6. Mass Movements
• Material moves downslope due to the pull of gravity
• Can happen almost anywhere
• Commonly associated with other events (heavy rainfall or earthquakes,
for example) and are therefore under-reported
• Movements can either be catastrophic (slope failure) or slow and steady
(creep)
• The rate of the mass movement can be increased by various erosive
agents (especially water)
8. Gravity & steepening of a slope
• Gravity causes the downward and outward movement of
landslides and the collapse of subsiding ground.
• Eventually it will flatten all slopes
• The force of gravity is the mass of a body x the sine of the slope.
• If can remove the initial resistance to motion the body will move.
• Earthquake, heavy rain could give initial energy
14. Water decreases rock/soil cohesion
Water circulating underground can dissolve cements
that hold sedimentary rocks together
15.
16. Internal Causes for Slope Failure
• Water (weight & interaction with clay minerals)
• Decreasing rock cohesion
• Incompetent/weak material
• Adverse geologic structures
17. Role of Earth Materials
• Slopes formed by weak rocks such as shale or
have thick soil deposits typically fail by
rotational slides
• Slopes formed by hard rocks typically fail by
translational slides
• Soil slips occur above bedrock and fail by
translational slides
18. Role of Slope and Topography
• Hillslope angle is a measure of the steepness
of a slope = slope gradient
• Steeper slope = increased driving forces
• Steep slopes associated with rockfalls
• Subarid to arid environments
19. Role of Vegetation
• In subhumid to humid environments, vegetation is thick and
abundant
• Landslide activity includes deep complex landslides,
earthflows, and soil creep.
• Vegetation influences slope stability by:
– Providing a cover that cushions the impact of rain falling
on slopes and retards erosion on surface
– Vegetation has root systems that tend to provide an
apparent cohesion which increases resistance to
landsliding
– Vegetation adds weight to the slope increasing the driving
forces
20. Role of Water
• Water can affect slope stability by:
– Shallow soil slips can develop during rainstorms
when slopes become saturated
– Slumps or translational slides can develop months
or years after slope is saturated
– Water can erode the base or toe of a slope
decreasing slope stability
21. Role of Climate
• Climate influences the amount and timing of
water in the form of water or snow
• Influences type and amount of vegetation
22. Role of Time
• Physical and chemical weathering can weaken
slope materials decreasing resisting forces
23. IMPORTANT CONCEPTS
CREEP
Movement down slope of soil and uppermost bedrock
Block diagram showing the
effects of creep.
Most commonly seen by its
effects on telegraph poles,
fences and trees.
The soil zone slips in ultra slow
movement as particles shift in
response to gravity
24. IMPORTANT CONCEPTS
CREEP
How creep works
Surface materials expand
perpendicular to slope (1 to 2) as a
result of freezing, wetting or heat
of sun.
Upon thawing,drying or cooling they
contract (2 to 3) under pull of
gravity vertically.
Do not go back to old position. Thus
have a slow movement down slope
i.e. creep
25. IMPORTANT CONCEPTS
LANDSLIDE
Fast moving mass-movement
Causes most fatalities.
Landslide is a mass whose center has
moved downwards and outwards.
Has a tear-away zone upslope where
material has pulled away and a
pile-up zone where material had
accumulated.
26. IMPORTANT CONCEPTS
LANDSLIDE
Major topographic features
Features include crown, head scarp, basal surface of rupture, transverse cracks,
transverse ridges and radial cracks. All created in the downward and outward
movement
27. IMPORTANT CONCEPTS
EXTERNAL PROCESSES CAUSING FAILURE
Three major ones
On arcuate failure surfaces have balance between the driving mass and the resisting
mass. Changing either can create a landslide
Processes include: 1) steepen slope, 2) remove support from bottom of slope, and 3)
add mass high up on slope.
28. IMPORTANT CONCEPTS
INTERNAL CAUSES OF SLOPE FAILURE
Clays
Clays form during chemical weathering due to acidic fluids such as water, CO2
charged water and organic acids decomposing minerals created at high
pressures and temperatures.
Creates totally different internal structure. Clay minerals are built like books and
have many unfilled atomic positions in the crystal structure.
Typically can have their strength dramatically reduced by adding water which also
causes expansion.
29. IMPORTANT CONCEPTS
INTERNAL CAUSES OF SLOPE FAILURE
Quick Clay: Ontario, Canada 1993
Fine grained rock flour left behind during the retreat of the glaciers and
deposited in a nearby sea. The clay and silt particles are loosely packed and
held together as a rock by sea salts.
When the sea retreats, the sediments are uplifted and the glue removed by fresh
water. Anything can cause the house of cards to collapse
30. IMPORTANT CONCEPTS
INTERNAL CAUSES OF SLOPE FAILURE
The five roles of water
1) Sediments have high porosities. When these void spaces are filled with
water the sediment is much heavier and the driving mass increased.
2) Water is easily absorbed and attached externally to clay minerals with a
major decrease in strength.
3) Water flowing through rocks can dissolve the minerals that bind the rocks
together. The removal of the cement makes the rock easier to move or a
slope easier to collapse.
4) Water can physically erode loose material creating caverns.
5) Pressure builds up in water trapped in the pores of sediments being buried
deeper and deeper. Sediments can compress but water does not
compress. Get abnormally high pore-water pressures which “jacks up” the
sediment and makes it very easy to move.
31. IMPORTANT CONCEPTS
INTERNAL CAUSES OF SLOPE FAILURE
The role of flowing water
Schematic cross section of ground water flowing through poorly consolidated
rock. The water will carry sediments to the stream creating a series of caverns
that seriously weaken a hill.
32. IMPORTANT CONCEPTS
INTERNAL CAUSES OF SLOPE FAILURE
Slope stability
Addressed by use of Coulomb/Terazaghi equation where
s = c + (p - hw) tan ø
Where s = resistance due to shear, c = the cohesion of the sediment layer
p = load of sediment and water above a slide surface
hw = weight of water above the potential surface.
F = internal angle of friction.
Strength comes from cohesion + the weight of the sediment.
Weakness from the pore water pressure and the internaL angle of friction.
Clays have high cohesion but a very low failure angle.
Sands have poor cohesion.
Granites have very high failure angle.
33. IMPORTANT CONCEPTS
INTERNAL CAUSES OF SLOPE FAILURE
Quick sand
Example of the Coulomb-Terazaghi equation.
The pore water pressure hw equals the weight of the sands p.
Leaves cohesionless sand with no shear stress.
With no shear stress you will sink into the sand when you walk on it.
34. IMPORTANT CONCEPTS
ADVERSE STRUCTURES
1) Ancient slip surfaces are weaknesses that tend to be reused over time. These
surfaces are especially slippery when wet.
2) The orientation of the sediment layers can create strong or weak conditions.
Sediment layers dipping into the hill are very stable, dipping in the same
direction but shallower than the slope have daylight bedding. Potentially
dangerous condition.
3) Rocks have inherent weakness that set-up slope failure. Lack of cement, clay
layers, soft rocks, splitting joints, faulting surfaces.
TRIGGERS
Basic causes bring slopes close to failure.
Rain, earthquakes or humans create trigger.
35. MASS MOVEMENTS
CLASSIFICATION
Speed of movement and water flow.
On left have mass movement speed versus moisture content.
On right have rates of travel for mass movements
36. MASS MOVEMENTS
CLASSIFICATION
Falls, Flows, Slides and Subsides.
Falls and subsides involve
vertical drops. Slides
and flows involve
downward and
outward motion.
Sliding involves a coherent
mass.
Flowing involves the
moving mass behaving
like a viscous fluid.
38. MASS MOVEMENTS
SLIDES
Rotational
Downward and outward movement on a
curved surface.
Note the rotation of head and the up
movement of the toe.
Swedish circle analysis of slope stability has
a compass set at the center of rotation.
Use this to compute driving and
resisting forces.
Note backward tilted head and bulged toe.
Toe helps stability, tilted head catches
water.
39. MASS MOVEMENTS
SLIDES
Rotational: 1976 Ensenada slide
The slide was associated with a
pronounced head scarp shown
here.
A rotation of coherent beds
downwards and outwards and the
formation of a pronounced bulge.
Not all rotational slides are as simple
as this because of discontinuities
on the surface.
40. MASS MOVEMENTS
SLIDES
Rotational slide: 1976 Ensenada
Top) View northward along
Highway 1 showing seaward
shift of the highway. This was
created by the downward and
outward movement of the
center of the slide.
Bottom) Toe of the Ensenada
slide. Note that the ocean
floor was lifted above sea
level stabilizing the slide.
41. MASS MOVEMENTS
TRANSLATIONAL SLIDES
Three types
1) Move as coherent blocks. Pt Fermin 1929
2) May deform and break-up as a debris slide.
1963 Viaont, Italy
3) Involve lateral spreading where the underlying
material fails and flows, Anchorage, Alaska 1964
42. MASS MOVEMENTS
SLIDES
Translational slide: Pt Fermin, Ca 1929
Cross section showing block on top of a inclined slippery layer which day lighted
under the ocean.
Block moves towards the unsupported offshore.
Though triggered by watering from yard irrigation seeping down to layers of weak
clays which expand and lose strength and sliding begins.
44. MASS MOVEMENTS
SLIDES
Translational Debris Slide: Vaiont Italy 1963
Reservoir built on a) sedimentary rock layers with beds dipping towards the
reservoir, b) fractures formed in the rocks due to expansion after retreat of
glaciers and river cutting Canyon, 3) weak clay layers and numerous
Limestone caverns.
After very heavy rains the slide slumped down the fracture surfaces into the
reservoir.
45. MASS MOVEMENTS
SLIDES
Translational slide: Debris Slide, 1963 Vaiont Italy
Huge area of slope slid into the reservoir. Debris at 150 M above the water level. A huge
wall of water climbed over the dam and swept down the river bed as a 70m high wall of
water causing extensive fatalities in the towns down stream. The Dam stood. Has been
called world’s worst dam disaster
46. MASS MOVEMENTS
FLOWS
Movements that behave like fluids with internal reorganization
Loess flows - the flow of loose silt, Gansu
Province China, 1920. The mountains
walked.
Earth flows - wet flows moving slowly on slick
surface, Portuguese Bend Ca, 1958
Debris flows (Sturtzstroms) - massive rock falls
that convert into highly fluidized rapidly
moving Debris flows. Elm, Switzerland, 1881
47. MASS MOVEMENTS
FLOWS
Earth flow: Portuguese Bend 1958 and following.
Cross section through Portuguese Bend showing the seaward dipping reflectors,
bentonitic clay layers, a preexisting slide surface and waves eroding the base
of the slope.
An ancient earth flow site which was unstable but not moving. 1950’s put
development on area by beach. No sewers. Within a few years lower slope
started to move.
Watering and sewage disposal seeped down to bentonite layers. They expanded,
lost strength and started to move.
49. MASS MOVEMENTS
FLOWS
Earth flow: Portuguese Bend
View of the toe of the bulged up earth flow in 1959. Note the remains of the
houses and the roads and the damaged pier.
50. MASS MOVEMENTS
FLOWS
Slumps and Debris flows, La Conchita
Heavy rains led to failure at depth triggering a slump and earthflow in 1995 which moved
very slowly during day. No loss of life.
Heavy rains led to a faster moving debris flow in 2005 which killed 10 people.
These flows typical of coastal flows near coast in southern California.
51. MASS MOVEMENTS
FLOWS
Debris flow: Elm, Switzerland 1881
Three stages, rock fall, a jump and fluidization of the debris, and then a flow of
the disintegrated rock mass down the valley floor using air and dust as the
internal fluid.
Many fatalities in Elm 2.25 km away.
52. MASS MOVEMENTS
FLOWS
Submarine flows: Hawaii
Left) Slump and debris-avalanche deposits cover more than five times the area of
the islands.
Right) Created by the collapse of flanks of the Islands. The zone of normal faults
associated with the injection of magma at Kilauea appear to be head scarps
for giant mass movements.
54. MASS MOVEMENTS
SUBSIDENCE
Slow: Mississippi Delta, New Orleans
Left) Loosely packed sand and clay have high porosities near the surface. When buried they
expel the water and contract over a period of time.
Right) As much as 100 ft of subsidence of the Mississippi Delta around New Orleans over
the past 20,000 years. Has been faster recently up to 3m in the past 50 years making
city very prone to damage from hurricanes.
55. MASS MOVEMENTS
SUBSIDENCE
Catastrophic: Sink Holes
View of sinkhole formed by the collapse of a
limestone cavern on May 31 1981 in
winter park Florida. Sinkhole is 100 m
across by 30 m deep.
Most of southern and eastern US lies on
limestone rocks created from the
deposition under seawater of soft shells
made of calcium carbonate.
Uplifted above seawater they are prone to
dissolution from circulating acidic fresh
ground water. Creates unstable caverns
underground.
56. Use Knowledge of Mass Wasting to
Avoid Risks
• Be able to recognize
geologically unstable
situations
57. Understanding Mass Wasting
Development causes:
• Increased shear force
– Steepened slope
– Added weight
• Decreased shear
strength
– Devegetation
– Reworking of fill
– Saturation of soil
