454 lecture 4
Mass Movements and Hillslopes
Erosion (or lack of) results from balance between internal
resistance of materials & magnitude of external forces
acting on them
Evolution of landscapes depends largely on regional slope
development
Mechanics of slope erosion are related to processes of
physical weathering – the forces disintegrating rocks also
lower the internal strength of the unconsolidated cover
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Resisting forces (the properties of matter that resist the stresses
generated by gravitational force)
Shear strength
1) overall frictional characteristic, expressed as
angle of internal friction, Φ
a) plane friction: grains sliding past one another on planar surfaces;
varies with moisture, smoothness of plane surface,
mineralogy
b) interlocking friction: particles move upward and over one another
(greater resistance than plane friction); varies
with moisture, mineralogy, density of packing
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2) effective normal stress, δ’, acts to hold material together and to
increase internal resistance to shear
total normal stress: δ = δ’ + μeffective pore
normal stress pressure
pore pressure can increase or decrease δ
in unsaturated zone, water molecules attached to surface
particles by tension increase weight of soil (eg. wet sand)
in saturated zone, water exerts hydrostatic pressure upward
& supports soil
3) cohesion, c, causes increase in shear strength when grains are packed
or cemented together (eg. clay)
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Properties of material change with increasing or decreasing
moisture:
water added to dry soil – voids fill – plastic behavior
more water decreases cohesion – all pores filled
liquid behavior
“Plastic” refers to the way the material responds to stress
(force per unit area), in terms of strain (deformation) resulting
from applied force
stress
strain
y B
y: yield stress (permanent
deformation begins)
B: breaking strength (rupture occurs)
plastic
failure
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Atterberg Limits: indicate transition from solid to plastic state, &
from plastic to liquid state
liquid limit expressed as moisture contents
plastic limit (wt. of contained water/wt. of dry soil)
Range of water contents between two limits is plasticity index
Atterberg limits function of
• types of clay minerals (eg. limits higher for montmorillonite
than kaolinite)
• size of particles (limits increase with smaller particles)
• history of wetting and drying
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debris flow along
I-70 corridor near
Georgetown, triggered
by rainfall
454 lecture 4
Soil slips along Rt. 287 triggered by rainfall, 8/97
landslide above
Horsetooth Reservoir,
8/97
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Factors influencing shear stress & resistance in slope materials
1) Factors increasing shear stress (promote failure)
removal of lateral support
erosion (rivers, ice, waves)
human activity (quarries, road cuts, etc)
addition of mass
natural (rain, talus, etc)
human (fills, ore stockpiles, buildings, etc)
earthquakes
regional tilting
removal of underlying support
natural (undercutting, solution, weathering …)
human activity (mining)
lateral pressure
natural (swelling, freezing expansion, water addition)
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Huascaran, Peru (1973 Yungay slide)
Seismically triggered slides
Hebgen Lake landslide,
Montana
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2) Factors decreasing shear strength (promote failure)
weathering
disintegration (lowers cohesion)
hydration
base exchange
solution
drying
pore water
buoyancy
capillary tension
structural changes
remolding
fracturing
Gs = resisting/driving = shear strength/shear stress
Gs > 1 stable
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Three basic types of mass movements:
slides: cohesive blocks of material move on a well-defined
surface of sliding, with no internal shearing within the
sliding block
flows: move entirely by differential shearing within the
transported mass – no clear plane at base of moving
debris; velocity decreases from the surface down
heaves: disrupting forces act perpendicular to the ground surface
by expansion of material – facilitates downslope
movement & is the forerunner of more rapid mass
movementsleads to seasonal or soil creep
very slow movement of material due to gravity when
cohesion & frictional resistance are spasmodically lowered
functions in upper few feet of soil
evidence includes stone lines, structures, trees
caused by swelling & contracting due to wetting/drying or
freezing/thawing
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Slides
failure, crest of
sand dune
arcuate soil slips along ridge crest,
northern California
slumps on landslide toe, southern Poland
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forest fire & resulting debris flow,
Huachuca Mountains, AZ
unburned swale
burned slope
upper channel
scoured to bedrock
Flows
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lower reaches of channels
& alluvial fan, Huachucas
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Buffalo Creek, Colorado
fire, debris flows, floods
1996
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Culebres cut, Panama Canal
debris flow fan, Langtang, Nepal
debris flow, Khumbu,
Nepal
debris flow, Idaho
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failures on dune face
Rio Quijos, Ecuador
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Falls/flows
debris cone, Oi River,
Japan
debris cone,
Banff National
Park, Canada
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Heave
tree response to soil creep, northern Montana
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Classification of Mass Movement Processes
slide heave
flow
wet
dry
fast slow
rockslide talus creepsoil
creep
landslide
river
mudflow
earthflow
solifluction
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Vajont dam overtopping, Italy, 1963
262 m high; 260 million m3 failure;
2,000 casualties
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Mitigation of mass movement hazards
slope stabilization, Japandebris flow monitoring site, Japan
attempted landslide
prevention, Seattle
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slope stabilization, Japan
Yoho National Park, Canada
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Individual Grain Movements
Sediment is moved on the surface of slopes by raindrop impact
(splash) and by overland flow (wash) – flow is shallow & spread
evenly across slope as uniform sheet
Amount of soil moved by splash depends on
i) kinetic energy of raindrops
ii) type & amount of soil exposed
iii) steepness of slope
particles are dislodged, detached, dispersed
Sheet wash doesn’t last long because natural flow irregularities
concentrate flow into deeper & shallower paths – variable flow
depths imply irregular eroding & transporting capabilities – small
rills begin to develop, but they periodically shift their position
so that erosion is fairly even in the long run
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Amount of soil eroded & transported is balance:
driving resisting
(gravity, force of vs (vegetation, soil
flowing water) shear strength)
Soil strength is referred to as erodibility – an estimate of the
ease with which soil can be eroded
Ie, index of erodibility
Ie = shear resistance x permeability
Soil loss can be estimated using empirical equations, eg:
Universal Soil Loss Equationerodibility slope length cropping
A = K R L S C Psoil loss rainfall steepness conservation
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Slope angles are not uniformly distributed, but tend to cluster
in groups – probably represent stability regimes for slopes
formed in particular climatic & lithologic settings
0
10
20
30
40
relative distribution of slope angles
slo
pe a
ngle
(in
degre
es)
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Major controls on slope form and evolution are
• time
• lithology
• climate
• process
Two contrasting models of slope development focus on
process and time
process model: slope angle is time-independent – depends
more on properties of slope materials & mechanics of
dominant slope processes; slope angle decreases with
increasing erodibility of rock regolith
evolutionary model: slope angle depends on time, &
decreases with time
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Influence of lithology on slopes
• coherent, resistant rocks = steeper slopes
• more massive bedding = steeper slopes
• alternating weak & strong strata = irregular profile
Resistance of a particular rock type varies with climate (eg.
limestone), and resistance depends on whether overlying slope
is controlled by
a) processes of weathering (resistance of rock = rapidity with
which rock is weathered)
b) processes of removal (resistance = rate at which regolith
is eroded)
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Influence of climate on slopes
1) Slopes in humid temperate regions tend to be
• upper convexity due to soil creep, lower concavity to soil wash
• applies after mass movements produce long-term angular
stability, so that creep & wash become dominant slope
processes
convexstraight
concave
cliff
debris
slope plain
2) Slopes in semiarid/arid regions
• less vegetation and precipitation
• mass movements occur at higher angles
• creep less important than wash
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stepped slope profiles,
Grand Canyon, Arizona
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Canyonlands, Utah
spheroidal granite
weathering & rounded
slopes, Missouri
Rt. 125, Colorado
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Slope Development with Time
1) slope decline: steep upper slope erodes more rapidly than
basal zone, flattening the overall angle, with a convexity on
the upper slope & a concavity on the lower slope
12 3
4
slope decline slope replacement parallel retreat
2) slope replacement: steepest angle is progressively replaced
by upward expansion of gentler slope developed near base;
enlarges overall concavity of profile, which can be
segmented or smoothly curved
3) parallel retreat: maintain constant angles on steepest part of
slope
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