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

454 lecture 4

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)

454 lecture 4

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

454 lecture 4

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

454 lecture 4

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

454 lecture 4

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

454 lecture 4

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

454 lecture 4

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

454 lecture 4

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)

454 lecture 4

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