Chapter 6: Mass Wasting

Chapter 6: Mass WastingChapter 6: Mass Wasting

Palos Verdes Hills LandslidePalos Verdes Hills Landslide

Landslide is a general term for rapid downslope movement/failure

Mass Wasting is even more general and includes slow movements (creep, slow flows…)

Subsidence is a vertical failure usually closed depression

A Classification of Mass WastingA Classification of Mass Wasting Nature of materialNature of material

RockRock Unconsolidated material (debris, mud)Unconsolidated material (debris, mud)

VelocityVelocity Nature of movementNature of movement

A Classification of Mass WastingA Classification of Mass Wasting

A Classification of Mass WastingA Classification of Mass WastingRockRock DebrisDebris

FallFall

(rare)(rare)

A Classification of Mass WastingA Classification of Mass WastingRockRock fall- Talusfall- Talus


AvalancheAvalanche


SlideSlide

A Classification of Mass WastingA Classification of Mass WastingEarthEarth Debris Debris Mud Mud

Flow Flow


Slump Slump CreepCreep


Solufluction Solufluction

Solufluction Solufluction

Angle of repose:Angle of repose: higher with coarser and more higher with coarser and more angular particlesangular particles

Slope stability:Slope stability: Driving forces:Driving forces:

MassMass GravityGravity

Resisting forces:Resisting forces: InertiaInertia FrictionFriction CohesionCohesion

What are the problems here?What are the problems here?

Consolidated material can slide or slump, Consolidated material can slide or slump,

Unconsolidated can slump or flowUnconsolidated can slump or flow

Role of Role of slopeslope Steeper Steeper less stable (incr DF and decr RF) less stable (incr DF and decr RF)

Role of Role of materialmaterial

Climate controls water and vegetationClimate controls water and vegetation Vegetation absorbs rain impact and roots incr cohesionVegetation absorbs rain impact and roots incr cohesion Vegetation adds weightVegetation adds weight LoggingLogging (clearcut): (clearcut):

Less transpiration Less transpiration more water to soil more water to soil Increased snowpack in clearingsIncreased snowpack in clearings Roots rot and lose cohesionRoots rot and lose cohesion Long vertical run for rill developmentLong vertical run for rill development Logging Logging roadsroads are perhaps worse than the cuts are perhaps worse than the cuts

themselvesthemselves

Role of Role of climateclimate and and vegetationvegetation

Role of Role of waterwater

Adds Adds weightweight (incr DF) (incr DF) Initially increases Initially increases cohesioncohesion, but decreases after , but decreases after

a point (decr RF)a point (decr RF) Often Often saturated zonesaturated zone at surface if above at surface if above

impermeable layer (frozen, clay, rock)impermeable layer (frozen, clay, rock) Promotes Promotes weatheringweathering ErodesErodes gullies and slope bases gullies and slope bases AvalanchesAvalanches are all water are all water

Role of Role of vibrationvibration Earthquakes, traffic, sonic boomsEarthquakes, traffic, sonic booms Can Can immediate immediate loss of cohesionloss of cohesion LiquefactionLiquefaction = shock + water saturation = shock + water saturation

UltimateUltimate cause: oversteepened slope, cause: oversteepened slope, overweighted...overweighted...

ImmediateImmediate cause: earthquake, rain storm... cause: earthquake, rain storm...

Human Use and LandslidesHuman Use and LandslidesLandslides are a growing problem.Landslides are a growing problem.

1. Incr population in landslide-prone areas1. Incr population in landslide-prone areas

2. Human activity that affects magnitude and 2. Human activity that affects magnitude and frequency of landslidesfrequency of landslides

Vaiont Dam DisasterVaiont Dam Disaster

Vaiont Dam DisasterVaiont Dam Disaster

LoggingLogging

Swanson & Dryness (1975) Cascade studySwanson & Dryness (1975) Cascade study

Found logging and roads increase landslide Found logging and roads increase landslide erosion by several times if performed on erosion by several times if performed on weak unstable slopesweak unstable slopes

Roads interrupt surface drainage (culverts Roads interrupt surface drainage (culverts blocked), alter subsurface movement, blocked), alter subsurface movement, redistribute massredistribute mass

UrbanizationUrbanizationPerson A undercuts slope for garage while person Person A undercuts slope for garage while person

B builds house above. Who is responsible?B builds house above. Who is responsible?

S. CaliforniaS. California Wave-cut terraces oversteepenedWave-cut terraces oversteepened Now lots of building on lower mtn slopes Now lots of building on lower mtn slopes

(most = old landslide deposits!)(most = old landslide deposits!)

Urbanization:Urbanization:Typical Puget Typical Puget

Sound HillSound Hill

Seattle Seattle TopographyTopography

Slope Slope Stability Stability



MapsMaps

Minimizing the HazardMinimizing the Hazard Identify potential hazard. Identify potential hazard.

Map unstable areas Map unstable areas

Minimizing the HazardMinimizing the Hazard Identify potential hazard. Identify potential hazard.

Map unstable areasMap unstable areas Land developers and buyers (YOU) look Land developers and buyers (YOU) look

for what??for what??

Minimizing the HazardMinimizing the Hazard If suspect, hire a consulting geologist to evaluateIf suspect, hire a consulting geologist to evaluate

Don’t buildDon’t build Minimize load or sink foundation deep to stable materialMinimize load or sink foundation deep to stable material Drainage controlDrainage control

Divert surface waterDivert surface water Surface drainsSurface drains Soil cement, asphalt, plastic to coverSoil cement, asphalt, plastic to cover

Cut-off trench to divert groundwaterCut-off trench to divert groundwater Drain internal water at base or even pump (Tukwilla)Drain internal water at base or even pump (Tukwilla)

Minimizing the HazardMinimizing the Hazard If suspect, hire a consulting geologist to evaluateIf suspect, hire a consulting geologist to evaluate

Grade slopeGrade slope Cut-and-fill: remove top and fill in at base Cut-and-fill: remove top and fill in at base Must do with care, since this often Must do with care, since this often causescauses problems problems Benching: series of cut-and-fills with diversion drainsBenching: series of cut-and-fills with diversion drains

Slope supportsSlope supports Retaining walls (permeable and/or drained)Retaining walls (permeable and/or drained) Rock boltsRock bolts

Landslide warning systemsLandslide warning systems Periodic inspectionPeriodic inspection Tilt metersTilt meters Well monitoringWell monitoring

Minimizing the HazardMinimizing the Hazard

Landslide correction (very tough…better to prevent)

Snow AvalanchesSnow Avalanches

1. Loose-snow avalanches1. Loose-snow avalanches in fresh cohesionless in fresh cohesionless snowsnow

Snow AvalanchesSnow Avalanches

2. Slab avalanches2. Slab avalanches above above subsurface hoar zonessubsurface hoar zones

SubsidenceSubsidenceSlow to rapid vertical sinking or settling Slow to rapid vertical sinking or settling

1. 1. Withdrawal of fluidWithdrawal of fluid (oil, gas, water)(oil, gas, water)

SubsidenceSubsidence

Slow to rapid vertical sinking or settling Slow to rapid vertical sinking or settling

2. Sinkholes2. Sinkholes Limestone solution, caves and collapse Limestone solution, caves and collapse

(also with salt deposits)(also with salt deposits)

Sink hole- Winter Park, FloridaSink hole- Winter Park, Florida

SubsidenceSubsidence

Slow to rapid vertical sinking or settling Slow to rapid vertical sinking or settling

1. 1. Withdrawal of fluidWithdrawal of fluid (oil, gas, water)(oil, gas, water)

2. Sinkholes2. Sinkholes Limestone solution, caves and collapse Limestone solution, caves and collapse

(also with salt deposits)(also with salt deposits)

3. Mining3. Mining

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Chapter 6: Mass Wasting