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 l Nature of material F Rock F Unconsolidated material (debris, mud) l Velocity l Nature of movement

A Classification of Mass Wasting

RockDebris Fall (rare)

A Classification of Mass Wasting Rockfall- Talus

A Classification of Mass Wasting RockDebris Avalanche

A Classification of Mass Wasting RockDebris Slide

A Classification of Mass Wasting Earth Debris Mud Flow

A Classification of Mass Wasting Slump Creep

Solufluction

Solufluction

Angle of repose: higher with coarser and more angular particles Slope stability: F Driving forces: s Mass s Gravity F Resisting forces: s Inertia s Friction s Cohesion

What are the problems here?

F Consolidated material can slide or slump, F Unconsolidated can slump or flow Role of slope F Steeper  less stable (incr DF and decr RF) Role of material

F Climate controls water and vegetation F Vegetation absorbs rain impact and roots incr cohesion F Vegetation adds weight F Logging (clearcut): s Less transpiration  more water to soil s Increased snowpack in clearings s Roots rot and lose cohesion s Long vertical run for rill development s Logging roads are perhaps worse than the cuts themselves Role of climate and vegetation

Role of water F Adds weight (incr DF) F Initially increases cohesion, but decreases after a point (decr RF) F Often  saturated zone at surface if above impermeable layer (frozen, clay, rock) F Promotes weathering F Erodes gullies and slope bases F Avalanches are all water

Role of vibration F Earthquakes, traffic, sonic booms F Can  immediate loss of cohesion F 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

Logging Swanson & Dryness (1975) Cascade study F Found logging and roads increase landslide erosion by several times if performed on weak unstable slopes F 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 F Wave-cut terraces oversteepened F Now lots of building on lower mtn slopes (most = old landslide deposits!)

Urbanization: Typical Puget Sound Hill

Seattle Topography

Slope Stability

Slope Stability Maps

Minimizing the Hazard l Identify potential hazard. F Map unstable areas

Minimizing the Hazard l Identify potential hazard. F Map unstable areas F Land developers and buyers (YOU) look for what??

Minimizing the Hazard l If suspect, hire a consulting geologist to evaluate F Don’t build F Minimize load or sink foundation deep to stable material F Drainage control s Divert surface water i Surface drains i Soil cement, asphalt, plastic to cover s Cut-off trench to divert groundwater s Drain internal water at base or even pump (Tukwilla)

Minimizing the Hazard l If suspect, hire a consulting geologist to evaluate F Grade slope s Cut-and-fill: remove top and fill in at base s Must do with care, since this often causes problems s Benching: series of cut-and-fills with diversion drains F Slope supports s Retaining walls (permeable and/or drained) s Rock bolts l Landslide warning systems F Periodic inspection F Tilt meters F 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 F 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 F Limestone solution, caves and collapse (also with salt deposits) 3. Mining