Chapter 6: Mass Wasting
Palos 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 Wasting

Nature of material
Rock
 Unconsolidated material (debris, mud)



Velocity
Nature of movement
A Classification of Mass Wasting
A Classification of Mass Wasting
Rock
Debris
(rare)
Fall
A Classification of Mass Wasting
Rock fall- Talus
A Classification of Mass Wasting
Rock
Avalanche
Debris
A Classification of Mass Wasting
Rock
Slide
Debris
A Classification of Mass Wasting
Earth
Flow
Debris
Mud
A Classification of Mass Wasting
Slump
Creep
A Classification of Mass Wasting
Solufluction
Solufluction
Angle of repose: higher with coarser and more
angular particles
Slope stability:
 Driving forces:
 Mass
 Gravity
 Resisting forces:
 Inertia
 Friction
 Cohesion
What are the problems here?
Role of material

Consolidated material can slide or slump,

Unconsolidated can slump or flow
Role of slope

Steeper  less stable (incr DF and decr RF)
Role of climate and vegetation

Climate controls water and vegetation

Vegetation absorbs rain impact and roots incr cohesion

Vegetation adds weight

Logging (clearcut):

Less transpiration  more water to soil

Increased snowpack in clearings

Roots rot and lose cohesion

Long vertical run for rill development

Logging roads are perhaps worse than the cuts
themselves
Role of water
Adds weight (incr DF)
 Initially increases cohesion, but decreases after
a point (decr RF)
 Often  saturated zone at surface if above
impermeable layer (frozen, clay, rock)
 Promotes weathering
 Erodes gullies and slope bases
 Avalanches are all water

Role of vibration

Earthquakes, traffic, sonic booms

Can  immediate loss of cohesion

Liquefaction = shock + water saturation
Ultimate cause: oversteepened slope, overweighted...
Immediate cause: earthquake, rain storm...
Human Use and Landslides
Landslides are a growing problem.
1. Incr population in landslide-prone areas
2. Human activity that affects magnitude and
frequency of landslides
Vaiont Dam Disaster
Vaiont Dam Disaster
Logging
Swanson & Dryness (1975) Cascade study

Found logging and roads increase landslide
erosion by several times if performed on
weak unstable slopes

Roads interrupt surface drainage (culverts
blocked), alter subsurface movement,
redistribute mass
Urbanization
Person A undercuts slope for garage while person
B builds house above. Who is responsible?
S. California
Wave-cut terraces oversteepened
 Now lots of building on lower mtn slopes
(most = old landslide deposits!)

Urbanization:
Typical Puget
Sound Hill
Seattle
Topography
Slope
Stability
Slope
Stability
Slope
Stability
Maps
Minimizing the Hazard

Identify potential hazard.
 Map unstable areas
Minimizing the Hazard

Identify potential hazard.
Map unstable areas
 Land developers and buyers (YOU) look
for what??

Minimizing the Hazard

If suspect, hire a consulting geologist to evaluate
Don’t build
 Minimize load or sink foundation deep to stable material
 Drainage control
 Divert surface water
 Surface drains
 Soil cement, asphalt, plastic to cover
 Cut-off trench to divert groundwater
 Drain internal water at base or even pump (Tukwilla)

Minimizing the Hazard

If suspect, hire a consulting geologist to evaluate

Grade slope




Slope supports



Cut-and-fill: remove top and fill in at base
Must do with care, since this often causes problems
Benching: series of cut-and-fills with diversion drains
Retaining walls (permeable and/or drained)
Rock bolts
Landslide warning systems
Periodic inspection
 Tilt meters
 Well monitoring

Minimizing the Hazard
Landslide correction (very tough…better to prevent)
Snow Avalanches
1. Loose-snow avalanches in fresh cohesionless snow
Snow Avalanches
2. Slab avalanches above
subsurface hoar zones
Subsidence
Slow to rapid vertical sinking or settling
1. Withdrawal of fluid
(oil, gas, water)
Subsidence
Slow to rapid vertical sinking or settling
2. Sinkholes

Limestone solution, caves and collapse
(also with salt deposits)
Sink hole- Winter Park, Florida
Subsidence
Slow to rapid vertical sinking or settling
1. Withdrawal of fluid (oil, gas, water)
2. Sinkholes

Limestone solution, caves and collapse
(also with salt deposits)
3. Mining