GSC 1620 Chapter 8

Mass Movements (Wasting)

Mass Wasting

Mass Wasting – downhill movement of Earth materials under the influence of gravity

Annually in the U.S. average mass wasting property damages exceed 4 billion dollars and typically 50 or more people are killed

Landslide is a colloquial term used by most people to describe all types of mass wasting (see slides)

Venezuela, 1999~30,000 deaths fromrain induced debris flows

Venezuela, 1999Note the largeboulders moved by the debris flow

Mass Wasting

The adjacent photos show the effects of mass wasting in Laguna Beach, CA (6/1/05)

The average price of homes in this community is 1.75 million dollars!

Mass Wasting

Gravity – force of attraction between two bodies; the magnitude of this force is dependent only on the masses of the bodies and their separation distance (see slide)

The Earth’s gravity pulls objects toward its center

BA

Distance

Gravity Force = Constant x (Mass A x Mass B) Distance2

Background

On sloped surfaces, the force of gravity can be conceptually subdivided into the shear force (stress) and normal force (see slides)

Gravity

Shear stress Normal force

Shear strength (friction)

If the shear stress actingon the slope exceeds theslope’s normal force plusshear strength, whatshould happen to the slope materials? Mass waste or not?

Gravity

Normal force

Shear stress

Shear strength (friction)

Notice how the shearstress increases and thenormal force decreasesas the slope angle increases

Angle of Repose

Angle of repose – maximum slope angle at which particles remain stable; for loose materials this angle typically varies from 25 – 40 degrees

If the slope material’s angle of repose is exceeded by natural or human forces, portions of the slope should mass waste (see slides)

Solid rock

Vertical cliff

Gravel slope Sand Clayey soil

In unconsolidated (loose) slope materials, larger and moreangular particles have a higher angle of repose

Slopes

Natural slopes should be viewed as landforms that have attained equilibrium with their surrounding conditions; if that equilibrium is disturbed by natural or human forces the slope may mass waste to reestablish its equilibrium

Other Mass Wasting Factors

The amount of water in the slope materials and the type and distribution of slope vegetation also affect slope stability (see slides)

Role of Water in Slope Stability

The more completelythe surface is coveredby vegetation and themore interlocking and denser the root system, the more stable a slope of unconsolidatedmaterials.

The removal of vegetation during land development or fire sets the stage for mass wasting.

Mass Wasting Triggers

Most mass wasting events are “triggered”Human and natural triggers existCommon human triggers:Slope oversteepeningVegetation removal Addition of excessive weight to slopeAddition of excessive water to slope (see

slide)

Mass Wasting Triggers

Four typical natural triggers:

1) Prolonged and/or intense rainfall, or rapid snow melting, that saturates the slope materials

Mass Wasting Triggers

2) Earthquakes – violent ground shaking induces ground failure

(5/17/2008)

FOX News

Mass Wasting Triggers

3) Volcanic eruptions – remember lahars?

Mass Wasting Triggers

4) Stream or wave undercutting – water erosion removes support for the overlying slope materials

Mass Wasting Classification

Rate: potentially quite fast; perhaps100 miles per hour

Rate: generally slow; a few feet toperhaps tens of feet per day

Rate: potentially quite fast; perhaps100 miles per hour

Rate: generally slow; a few feet toperhaps tens of feet per day

Californian slumps and flows

1995

2005 – renewed mass wasting claims ten lives

Rate: quitefast; up toabout 125miles per hour

Wave undercutting of slope

RockfallBlocks

Waveundercutting

Sediment Fall

Many subcategories: debris, mud, earth, lahar, snow avalanche,creep

Chaotic, turbulentinternal motion

Rate: vast range; extremely slow (inchesper year) toperhaps 600miles per hour

All fast-moving, extensive travel distance(tens of miles) flows possess ahigh degree of lubrication fromwater and/or air

California Mudflow

In 1970, a verypowerful earthquakein Peru triggered a debrisavalanche that buriedmost of the towns ofYungay and Ranrachircaand killed about 20,000 people. Estimatedmaximum speed offlow: 600 miles perhour!

SnowAvalanche

Why have deathsfrom snow avalanchesin North America been increasing?

Mass Wasting Types

Mass wasting movement rates vary from exceedingly slow to exceedingly fast

Don’t be fooled – even the very slow mass wasting processes can threaten human development over long time periods (see slides)

Rate: maybe12 inchesper year

Submarine Slumps

Mass Wasting Also Occurs Beneath Water

Recognition of Mass Wasting Hazards

The steeper the slope the higher the riskThe less vegetated the slope the higher

the riskKnowledge of the subsurface geology and

local/regional geologic and climatic conditions helpful

Geologically recent mass wasting events are often recognized by slope scars – sections of oversteepened slopes mostly devoid of vegetation (see slides)

1925

Recognition of Mass Wasting Hazards

1925, Wyoming

CNN.com; 5/26/08

Mass wasting slope scar

Slope scars

Human Intervention

Humans have used various methods to attempt to reduce the risk of mass wasting including: slope retention structures; mass wasting diversion structures, slope drainage; slope reduction; and slope stitching (see slides)

None of these methods is failsafe!

Concreteblock retentionstructure

Retention structuresmade of rock rubbleencased in wire mesh

Mass wasting diversion structure builtto divert snow avalanches, Canadian Rockies

(SlopeDrainage)

Drain Pipes

Slope Reduction

Figure 8.30A

Slope Stitching

U.S. Geological Survey (USGS) Landslide Warning System

Constructed in response to 1982 rain-triggered landslides that killed 25 in the San Francisco, CA area and caused $66 million damage

The USGS method is a mathematical prediction of slope stability based on: rainfall intensity and rainfall total, storm duration, slope angle and slope stability characteristics (e.g., soil/rock type, slope fluid content)

U.S. Geological Survey (USGS) Landslide Warning System

The mathematical relationships between the variables are determined from observations of previous landslides

First test of model: February, 1986; of ten landslides with known occurrence times, eight took place when forecast

Other Work

In addition, ground tilt meter and GPS measurements can be used to assess precursor changes to slope stability

Slope pore water pressures can also be measured – remember, the higher the slope material water pore pressure the greater the risk of mass wasting (see slides)

Slopestabilitymonitoringsite

Woodway, WA

Other Work

Attempts to improve these methods continue – see picture describing the USGS and US National Forest Service experimental flume and the use of “smart” rocks

Then review U.S. landslide potential map

U.S. Landslide Potential

United States Geological Survey