actions add water to slopes • Lawn-watering, crop irrigation • Leaking water or sewer pipes, cracked swimming pools • Filling reservoir behind dam

23. Overlapping Causes • Worst-case scenario for Mount Rainier, with steep, weak sides: • Next giant megathrust earthquake in winter, with heavy snowpack soaking soil • Strong shaking for three minutes or more  collapse of large part of flank of mountain • If collapse is toward communities to northwest, tragic consequences

24. Types of Downslope Movement • Classified on basis of type of material: • Categories of blocks of solid bedrock • Debris of various sizes coarser than 2 millimeters • Earth or soil finer than millimeters • Rates of downslope movements are highly variable, depending on • Slope steepness • Grain size • Water content • Thickness of moving mass • Clay mineral type • Amount

25. Types of Downslope Movement • Styles of downslope movements include: • Falls from cliffs • Topples • Slides • Lateral spreads • Flows • Described by moving material and style of movement • Rockfalls • Rock slides • Debris slides • Debris flows • Earth flows • Mudflows • Snow avalanches

26. Types of Downslope Movement

27. Rockfalls • Old Man of the Mountain was granite outcrop symbol of New Hampshire • Broke off onto lower slopes in spring 2003 • Rockfalls develop in steep, mountainous areas of cliffs with nearly vertical fractures or weaknesses • May be pried loose by freezing water or triggered by ground shaking • Large boulders may bounce or roll far out from cliff, where smaller fragments collect in talus slope • Base of steep slope capped by vertical cliffs that have shed big boulders in past is dangerous

28. Debris Avalanches • Debris avalanche: Rockfall in which material breaks into numerous small fragments that flow at high velocity as coherent stream • Elm, Switzerland • Material breaks up, entrains air and water, flows downslope at speeds of 100 to 300 km/hr • Can run out up to 5-20 times their vertical fall distance • 1 billion cubic meter mass falling 100 meters would run out to distance of a kilometer

29. By the Numbers • Potential Energy of a Rock on a Slope • Depends on mass of rock and height on slope • Potential Energy = m x g x h • m = mass (kilograms) • g = gravitational acceleration (meters per second per second) • h = height (meters) • When rock falls, potential energy becomes kinetic energy (movement) • Highest velocity and kinetic energy at bottom of fall • Kinetic energy = ½ m x v2 • m = mass (kilograms) • v = velocity (meters per second)

30. Debris Avalanches Magnitude 7.7 earthquake on subduction zone off Peru Triggered landslide 130 km away on Mount Nevados Huascaran (highest mountain in Peru) 50-100 million cubic meters of granite, glacial debris and ice fell 400-900 meters off vertical cliff Debris flow raced down valley, 14 km to Yungay with average speed of 270 km/hr

31. Debris Avalanches • Mass of rock that falls but does not disintegrate will travel shorter distance • Mechanism for travel of debris flows debated • Cushion of compressed air beneath flow? • Major debris avalanches (like Elm, Switzerland) scoured ground beneath them, so could not have moved atop cushion of air • May flow as fluid composed of rock fragments suspended in air • Acoustic fluidization, if air can not escape spaces between fragments

32. Rotational Slides and Slumps • Homogeneous, cohesive, soft materials often slide on curving slip surface concave to sky • Curvature rotates slide mass as it slips • Upper end of slide block tilts back as it moves • Lower part of slide moves outward from slope • Engineers find center of rotation by projecting perpendicular to any exposed part of surface • Calculate and compare driving mass to resisting mass and friction – if driving mass is larger, then mass will slide

33. Rotational Slides and Slumps

34. Translational Slides • Move on preexisting weak surfaces more or less parallel to slope • Inherently weak layers such as shale • Old fault or slide surfaces • Fractures • Often move faster and farther than rotational slides • May move as coherent mass or break up into debris flows

35. Lateral-Spreading Slides • If loose, water-rich sands or quick clays are present at shallow depth, liquefaction or collapse can send mass moving downslope • Some parts sink, some blocks left standing higher • Liquefaction can occur on flat surface • Does not cause slide but can still collapse buildings

36. Soil Creep • Slow, downslope movement of soil and weak rock • Involves near-surface movement by alternate expansion and shrinkage of soil • Expansion pushes out perpendicular to slope • Shrinkage collapses particles straight down • Net change is slow movement downhill

37. Soil Creep

38. Snow Avalanches • Boundary between layer of dry snow and layer of tightly packed or frozen snow can be zone of weakness • Conditions for avalanche formation depend on • Slope steepness • Weather • Temperature • Slope-facing direction • Wind speed and direction • Vegetation • Conditions within snowpack

39. Snow Avalanches • Trigger for avalanche could be • Weight of skier crossing slope • Vibrations of snowmobile • Movement of glacier • Changes in temperature • Earthquake

40. Hazards Related to Landslides: Earthquakes • Eyewitness accounts of great clouds of dust rising from hillsides during and after earthquake • Sudden shaking of earthquakes above magnitude 4 can trigger failure in unstable slopes • Most earthquake-triggered landslides are rockfalls • Less than 1% are debris avalanches or rapid soil flows, but these are most deadly • Shaking can cause liquefaction of clays, even in relatively flat ground • Buildings tilt or fall over as liquefied ground settles Landslides are closely related to other hazards Can be triggered by storms and flooding or by earthquakes When landslide blocks waterway, can cause flooding

41. Hazards Related to Landslides: Failure of Landslide Dams • Any moderately fast-moving landslide can block a river or stream to create a dam and temporary lake before eventually failing • Time before failure and size of flood depends on • Size, height and geometry of dam • Material making up dam • Rate of stream flow, how fast lake rises • Use of engineering controls (artificial breaches, spillways or tunnels) • Dams from mudflows, debris flows and earth flows are non-cohesive and erode quickly

42. Failure of Landslide Dams • Most landslide dams fail when water overflows and erodes spillway that drains lake • If dam-failure flood incorporates significant sediment, can turn into debris flow – much more dangerous • Useful dams can be constructed on top of landslide dams • Rockfalls or rock slides are most stable • 1928 St. Francis high-arch concrete dam failed – built on toe of old landslide

43. Mitigation of Damages from Landslides Damages can be extremely costly Not covered by most insurance policies In U.S., landslides cost more than $2 billion and cause 25-50 deaths per year Globally, cost more than $20 billion, cause about 7500 deaths per year Major landslide disasters increase with growth in population in dangerous areas

44. Mitigation of Damages from Landslides

45. Mitigation of Damages from Landslides

46. Landslide Hazard Maps • Best strategy is to avoid building in places that are prone to landslides • Geographic Information System can be used to build debris-flow and landslide-hazard maps • Prescribe restrictions in land use • Area divided into polygons with consistent internal attributes such as characteristics of slope, composition of material, presence of previous failures

47. Record of Past Landslides Existence of past landslides in area indicates circumstances for more in future Hummocky ground surface may be indication Cracks or broad waves in pavement Building roads or structures on slide aids further movement of slope

48. Engineering Solutions Engineers can sometimes restore balance among forces to keep slope stable Add load to lower part of slope that is overloaded at top Rock cliffs or slopes sprayed with shotcrete, draped with heavy wire mesh or anchored with rockbolts Plant vegetation to take moisture out of soil through evapotranspiration Artificially drain slopes by increasing permeability with perforated pipes or geotextile fabric

49. Shotcrete Installed, Colorado

50. Wire Mesh, Colorado