09 Mass Wasting and Hill Slopes Feb 17f

8/4/2019 09 Mass Wasting and Hill Slopes Feb 17f

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Mass Wasting and HillslopesMass Wasting and Hillslopes

Steep slope

Gd>F

Gentle

slope

Gd< F

Gp

Gp

Gp

Moderate

slope

Gd=F

Boulder

on verge

of moving

Boulder is

stable

BouldermovesdownslopeF

F

F

Gd

Gd

Gd

W

W

W

Gravity overcomes Friction


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Sliding Threshold whenSliding Threshold when gravity componentgravity component ==

friction componentfriction component, both parallel to slope, both parallel to slopeShear Forces are parallel to 2 touching surfaces.

If the slab is about to move, then the

downhill force = resisting force pointing uphill

Downhill force = mass x gravity x sine of dip

FF00

= mg sin (dip)= mg sin (dip) (1)(1) E is the same as the dip

F0 = mg sin()

mg

h

dip


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Your book uses mg = weight "w"Your book uses mg = weight "w"

Downhill force = mass x gravity x sine of dip

FF00

= w sin (dip)= w sin (dip) (1)(1) E is the same as the dip

ShearForceShearForce =F0= w sin()

w

h

dip

Bloom confuses shear force with shear stress


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Stress has units Force/areaStress has units Force/areamass =V V because V has units mass/volume

so F0= Vg sin()

If calculated over a unit area, F0 / a "force per unit area"

F0 / a = s =V(V/a) g sin() but V/a has units lengthssdd'' = gh sin() (2)

when we use the height h as the characteristic length

Equation (2) is the shear stress

mg

h

dip

Notice Vgh is the potential energy per

unit volume

ssdd'' is its component along the slope

pulling downhill


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Role of water for slabsRole of water for slabs

Friction ForceFriction Force is proportional to Normal ForceNormal Force

It is the amount of Force needed to lift the

surfaces apart

Increased water pressure between the surfaces

lifts the upper slab, and it will slip at a lower dip

angle.

mg

h

dip

N


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Water's role for slabs:Water's role for slabs:

BeforeFall

BeforeFall


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Water's role for slabs:Water's role for slabs:

AfterFallAfterFall

Of course, in our

area, winter freezing

causes frost wedging

breaks loose any

remaining bonds


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Classification of slope movementsClassification of slope movements

Slides

FallsSlumps

Flows

(note rotation)


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Slow mass movement indicatorsSlow mass movement indicators

Example: Soil Slump


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Scarp

Scarp

Scarp

CD

CD

Lobe

LobeDF

Soil CreepSoil Creep


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Signs ofSoil CreepSigns ofSoil Creep

Vertical features

exposed in new roadcut

Vertical features (if available) curved near surfaceVertical features (if available) curved near surface


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Creep Typical FeaturesCreep Typical Features

Drunken forestDrunken forest


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SolifluctionSolifluction

Wet and Dry


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Gelifluction: FreezeGelifluction: Freeze


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Gelifluction: ThawGelifluction: Thaw


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Rapid Mass MovementRapid Mass Movement

Flows: mixture moves downslope as aFlows: mixture moves downslope as aviscous fluidviscous fluid

Slumps: move downslope along a concaveSlumps: move downslope along a concaveslip surfaceslip surface

Slides: move downslope along preexistingSlides: move downslope along preexisting

plane of weakness as a single, intact massplane of weakness as a single, intact mass Falls: rock drops from steep slopeFalls: rock drops from steep slope

http://geology.com/news/2008/spectacular-yosemite-rockfall-and-debris-avalanche-photos.shtml


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Rapid Mass MovementRapid Mass Movement


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FlowsFlows

Flows with a high water content are fasterand more dangerous

Debris avalanches- rain- regolith detaches

200 kilometers per hour Lahars

Liquifaction- Quick Clay earthquake -increased pore water pressure - grains

separate - liquefies instantaneously Mudflow swift slurry- heavy rains

Earthflows dry masses of clayey regolith 1-2 meters per hour


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Yungay AvalancheYungay AvalancheSource: Lloyd S. Cluff

Debris Avalanche

Town in Peru

Earthquake dislodgedSlab ice => landslide


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LaharLahar

http://www.massey.ac.nz/~trauma/ishttp://www.massey.ac.nz/~trauma/is

sues/2004sues/2004--1/galley.htm1/galley.htm


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LiquifactionLiquifaction -- Quick ClayQuick Clay

http://cce.oregonstate.edu/geotech/research.html

Caused by Earthquakes


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Mudflow in Sarno, Italy, 1998Mudflow in Sarno, Italy, 1998


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SlumgullionSlumgullion

EarthflowEarthflow San Juan

Mtns, CO

Volcanics

Dams

Lake Fork

of the

Gunnison


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SlidesSlides

Slumps

Mudslides

Rock Slides Avalanche and Debris Slides

www.pdc.org/.../2007/R

io-de-Janeiro/rio.htm


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SlumpingSlumpingand Scarpsand Scarps

in Dorset,in Dorset,

EnglandEngland

These areThese arerotationalrotational

SlumpSlump


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Little Hat Mountain Slump, CALittle Hat Mountain Slump, CA

scarpscarp

Toe,Toe,

no veg.no veg.


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La ConchitaLa Conchita

SlumpSlump

Typical urbanlandslide

Preexisting slide

masses Development to the

edge

Lawsuits

9 houses destroyed

Property valuesdown


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slump scar

Snow Avalanche


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Turtle Mountain Debris SlideTurtle Mountain Debris Slide


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Frost heave, Yosemite NP.Frost heave, Yosemite NP.Glacier Point climbing area.Glacier Point climbing area.

1996 Rockfall1996 Rockfall

162,000162,000--ton granite slab.ton granite slab.

160 mph speed.160 mph speed.Killed several people.Killed several people.

Falls:Falls:

RockfallRockfall


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Angle ofReposeAngle ofRepose

For loose materials, the angle of repose

dictates the maximum steepness a

material can be arranged before it will

move downslope

Bloom claims: coarser materials tend to

have steeper slopes

http://www.quanterra.org/guide/guide1_5.htm

Rockfall Talus Slope


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Slope StabilitySlope Stability

Slope characteristics such as composition,

vegetation, and water content also

influence slope stability.

Haiti is plagued by slides after many trees

were cut down.


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Natural TriggersNatural Triggers

Natural triggers such as:Natural triggers such as:

torrential rainstorms 1967 central Braziltorrential rainstorms 1967 central Brazil Earthquakes 1812 New Madrid, MissouriEarthquakes 1812 New Madrid, Missouri

volcanic eruptions 1980 Mount St. Helensvolcanic eruptions 1980 Mount St. Helens

produce damaging mass movementsproduce damaging mass movements

http://www.terradaily.com/reports/At_least_26_dead_in_China_mudslide_999.html


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09 Mass Wasting and Hill Slopes Feb 17f

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