Sep 2009M7.6

Landslidesand other

forms of

masswasting

Particles create an angle of repose based on their size and angularity

Angle ofrepose

Fine sand Coarse sand Angular pebbles

45°40°35°

Damp sand Dry sand Water-saturated sand

More cohesive Less cohesive

Surface tension binds particles

Dry particles are bound only by their size and friction.

Saturated particles are separated by water, which acts as a lubricant, allowing them to flow.

Water can fill pores in soil• Cohesion: force holding soil grains together

• Loose soils have 10-45% pore space– Small amount of water increases cohesion– Too much water pushes grains apart, reducing

cohesion

Frictional Resistance Prevents Sliding• Gravitational Force pressing down on slope

• Friction is the ‘Roughness’ of slippage surface

• Area of contact does not affect friction coefficient

• Slide occurs when gravitational force exceeds frictional resistance

Slope Material influences sliding

• Loose materials slide easier:– Soil– Loose sediment – Soft sedimentary rocks such as clay or shale

Clays can increase chance of landslide• Clays absorb water and expand to weaken rock• Kaolinite: soaks up water• Smectite: forms from volcanic ash, with open

structure between layers that fills with water swelling soils

Mud in bays, lakes is likely to fail

“Quick-clays”

Before earthquakeSand andgravel

ClayClay

Water-saturatedsandy layer

Landslide associated with 1964 Alaska Earthquake

After earthquake

Pre-earthquake profile

Landslide Triggers

(1) Oversteepening(2) Earthquakes (3) Rainfall(4) Volcanic eruptions

1. Oversteepening decreases stability• Steeper slopes are less stable• Slope angle is increased when

– Fill is added above• Construction of homes with magnificent views

– Slopes are undercut below• Erosion at base of slope, by waves at coast• Excavation of road at base of slope

Oversteepening decreases stability

Earthquake

2.Earthquakeloosens large masses of rock

Denali Earthquake 2002

3. Adding Water• Water reduces strength of slope • Heavy or prolonged rainfall saturates soil, • Human actions add water to slopes

– Lawn-watering, crop irrigation– Leaking water/sewer pipes, swimming pools– Filling reservoir behind dam

Water-permeablesoil

Water-impermeablerocks

Rain has soakedfine-grainedpermeable soils,… …which quickly loosen…

…and flow downhill.

Clear-cut slopes

Shale

Jointedbedrock

Rain soaks muds and rubble…

…resulting in a flow of mixed mud, rock, and surface debris.

Irrigation caused this slide in So. Cal.

Venezuela 1999

Venezuela 1999

Venezuela 1999

Venezuela 1999

Venezuela 1999

Snow and ice

Water-permeablevolcanic ash

Water-impermeablelava

A volcanic eruption has melted snow and ice that soaks volcanic ash over impermeable lavas.

The resulting mud moves quicklydownhill.

4. Eruption causes landslides

Types of Landslides

Landslides are classified by:-Material type-Movement type-Movement velocity

Velocity can range from <1 mm/yr to 100 m/sec

Ice wedging preparesrocks to loosen and fall away.

Individual blocks free-fall down slope.

Rockfall: Extremely rapid

Rockfall creates talus slopes of loose rock

Debris Avalanches: extremely rapid• Triggered by Peru

earthquake

• Can begin as rockfall, but become larger and run further.

• Avalanche went 14 km to with average speed of 270 km/hr

Earthquake

An earthquake has loosened large masses of rock…

…that flow downhillat high velocity on acushion of air.

Debris Avalanche

Snow and ice

Water-permeablevolcanic ash

Water-impermeablelava

A volcanic eruption has melted snow and ice that soaks volcanic ash over impermeable lavas.

The resulting mud moves quicklydownhill.

Debrisflows, mudflows, and LAHARS: Rapid

Frost wedging has loosened jointed bedrock layers…

…that move downhill asa unit.

Rockslide: moderately rapid

Translational Slide: goes along existing weaknessThis one cost taxpayers $400 million

Scar

Unconsolidated material slowly slides as a unit.

Slump: Slow to moderate

Scar

Buildingfoundationsshear and crack

Gravestones andfence posts lean Trees grow with

curved trunks

Road cracksPower poleslean

Creep: extremely slow

Soil Creep• Slow, downslope

movement of soil and weak rock

• Involves near-surface movement by alternate expansion and shrinkage of soil

Snow Avalanches: usually rapid• Trigger for avalanche could be

– Weight of skier crossing slope– Vibrations of snowmobile– Movement of glacier– Changes in temperature– Earthquake

Failure of Landslide Dams

• Any moderately fast-moving landslide can block a river or stream to create a dam and temporary lake before eventually failing

• Time before failure and size of flood depends on– Size, height and geometry of dam– Material making up dam– Rate of stream flow, how fast lake rises– Use of engineering controls (artificial breaches, spillways or

tunnels)• Dams from mudflows, debris flows and earth flows are

noncohesive and erode quickly

Failure of Landslide Dams

• Most landslide dams fail when water overflows and erodes spillway that drains lake

• If dam-failure flood incorporates significant sediment, can turn into debris flow – much more dangerous

• Useful dams can be constructed on top of landslide dams – Rockfalls or rock slides are most stable– 1928 St. Francis high-arch concrete dam failed – built on toe of old

landslide

Mitigation of Damages from Landslides

• Damages can be extremely costly• Not covered by most insurance policies• In U.S., landslides cost more than $2

billion and cause 25-50 deaths per year• Globally, cost more than $20 billion, cause

about 7500 deaths per year• Major landslide disasters increase with

growth in population in dangerous areas

Mitigation of Damages from Landslides