1. Landslides and Other Downslope Movements

2. Unstable Hills
1978: large slab landslide destroyed 24 homes in hills above Laguna Beach
Followed heavy rains of El Nino winter
No earthquakes or leaking pipes contributed to slide
1979: slope was regraded and homes were rebuilt
2005: new slide destroyed 19 homes on same slopes

3. Forces on a Slope
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

4. Forces on a Slope 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
When slope will fail depends on
Slope steepness
Material weight
Moisture content

5. Slope and Load
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
Angle of repose: steepest angle at which any loose material is stable
Depends on angularity and size of grains and moisture content

6. Figure 8-2: Driving Force and Resisting Force
These two diagrams show the forces on a mass resting on a slope. A steeper slope has a larger force parallel to the slope (red arrow) and is therefore
more likely to slide. Note that for the gentler slope, the friction force (f) is larger than the force pulling parallel to the slope (F), whereas for the
steeper slope the opposite is true. Clearly the mass on the steeper slope is more likely to slide.
Fig. 8-2, p. 186

7. Frictional Resistance and Cohesion
Frictional resistance depends on
Slope angle (a)
Load (L)
Area of contact does not affect frictional resistance
Small mass will slide on same slope as large mass, given same material
Mass will slide (or slope will fail) when force (F) exceeds frictional resistance (f)
If friction is high enough, mass will stay in place
Anything that reduces friction increases likelihood that rock will move

8. Frictional Resistance and Cohesion
Cohesion: important force holding soil grains together
From surface tension of water between loose grains or cement between grains
Can be overcome if sliding force if large enough

9. Figure 8-3: Surface Tension
Fig. 8-3, p. 186

10. Slope Failure Mass will slide if: f + C < F
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)

11. Slope Material
Material that makes up a slope, topography and moisture content all play a role in slides
Loose material above bedrock is inherently weak
Sedimentary materials are inherently weak

12. Moisture Content
Moisture content determines effect of water on strength of slope
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. Figure 8-4: Water in the Ground
A. Water pressure at depth is equal to the load or weight of the overlying water. B. Water (dark) seeps out of the soil below the sharply defined
saturation level exposed in a road cut in Glacier National Park, Montana.
Fig. 8-4, p. 187

14. 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
Daylighted beds: no resisting mass to hold them back
Layers that dip at gentler angle than slope of hill

15. Figure 8-6: Daylighted Beds
A. Rock layers dipping about parallel to a slope, with their edges coming to the surface, are said to “daylight.” B. Steeply
dipping sheets of granite daylight over Highway 3 in southern British Columbia.
Fig. 8-6, p. 188

16. Clays and Clay Behavior
Clays absorb water and expand, and so 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 flakes: form 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 undergoes liquefaction 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. Figure 8-7: Collapse of Quick Clays
A. Clay grains deposited in random orientation have especially large pore spaces—a “house of cards” arrangement. B. After collapse,
the compacted clays take up much less space, so water in the pore spaces must escape.
Fig. 8-7, p. 188

19. Figure 8-8: Flowing Ground
The Lemieux flow in a horizontal terrace of the Leda Clay of the St. Lawrence River valley near Ottawa, Ontario, settled and flowed into the
adjacent river on June 20, 1993.
Fig. 8-8, p. 189

20. Causes of Landslides Landslides can be caused by
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
Removal of vegetation
Instability of slope material
Jarring by earthquakes

21. Oversteepening and Overloading
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
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:
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, type of movement and rate of movement
Categories of blocks of solid bedrock
Debris of various sizes coarser than 2 millimeters
Earth or soil finer than 2 millimeters
Rates of downslope movements depend on
Slope steepness
Grain size
Water content
Thickness of moving mass
Clay mineral type
Amount of material

25. Figure 8-T1
p. 193

26. Rockfalls
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 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

27. Figure 8-17: Rockfall Hazard
Some rockfall problems arise where a strong layer, such as sandstone, overlies a weak layer, like shale or clay, as in this southwestern Utah
photo. The largest rocks tend to roll well out from the base of the cliff. Is this a safe place to live?
Fig. 8-17, p. 194

28. Rockfalls
Rockfall runout is distance rockfall will travel, related to height from which rock falls and its mass
Potential energy of rock on slope is converted to kinetic energy when it falls
Figure 8-20: Potential Energy to Kinetic Energy

29. 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 greatest kinetic energy at bottom of fall
Kinetic energy = ½ m x v2
v = velocity (meters per second)

30. Rockfalls
Debris avalanche: rockfall material breaks into small fragments and flows at high velocity in coherent stream
1970: Magnitude 7.7 earthquake on subduction zone off Peru
Triggered huge rockfall 130 km away on Mount Nevados Huascaran (highest mountain in Peru)
million cubic meters of granite, glacial debris and ice fell meters off vertical cliff
Debris avalanche raced down valley, 14 km to Yungay with average speed of 270 km/hr
Figure 8-18

31. Figure 8-23: Slide Buries a Town
The Mount Nevados Huascaran debris avalanche in 1970 fell from the peak at the top of the photo
and raced down the valley to bury the town of Yungay, which occupied the lower half of the photo.
Fig. 8-23, p. 196

32. Rockfalls
Mass of rock that falls but does not disintegrate will travel shorter distance as debris avalanche
Mechanism for travel of debris avalanches is 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 cannot escape spaces between fragments

33. Rotational 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
Vertical part of slip surface is headscarp
Lowest end of slide mass is toe
Calculate and compare driving mass to resisting mass and friction – if driving mass is larger, then mass will slide

34. Figure 8-25: Anatomy of a Rotational Slump
This cross section shows a rotational slump. Note that the failure surface for the driving mass slopes downward (right side of vertical line), and the failure surface for the resisting
mass (left side of vertical line) slopes back into the slope. Rotation of a slump block on an arcuate surface also rotates everything on the block. Trees growing on an originally
horizontal upper terrace (the Blackfoot landslide in Montana) now tilt back toward the headscarp as a result of rotation. Note the person standing at the left. The headscarp is on
the right. Crushed rock is scattered along the failure surface of a landslide at Newport, Oregon.
Fig. 8-25, p. 198

35. 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

36. Translational Slides 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

37. Figure 8-26: Rotational Slumps Versus Translational Slides
A rotational slide penetrates deep into the ground; the pre-slide surface and growing trees tilt backward as movement progresses.
A translational slide is shallow and parallel to the ground surface; the pre-slide surface may break up, especially at the slide toe.
Growing trees remain vertical on the slope but jumble erratic orientations at the toe.
Fig. 8-26, p. 198

38. 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

39. Figure 8-29: Creeping Ground
These trees along California’s Highway 1 west of Leggett have been bent by soil creep, the slow downslope movement
of the upper layers of soil or soft rocks. They originally grew upright but were tilted outward by creep. Continued
upward growth produced the bending. Creep of sedimentary layers near the ground surface bends their upturned ends
downslope (to the right, in this photo).
Fig. 8-29, p. 200

40. 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

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

42. Snow Avalanches Unstable layers can result from
New snow
Temperature changes
Buried hoar frost
Increased risk should be recognized with
Previous dangerous weather conditions
Downwind-oriented slopes
Cornices with cracks or breakaway zones
Gullies

43. Snow Avalanches Surviving an avalanche rules Avoid traveling alone
Carry shovel, probe and avalanche beacon
Take deep breath and make air space around face
Look for poles or clothing to locate victims

44. Hazards Related to Landslides
Landslides are closely related to other hazards
Can be triggered by storms and flooding or by earthquakes
When landslide blocks waterway, can cause flooding

45. 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

46. 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 noncohesive and erode quickly

47. 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

48. Table 8-2:
Table 8-2, p. 206

49. 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 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

50. Figure 8-40: Landslide Hazard Map
Landslides are widespread, not only in mountainous areas of the western United States and Canada and in the Appalachians, but also
in the subdued terrain of the central United States and Canada. Canadian landslide areas shown in red.
Fig. 8-40, p. 207

51. Records 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

52. Landslide Hazard Maps
Best strategy is to avoid building in places that are prone to landslides
Geographic Information System (GIS) 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

53. 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

54. Figure 8-42: Weight the Bottom
Heavy boulders are often piled on the lower part of a slide to resist movement. This road, cut through a landslide on U.S. Highway 101
near Garberville, in northern California, has been stabilized by loading its toe area.
Fig. 8-42, p. 208

55. Ongoing Landslide Problems: Coastal Area of Los Angeles
Coastal bluffs composed of young sediments, sand and shale
Easily break up and landslide, especially when
Soaked by unusual amounts of water
Undercut by waves or road construction
Overloaded by addition of fill
Disturbed during almost any kind of construction

56. Ongoing Landslide Problems: Coastal Area of Los Angeles
Portuguese Bend area of Palos Verdes Hills began moving slowly downslope in 1956 after construction
In 1979 homeowners placed moratorium on building and hired engineering geologist
Determined that movements were reactivation of old translational slide on volcanic ash smectite layer
Dewatering wells and storm-drain culverts reduce movement

57. Ongoing Landslide Problems: Coastal Area of Los Angeles
‘Sunken City’ in San Pedro area of Point Fermin began sliding in 1929
In 1941, broken water main accelerated movement
July 2011: section of bluff separated, dropped and moved seaward

58. p. 211

59. Slippery Smectite Deposits Create Conditions for Landslide: Forest City Bridge, South Dakota
U.S. Army Corps of Engineers built Oahe Dam across Missouri River in 1950s
Smectite layers in Pierre Shale swelled, became extremely slippery
Slides began moving down toward reservoir
Repaving of roads accumulated 2.1 m of asphalt
Engineers excavated broad area of material upslope and piled heavy load on toe to stabilize slopes

60. Figure 8-CIP02b: A huge landslide at the U. S
Figure 8-CIP02b: A huge landslide at the U.S. 212 Forest City Bridge across the Missouri River was reactivated by raising the reservoir level behind Oahe Dam.
p. 212

61. House destroyed and family killed
Water leaking from a canal saturates a steep slope and triggers a deadly landslide: Logan Slide, Utah, 2009
20-meter section of water-filled irrigation canal collapsed with hillside
House destroyed and family killed
Many reports had been filed about leaks in the canal walls but little was done
Many questions remained after slide as to exact cause or combination of causes

62. Slide Triggered by Filling a Reservoir: The Vaiont Landslide, Italy, 1963
Filling reservoir behind new Vaiont Dam caused a catastrophic mountainside collapse
Unstable sides of valley included ancient slide plane
For three years, engineers monitored slope above reservoir, expecting occasional small slides
Heavy rains in summer 1963 filled reservoir to within 12 m of spillway, increasing water pressure in surrounding rocks
Slope was creeping at 1-30 cm/week, then 25 cm/day, then 100 cm/day
Engineers recognized danger and began lowering water level in reservoir, but too late

63. Slide Triggered by Filling a Reservoir: The Vaiont Landslide, Italy, 1963
October 9, 1963, 10:41 pm:
238 million cubic meters of rock and debris collapsed off mountain face just upstream of dam, at 90 km/hr
Slide generated earthquakes, violent blast of air
Filled 2 km of reservoir, went 260 m up other side of reservoir
Displaced water swept over dam in wave 125 m high
Wave destroyed 5 villages downstream of dam
Second wave upstream through reservoir, destroying town
2,533 people killed within few minutes
Dam remained almost undamaged

64. Slide Triggered by Filling a Reservoir: The Vaiont Landslide, Italy, 1963
Engineering warning signs had been disregarded:
Drilling encountered broken rock with much pore space
Tunnel was excavated through strongly sheared zone, but no further investigation
Solution fractures upslope collect surface runoff  increase internal pore-water pressure in rocks above reservoir
Lesson: Slopes that seem to be only moving slowly may suddenly fail catastrophically

65. Figure 8-CIP04b: The 1963 Vaiont slide moved catastrophically on weak layers of shale within limestone beds parallel to the mountain face. A. Cross
section of the valley. B. The slip surface is in the upper right; the slide mass fills the center of the view, and the dam is just out of sight in
the lower right. The highway looping around the toe of the slide mass in mid-view provides scale. C. The dam, viewed from downstream,
survived even though a high-velocity wave of water 125 meters high swept over it (up to the yellow line) and killed more than 2,500
people downstream.
p. 215

66. A Rockfall Triggered by Blasting: Frank Slide, Alberta, 1903
Coal mine cuts low on mountainside destabilized rock face
Began as translational slide, developed into debris avalanche as it gained speed
Swept across and buried town, killing 70 of 600 inhabitants
Miners working in mountain at time were below slide scarp and were later rescued

67. Figure 8-CIP05b: A. The Frank slide peeled off the whole east face of Turtle Mountain and spread a spectacular bouldery deposit across the valley and up
the slope to the west. Huge limestone boulders are part of the Frank slide deposit. B. This cross section shows the slope before and after
the Frank slide. Fracture sets in some rocks and approximately parallel bedding surfaces in other rocks provided zones of weakness.
p. 216