instructions to follow 1 page single spaced 10 font You have been asked to write an article for a magazine (or newspaper ? your choice) about a major mass wasting event (a landslide, muds

Mass wasting is the downslope movement of rock and soil under the

direct influence of gravity.

This is a particularly important geologic

activity in the S.F. Bay Area – we deal with

"landslides" every year during the rainy

season (Oct. -April).

Although these are often small-scale

events — some are very large and are

responsible for thousands of deaths.

Examples:

Peru (1970) – 20,000 people died during a gigantic

rock avalanche.

Colombia (1985) – 25,000 people died in the lahar

flows after the eruption of Nevado del Ruiz.

Italy (1960) – a rock slide caused a 90 meter high

wave of water in a mountain reservoir that killed

2,600 people. USGS

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I. Slope Stability

One key factor that affects slope stability is

the steepness of the slope.

In A, the pull of gravity is downward.

On a slope, the force of gravity can be

resolved into two components: a component

acting perpendicular to the slope (gp), and a

component acting parallel to the slope (gs).

In B and C, more of

the rock’s weight

(gs) is directed

downslope as the

slope slope

increases (vector gs

becomes larger).

Stephen A. Nelson, Tulane U.

A

B C

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Another important factor in slope stability

is the nature of material – some materials

are inherently stronger (granite v. shale).

The figure shows the slope profile for the

walls of the Grand Canyon. Differences in

the strength of the different rock types

where resistant layers (limestone and

sandstone) form steeper walls than

weaker rocks such as shale.

National Park Service

National Park Service

Although gravity is the controlling

force in mass wasting, other factors

may play an important role in mass

wasting:

1. Water

2. Oversteepened slopes

3. Vegetation

4. Earthquakes

USGS

The Role of Water in Mass Wasting

When the pore spaces between grains in a

sediment become filled with water – cohesion

between the grains is destroyed. Water

saturation reduces the internal resistance

(friction?) of materials to flow – the sediments

are less coherent and may flow under the

influence of gravity.

Water adds considerable weight to a package

of sediments – the added weight may be

enough to initiate flow under the influence of

gravity.

In the S.F. Bay Area, CalTrans and other

government agencies install plumbing systems

in steep slopes to help drain them (prevent

oversaturation) in hopes to prevent slope

failure (ex. Highway 92 near Half Moon Bay). Copyright by Andrew Alden, geology.about.com, reproduced

under educational fair use.

Loss of cohesion in

unconsolidated sediments by

oversaturation from water.

Images by Stephen A. Nelson, Tulane U.

The Role of Oversteepened Slopes in Mass Wasting

Unconsolidated particles assume a stable slope called the angleof

repose. This is the steepest angle at which the material remains stable

(usually varies from 25 to 40°).

If a slope angle is greater than the angle of repose for a particular

material, the slope is oversteepened and is subject to failure.

Many activities are responsible for

oversteepening of slopes:

1. A stream may undercut a valley wall.

2. Pounding waves against the base of a cliff may

undercut the cliff (we are seeing this happen

along the CA coast (Daly City).

3. People commonly oversteepen slopes by

construction of roads and landscaping.

USGS

USGS Vetiver Network

The Role of Vegetation in Mass Wasting

Plants protect against erosion and help stabilize slopes – their roots bind soil together.

Mass wasting is enhanced where plants are lacking – by fire, urban development, logging, etc. The image shows multiple landslides resulting from deforestation in Colombia.

In the S.F. Bay Area, CalTrans plants "at risk" slopes in the hope to help stabilize them.

El Colombiana

CalTrans

The Role of Earthquakes in Mass Wasting

Earthquakes play a major role in triggering landslides.

Although the conditions for a slope may be favorable for a failure

(slide), it sometimes requires an additional factor to "trigger" the

movement.

Earthquakes can provide the "nudge" that is required for an

oversteepened slope to fail.

Remember that the

eruption of Mount St.

Helens was triggered by

a giant landslide. In turn,

the landslide that was

responsible for the

eruption was itself

triggered by an

earthquake due to the

movement of magma

underground. USGS

The general types of mass wasting processes and the classification scheme is based upon

1. Type of Material – classification depends

upon whether the material began as

unconsolidated sediments (and soil) or as

bedrock.

2. Type of Motion – Kind of motion may be

described as fall, slide or flow

• Fall – free-fall of detached material. Common

on slopes so steep that there is no

accumulation of material on the slope. Many

falls may result from mechanical weathering

such as frost wedging and the action of tree

roots.

• Slide – material remains fairly coherent and

moves along a well-defined surface such as a

joint, fault, or bedding plane.

• Flow – material moves downslope as a viscous

fluid.

3. Rate of Movement – can be very rapid (>

125 miles/hour) or imperceptibly slow –

ranges from sudden to gradual movement.

II. Classification of Mass Wasting Processes

USGS

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Falls

A fall involves rapid movement of material in free

fall.

Rock falls occur when a slab of rock travels

mostly vertically through the air, then after

hitting the ground, rolling and bounding.

Triggers tend to be heavy rain, frost wedging,

earthquakes, animals and people. Stephen A. Nelson, Tulane U.

Although falls are a hazard, they are

an important process that helps to

widen valleys and erode mountains

over geologic time.

Tom Trujillo, Associated Press (SFGate)

The above image shows the damage at

Curry Village after a rockfall.

Lloyd DeForest, USGS

Slides

Slidesoccur when blocks of

bedrock and/or soil break loose

and slide down a slope – these are

fast and destructive types of mass

wasting events.

Rockslides occur where layers

are inclined or where joints or

fractures are parallel to the slope.

These are most commonly

triggered by heavy rains or snow

melts which "lubricate" the slide

surface.

Herbythyme, Wikimedia Commons

On July 10, 1996, a giant rockslide

occurred in Yosemite. The left image

shows the detachment zone of the

rockslide and the impact areas (C/D, B).

In addition, an air blast from the impact

flattened 10 acres of forest. The blast

area is shown in the lower left region.

The right figure shows a

close-up of the blast zone.

Rock fragments were

embedded in trees for

hundreds of feet into the

forest. UCB Seismological Lab

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Chaos Crags

Chaos Jumbles

The Chaos Jumbles in Mt. Lassen

NP were formed by a huge

rockslide that occurred ~300

years ago when the volcanic peaks

of the Chaos Crags collapsed. The

slide had a lot of horizontal

motion because the debris rode

on a bed of compressed air.

John Soares

Slump

A slump is a downward sliding mass of rock or unconsolidated

sediment as a unit along a curved surface — this is very common in the

S.F. Bay Area.

A slump commonly occurs because a slope has been oversteepened.

A crescent-shaped scarp is created and the block's upper surface is

sometimes tilted backwards.

The slump may occur as a

single mass or as a series

of blocks.

Water can percolate down

through the fracture and

cause more movement.

USGS

Slumps on Mars.

NASA

Flows

A flow is a relatively rapid type

of event involving a flow of

soil &/or rock &/or mud

containing a large amount of

water.

Because of their fluid

properties, debris flows follow

canyons and stream.

Clatskanie, OR PUDUSGS

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Two main types of occurrences of

flows:

1.In semiarid regions – cloudburst

or snowmelt creates a sudden flood

that entrains a lot of soil because

there is no vegetation to anchor

the surface material. The result is a

lobe- or tongue-shaped fan of well-

mixed soil, mud and rock.

2.Lahars – occur on

stratovolcanoes. Eruptions suddenly

melt snow and glaciers that release

a tremendous amount of material –

ex. Mount St. Helens.

USGS

USGS

Debris flows are very

common in the Coast Range

of California.

The images show

Mt. Kazbek in S.

Russia before and

after the recent

collapse of the

Kolka Glacier that

triggered a debris

flow that traveled

more than 15

miles. The

avalanche buried

small villages killing

dozens of people.

NASA

Oct. 3, 2001 Sept. 27, 2002

The long, dark grey streak running upward through the center of the

scene shows the gorge that was overrun by the debris flow. The deep

reds show vegetated land surfaces, grey areas are bare rock, and white

shows ice-covered lands.

Earth flows are common on

hillsides in humid areas, during

times of heavy precipitation.

When the soil saturates, the

material may break away, leaving a

scar on the slope and forming a

tongue- or teardrop-shaped mass

downslope -common in the S.F.

Bay Area.

The materials are most commonly

rich in clay and silt.

Since earth flows are viscous, they

move slower than debris flows

and may remain active for a long

period of time.

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There are several ways to try to

prevent landslides. Most prevention

methods employ various engineering

controls to minimize the hazard and

stabilize a slope.

If a landslide occurs, mitigation means

reducing the effects or the intensity of

the landslide.

After a landslide occurs, the first task is

to remove the landslide material and

stabilize the slope.

Most methods of mitigation overlap

with preventive measures.

La Conchita, CA

Pacifica, CA

USGS

USGS

III. Landslide Prevention and Mitigation

1. Vegetation. Planting vegetation is

particularly effective in stabilizing

slopes that consist of sediment. The

roots bind the loose sediment and

may penetrate to the underlying rock

to anchor the sediment. Vegetation

is deep roots is more effective.

Vegetation also helps stabilize slopes

by absorbing water from the soil Sathiyam TV

Toyo Greenland

Ach Consulting Engineers

Caltrans

2. Retaining Walls. The purpose

behind a retaining wall is to

strengthen an oversteepened slope.

Retaining walls are especially

common along roadsides where a flat

or level surface has been cut into a

slope for the roadway.

Orange County Transportation Authority

3. Controlling Water. We saw

that water plays an important

role in mass wasting. Various

engineering controls are

employed to “dewater” a slope

to increase its stability in wet

conditions.

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4. Terracing. Terracing involves a series of benches on a hillside.

Frequently, retaining walls are also used to stabilize the steepened

portions of the slope.

Terracing is effective where it is not feasible to build a single large

retaining wall.

Gerald Tyner, CSULBSFGate

Janod Corp.

Geobrugg

Earth Stability Limited

5. Rock Bolts. Rock slides and falls are

common where the slope consists of

fractured rock. Rock bolts are used to

anchor fractured rock to more massive

rock.

Rock bolts are installed by drilling a hole

through the slope then inserting and

anchoring the bolt.

Rock bolts are frequently installed with

flexible metal mesh to help stabilize the

slope and prevent material from falling.