Lecture 14 Mass Movement
What is mass movement?
 Classification of mass wasting
 Type of movement
 Causes of slope movement

Robert L. Schuster/U.S. Geological Survey
Mass Movement
Mass movement (or mass wasting) is the movement
of soil or rock mass downslope under the influence of
gravity.
Venezuelan Mudslides
(December, 1999)
Venezuela’s worst natural disaster of
the last century.
 As much as 30,000 people were killed.
 Triggered by torrential rains and floods
over Venezuela’s Caribbean central
coastal area.

1999 Venezuela Mudslids

A mudslide ravaged neighborhood of Los Corales,
north of the capital Caracas.

Caracas, the capital. A woman tries to gather what she could
find in the remains of her destroyed house.

Caracas outskirts. A Venezuelan man surveys the destruction of
a mudlslide as he leaves his home with a suitcase.

A lone statue in a
destroyed plaza along
Venezuela's Caribbean
coast.

Classification of mass wasting

Mass movements are classified based on the
type of materials involved, the type of
movement, and the rate of movement. Thus we
have rock fall, debris flow, earth slump, etc.

type of material
Slope materials can be divided into bedrock
("rock"), or unconsolidated materials/soil
("debris", "mud", "earth").

Type of movement

fall





The mass pieces fall through the air. It is a common form of
movement on steep slopes. For example, this is the primary way
in which talus slopes are built from frost wedging.
slide
Slides occur when material remains fairly coherent and moves
along a weakness surface, such as a joint, a fault, or a bedding
plane.
flow
Flows occur when material moves downslope as a viscous fluid.
Most flows are saturated with water.

Rockfall: individual blocks drop in a free fall
from a cliff or steep mountainside.

An illustration of frost wedging. (Tarbuck and Lutgens)

Rockslide. Large masses of bedrocks move as
a unit in a fast downward slide.

Flows occur when material moves downslope
as a viscous fluid. Debris flow contains
material that is coarser than sand and can
travel a few km/hour. (Press and Siever)

Soil creep

Creep is the slow downslope migration of
soil and loose rock fragments.

One cause of creep is frost wedging.
Freezing lifts particles at right angles to
the slope, and thawing allows the particles
to fall back to a slightly lower level. Thus
each cycle moves the material a short
distance downhill.

The repeated expansion and contraction of
the surface material (frost wedging) causes a
net downslope migration of rock particles -- a
process called creep. (Tarbuck and Lutgens)

Creep is a slow movement, but its effects are
often visible. (Tarbuck and Lutgens)

A fence offset by creep in California (T. Amos)
Trees that grow in creeping soil gradually develop pronounced
curves. (Martin Miller)

Slump

similar to sliding movement, but the
descending material move along a curved
(circular) surface of rupture. Slumps are
common in soils or rocks of low shear
strength.

Slump occurs when material slips downslope
along a curved surface of rupture. Earthflows
and debris flows often form at the base of the
slump. (Tarbuck and Lutgents)

Slump is often triggered when slopes become
oversteepened by erosional processes such as
wave action. Point Fermin, CA (J.S. Shelton)

Debris flow
Debris flow contains material that is
coarser than sand, generally restricted to
channels and commonly result from
unusually heavy rainfall.
 Debris flows move faster than creeps and
slumps


Flows occur when material moves downslope
as a viscous fluid. Debris flow contains
material that is coarser than sand and can
travel a few km/hour. (Press and Siever)

Debris flows are common on the slopes of some
volcanoes (such flows are termed lahars). A lahar
along a river northwest of Mt. St. Helens.

Mudflow

Mudflow contains materials finer than sand and
large quantities of water. Mudflows are generally
even faster than debris flow.

Mudflows are rapid and are common in semiarid
regions and are confined to channels. Soil and
debris accumulate until infrequent rainfall
triggers high runoff. Water decreases shear
strength, and the wet mass flows rapidly down
the channel.

Mudflow in Mill Valley, California in Jan 1997,
triggered by heavy rains from a powerful
Pacific storm. (Tarbuck and Lutgens)

Rockslide

Rockslides occur when blocks of bedrock
break loose and slide down a slope.
Rockslides are among fastest and most
destructive mass movements.

Rockslides typically take place where the
rock strata are inclined.

Rock slides
along beding
planes. (West,
p.303)

A massive rockslide along the Madison River
of Montana triggered by Aug 17, 1959
earthquake. The rocks blocked the canyon
and created Earthquake Lake (J. Montagne)

Landslides

The term "landslide" has been used
vaguely by popular media for almost any
kind of slope failure.

True landslides occur along well-defined
slippage surfaces. Large blocks slump and
rotate downslope, and many grade into
debris flows at their lower margins.

Landslides occur along well-defined slippage surfaces.
Large blocks slump and rotate downslope and many
grade into debris flows at their lower margins. (Hamblin
and Christiansen)

Landslides are significant mass movements that are
often extremely destructive where development
encroaches on hill slopes. Hong Kong (C. Flecter)
 Speeds
of mass movement:
slow->fast
soil creep -> earth slump ->
debris flow, mud flow ->
rockslide -> rock fall, rock avalanche


Causes of slope movement
Slope stability analysis (P.297)
The controlling force of mass wasting is gravity. But
slope failure often involves many variables. A basic
approach to evaluating slope stability is to identify
the driving forces and the resisting forces.
factor of safety (FS)=
(total resisting forces)/(total driving
forces)
Mass Movement
Mass movement (or mass wasting) is the movement
of soil or rock mass downslope under the influence of
gravity.

Influence of water

Water is an important factor in the occurrence of mass
movement. Most rapid mass movements occur during or
after periods of heavy rainfall.

The addition of water increases pore pressure, decreases
the effective normal stress acting in the slope, and thus
decrease the shear resistance of the soil according to the
Mohr-Coulomb equation:
S = C + se tan(f)

On the other hand, the driving force increases with the
weight of water added to the soil.

Removal of lateral support

A common way to increase the driving
force on a potential failure plane is to
remove materials from the base of the
slope, by river erosion, previous slope
movements, or human modification such
as road cuts.
A river cuts into the base of a slope, steepening the sides of
the valley, making the slopes unstable.
Cutting terraces in a hill slope creates a steeper slope. (W.W.
Norton)
The Gros Ventre landslide, 1925 (near Jackson Hole, Wyoming).
River erosion cuts down to the weak shale, which dips parallel to
the slope. Rainfall in the weeks before seeped into the ground,
weakening the Amsden Shale and making the overlying rock layer
(Tensleep Sandstone) heavier. (W.W. Norton)

Vegetation
Plants protect against erosion and
contribute to the stability of slopes
because their root systems bind soil
together. Plants also absorb subsurface
water.
 Where plants are lacking (e.g. removed by
fire), mass wasting is enhanced, especially
when slopes are steep and water is
plentiful.


A fire that stripped of
vegetation and root
systems in
Yellowstone National
Park caused a
weakening of the soil,
making it susceptible
to erosion and mass
movement. (G. Meyer)
 Vegetation
…
Several decades ago, near steep slopes of
Menton, France, farmers replaced olive
trees (with deep roots) with a more
profitable carnations (with shallow roots).
When the slope failed, the landslide killed
11 people.

Earthquakes as triggers

Conditions that favor mass wasting may exist in
an area for a long time without movement
occurring. Earthquakes are dramatic triggers
that can dislodge enormous volumes of earth
materials.

In many areas, it is not ground vibrations
directly, but landslides and ground subsidence
triggered by the vibrations that cause the
greatest damage.

This 4-km long tongue of rubble was deposited
atop a glacier by a rock avalanche after the great
1964 Alaska earthquake. A rock avalanche is a
high velocity flow of large masses of broken rocky

Various forms of mass wasting can be
triggered by earthquakes. This landslide in
Pacific Palisades, CA was triggered by 1994
Northridge earthquake.
Preventing Mass Movements
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In many cases, the best solution is avoidance.
Geologists look for landform of past
movements, detect regions that are
beginning to move, or identify potential
hazards, to construct landslide-potential
maps.
Certain steps can be taken to remediate the
problem and stabilize the slope.
Revegetation:
Revegetation
removes water,
and tree roots bind
regolith. (W.W.
Norton)
Regrading:
Redistributing the
mass on a slope
eases the load
where necessary,
adds support
where necessary,
and decreases
slope angles.
Terracing a steep
slope may decrease
the load and provide
benches to catch
debris.
Reducing
subsurface water:
Lowering the level
of water table may
allow a glide
horizon to dry out.
Preventing
undercutting:
Relocating a river
channel away
from cliff stops
undercutting, and
filling the channel
adds support.
Reprap (lose
boulders or
concrete)
absorbs wave
energy along the
coast.
Constructing safety
structures: A
retaining wall traps
falling rock.
Bolting holds loose blocks in place. An avalanche shed diverts
avalanche debris over a roadway.