LANDSLIDES AND MASS ON SLOPE
evolution of many of Earth’s varied
landforms
Earth Slope
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Earth’s surface is never perfectly flat but
instead consists of slopes of many
different varieties.
Taken together, slopes are the most
common elements in our physical
landscape. Although most slopes may
appear to be stable and unchanging, the
force of gravity causes material to move
downslope. At one extreme, the
movement may be gradual and
practically imperceptible.
At the other extreme, it may consist of a
roaring debris flow or a thundering rock
avalanche. Landslides are a worldwide
natural hazard. When these hazardous
processes lead to loss of life and
property, they become natural disasters.
The Role of Mass Wasting in Landform
Development.
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Slopes Change Through Time
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Landslides as Geologic Hazards
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The term “landslide” has no specific
definition in geology. Rather, it is a
popular, nontechnical word used to
describe any or all relatively rapid forms
of mass wasting.
Landslides are spectacular examples of
a basic geologic process called mass
wasting.
Mass wasting, also known as mass
movement, is a general term for the
movement of rock or soil down slopes
under the force of gravity. It differs from
other processes of erosion in that the
debris transported by mass wasting is
not entrained in a moving medium, such
as water, wind, or ice
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The Importance of Mass Wasting
Some mass-wasting processes are
dangerous events that represent
significant geologic hazards. Perhaps
less well known is the fact that
mass-wasting processes play an
important role in the development and
In the evolution of most landforms, mass
wasting is the step that follows
weathering.
Weathering does not produce significant
landforms. Rather, landforms develop as
products of weathering are removed
from the places where they originate.
Once weathering weakens and breaks
rock apart, mass wasting transfers the
debris downslope, where a stream or
glacier, acting as a conveyor belt,
usually carries it away.
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It is clear that if mass wasting is to
occur, there must be slopes that rock,
soil, and regolith can move down.
It is Earth’s mountain building and
volcanic processes that produce these
slopes through sporadic changes in the
elevations of landmasses and the ocean
floor.
If dynamic internal processes did not
continually produce regions having
higher elevations, the system that moves
debris to lower elevations would
gradually slow and eventually cease.
Most rapid and spectacular
mass-wasting events occur in areas of
rugged, geologically young mountains.
As mountain building subsides, mass
wasting and eronal processes lower the
land. Through time, steep and rugged
mountain slopes give way to gentler,
more subdued terrain. Thus, as a
landscape ages, massive and rapid
mass-wasting processes give way to
smaller, less dramatic downslope
movements that are often imperceptibly
slow.
Controls and Triggers of Mass Wasting
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Gravity
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Gravity is the controlling force of mass
wasting, but several factors play an
important role in overcoming inertia and
creating downslope movements.
Long before a landslide occurs, various
processes work to weaken slope
material, gradually making it more and
more susceptible to the pull of gravity.
During this span, the slope remains
stable but gets closer and closer to
being unstable.
Eventually, the strength of the slope is
weakened to the point that something
causes it to cross the threshold from
stability to instability. Such an event that
initiates downslope movement is called a
trigger.
Water also adds considerable weight to a
mass of material. The added weight in
itself may be enough to cause the
material to slide or flow downslope.
Oversteepening of Slopes
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Water
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Mass wasting is sometimes triggered
when heavy rains or periods of
snow-melt saturate surface materials.
The water does not transport the
material. Rather, it allows gravity to more
easily set the material in motion.
When the pores in sediment become
filled with water, the cohesion among
particles is destroyed, allowing them to
move past one another with relative
ease.
For example, when sand is slightly
moist, it sticks together quite well.
However, if enough water is added to fill
the openings between the grains, the
sand will ooze out in all directions
(Figure 15.4).
Thus, saturation reduces the internal
resistance of materials, which are then
easily set in motion by the force of
gravity. When clay is wetted, it becomes
very slick—another example of the
“lubricating” effect of water.
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There are many situations in nature
where oversteepening takes place. For
example, as a stream cuts into a valley
wall, it removes material from the base of
the wall.
This causes the slope to become too
steep and material to fall or slide into the
stream. Furthermore, through their
activities, people often create
oversteepened and unstable slopes that
become prime sites for mass wasting.
Removal of Vegetation
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Plants protect against erosion and
contribute to the stability of slopes
because their root systems bind soil and
regolith together.
In addition, plants shield the soil surface
from the erosional effects of raindrop
impact. Where plants are lacking, mass
wasting is enhanced, especially if slopes
are steep and water is plentiful.
When anchoring vegetation is removed
by forest fires or by people (for timber,
farming, or development), surface
materials frequently move downslope
EARTHQUAKE AS TRIGGERS
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Among the most important and dramatic
triggers are earthquakes. An earthquake
and its aftershocks can dislodge
enormous volumes of rock and
unconsolidated material.
An earthquake can cause a slope to
become unstable by the inertial loading
it imposes or by causing a loss of
strength in the slope materials.
Landslides Without Triggers?
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Do rapid mass-wasting events always
require some sort of trigger, such as
heavy rains or an earthquake? The
answer is no; such events sometimes
occur without being triggered.
Study concluded that the landslide
occurred without triggering from any
discernible external conditions. Many
rapid mass-wasting events occur without
a discernible trigger. Slope materials
gradually weaken overtime under the
influence of long-term weathering,
infiltration of water, and other physical
processes. Eventually, if the strength
falls below what is necessary to maintain
slope stability, a landslide will occur. The
timing of such events is random, and
thus accurate prediction is impossible.
CLASSIFICATION OF MASS-WASTING
PROCESSSES
Rate of Motion
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The rate at which a mass is moving.
Typically measured in length per time
(centimeters/year, meters/second).
Type of Material
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Mass wasting processes are
distinguished by identifying the type of
material the descending mass is.
Usually identified as bedrock (in a rock
slide) or as unconsolidated material.
Amount of water (ice or snow) affects the
rate and type of movement
SOIL
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Also called as Engineering Soil
can be either debris, earth or mud.
Debris - coarse-grained fragments.
earth (not capitalized) - fine-grained.
sand, silt, clay.
Mud - combination of water, clay, and
silt.
Type of Movement
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Classified as flow, slide, or fall.
Amount of water (ice or snow) affects the
rate and type of movement.
Slide
Slump
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remains relatively intact, moving along
one or more well-defined surfaces.
Rotational Slide - movement along a
curved surface, the upper part moving
downward while the lower part moves
outward.
Translational Slide - movement
descending mass moves along a plane
approximately parallel to the slope of the
surface.
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Flow
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descending mass is moving downslope
as a viscous fluid.
Fall
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Slump in Lethbridge Area of Alberta
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occurs when material free-falls or
bounces down a cliff.
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RAPID MOVEMENT OF MASS WASTING
Type of slide (movement as a mass) that
takes place within thick unconsolidated
deposits (typically thicker than 10
metres)
involve movement along one or more
curved failure surfaces, with downward
motion near the top and outward motion
toward the bottom
They are typically caused by an excess
of water within these materials on a
steep slope
Slump in Lethbridge area of alberta
shown in the picture.
This feature has likely been active for
many decades, and moves a little more
whenever there are heavy spring rains
and significant snowmelt runoff
This feature has likely been active for
many decades, and moves a little more
whenever there are heavy spring rains
and significant snowmelt runoff.
The toe of the slump is failing because it
has been eroded by the small stream at
the bottom.
Mudflow and Debris Flow
Rockfall and Rockslide
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Rockfall
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when a mass of sediment becomes
completely saturated with water, the
mass loses strength, to the extent that
the grains are pushed apart, and it will
flow, even on a gentle slope.
This can happen during rapid spring
snowmelt or heavy rains, and is also
relatively common during volcanic
eruptions because of the rapid melting of
snow and ice. (A mudflow or debris flow
on a volcano or during a volcanic
eruption is a lahar.)
If the material involved is primarily
sand-sized or smaller, it is known as a
mudflow.
If the material involved is gravel sized or
larger, it is known as a debris flow.
typically forms in an area with steeper
slopes and more water than does a
mudflow.
In many cases, a debris flow takes place
within a steep stream channel, and is
triggered by the collapse of bank
material into the stream. This creates a
temporary dam, and then a major flow of
water and debris when the dam breaks.
This is the situation that led to the fatal
debris flow at Johnsons Landing, B.C., in
2012.
This event took place in November 2006
in response to very heavy rainfall. There
was enough energy to move large
boulders and to knock over large trees
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Rock fragments can break off relatively
easily from steep bedrock slopes, most
commonly due to frost-wedging in areas
where there are many freeze-thaw cycles
per year.
Happens because the water between
cracks freezes and expands overnight,
and then when that same water thaws in
the morning sun, the fragments that had
been pushed beyond their limit by the
ice fall to the slope below
Rockslide
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The sliding motion of rock along a
sloping surface.
In most cases, the movement is parallel
to a fracture, bedding, or metamorphic
foliation plane, and it can range from
very slow to moderately fast.
The word sackung describes the very
slow motion of a block of rock
(millimetres per year to centimetres per
year) on a slope
Earth flow
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Earth moves downslope as a viscous
fluid; the process can be slow or rapid.
Usually occur on hillsides that have a
thick cover of soil in which finer grains
are predominant, often after heavy rains
have saturated the soil.
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Typically, the flowing mass remains
covered by a blanket of vegetation, with
a scarp (steep cut) developing where the
moving debris has pulled away from the
stationary upper slope.
A landslide may be entirely an earthflow
with soil particles moving past one
another roughly parallel to the slope.
Commonly, rotational sliding (slumping)
takes place above the earthflow.
The figures each show a rotational slide
(upper part) and an earthflow (lower
part), and each can be called a
slump-earthflow.
soil remains relatively coherent block or
blocks that rotate downward and
outward, forcing the soil below to flow.
In March 1995, following an
extraordinarily wet year, a
slump-earthflow destroyed or severely
damaged fourteen homes in the
southern California coastal community
of La Conchita ( figure 9.8 B ).
In January 2005, following 15 days of
record-breaking rainfall, around 15% of
the 1995 landslide remobilized. A rapidly
moving flow of soil killed 10 people and
severely damaged or destroyed 36
houses. Because future landslides are
likely, the town of La Conchita was
abandoned.
People can trigger earthflows by adding
too much water to soil from septic tank
systems or by overwatering lawns.
in Los Angeles, a man departing on a
long trip forgot to turn off the sprinkler
system for his hillside lawn. The soil
became saturated, and both house and
lawn were carried downward on an
earthflow whose lobe spread out over
the highway below.
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Solifluction ("Soil flow")
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Creep(or soil creep )is very slow,
downslope movement of soil. One factor
It is a type of mass movement that is
common wherever water cannot escape
from the saturated surface layer by
infiltrating to deeper levels.
As solifluction movement is not rapid
enough to break up the overlying blanket
of vegetation into blocks, the
watersaturated soil flows downslope,
pulling vegetation along with it and
forming a wrinkled surfac
Permafrost
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SLOW MASS MOVEMENT
Creep
that contributes to creep is the
alternating expansion.
The rate of movement is usually less
than a centimeter per year and can be
detected only by observations taken
over months or years.
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Permafrost refers to a type of ground
that remains continuously frozen for at
least two consecutive years. It consists
of soil, sediment, and rock that remains
below 0°C (32°F) for extended periods,
Permafrost occurs where summers are
too short and cool to melt more than a
shallow surface layer. Deeper ground
remains frozen year-round.
Permafrost plays a crucial role in
maintaining the stability of ecosystems
and landscapes, but it is vulnerable to
thawing due to climate change and
human activities.
Preventing, Delaying, Monitoring, and
Mitigating Mass Wasting
We can't stop mass wasting completely
because it's a natural process that keeps
happening. But in lots of cases, we can
do things to make its harmful effects on
people and buildings less severe. And if
we can't do anything to slow it down or
make it less harmful, it might be best to
just move away from where it's
happening.
Underwater Landslide
A geological phenomena known as an
underwater landslide occurs when a pile of
rock, silt, or other material slides downslope
while being pulled downward by gravity on
the seafloor. These kinds of things may
happen in a lot of different underwater
environments, such undersea canyons,
continental slopes, and even the sides of
underwater volcanoes.
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Several factors can trigger underwater
landslides:
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Seismic Activity
Sediment Instability
Underwater Volcanism
Oceanographic Processes
Effects of underwater landslides:
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Tsunamis
Disturbance of Marine habitat
Underwater Infrastructure Damage
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Delaying mass wasting crucial for saving
lives and reducing property damage and
infrastructure loss.
Effective measures can be lifesaving and
mitigate extensive damage.
Caution needed to avoid activities that
could exacerbate mass wasting risks.
Common anthropogenic cause: Road
construction.
Applies to remote gravel roads for
forestry/mining and large urban/regional
highways.
Balance between delaying mass wasting
and avoiding actions that increase
susceptibility.
Responsible planning and construction
essential for minimizing human-induced
hazards.
Mass wasting usually can be prevented.
Proper engineering is essential when the
natural environment of a hillside is
altered by construction.
Some preventive measures can be taken
during construction.
A retaining wall is usually built where a
cut has been made in the slope, but this
alone is seldom as effective a deterrent
to downslope movement as people hope.
If, in addition, drain pipes are put
through the retaining wall and into the
hillside, water can percolate through and
drain away rather than collecting in the
soil behind the wall. Without drains,
excess water results in decreased shear
strength, and the whole soggy mass can
easily burst through the wall.
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Rockslides and rockfalls pose significant
threats on highways in mountainous
terrain.
Highway construction often involves
blasting and bulldozing into
mountainsides, creating steep slopes
and cliffs.
Bedrock with planes of weakness (e.g.,
joints, bedding planes, foliation planes)
determines rockslide hazards.
If planes of weakness are inclined into
the hill, there's no rockslide hazard (as in
Figure 9.23A).
However, if planes of weakness are
parallel to the slope of the hillside, a
rockslide may occur.
Understanding the orientation of planes
of weakness relative to road cuts is
crucial for assessing rockslide risks.
Various techniques employed to prevent
rockslides on highways.
Detailed geological studies conducted
before road construction to identify
potential hazards.
Choosing the least dangerous route
helps mitigate risks.
If road cuts are necessary through prone
bedrock, all potentially sliding rock can
be removed (as in Figure 9.23B).
Removal can be costly but significantly
reduces the risk of rockslides.
Prioritizing safety through proactive
measures ensures highway resilience and
traveler safety.
Monitoring Mass Wasting
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Necessity for warning systems in areas
prone to mass wasting events.
Purpose: Alert to changes in conditions
or imminent debris flow.
Critical for mitigating risks and ensuring
timely evacuation.
Early detection enhances preparedness
and saves lives.
Objective: Establish effective warning
systems to monitor and respond to potential
hazards.
Radio-transmitted, real-time monitoring
crucial for predicting dangerous mass
movements.
Example: U.S. Highway 50 in California's
Sierra Nevada monitored by U.S.
Geological Survey.
Instruments include buried pore
pressure gauges and motion sensors.
Data immediately available online, aiding
in timely responses to changing
conditions.
Importance of obtaining information on
land susceptibility before building or
buying to avoid mass wasting damage.
Mitigating Mass Wasting Risks
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Mass wasting events are often
unpredictable and unavoidable.
Effective measures can minimize
associated risks.
Example: Avalanche shelters on
highways in B.C. and western Alberta.
Similar features exist globally to protect
infrastructure from various types of
mass wasting.
Objective: Enhance safety and protect
infrastructure against natural hazards.
Debris Flow Mitigation along the Sea-to-Sky
Highway
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Debris flows are inevitable,
unpreventable, and unpredictable along
the Sea-to-Sky Highway.
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Deadly and costly incidents necessitate
proactive measures.
Developing a new route is financially
impractical.
Provincial authorities have constructed
debris-flow defensive structures in
drainage basins.
Strategies include:
Allowing debris to flow quickly through a
smooth channel to the ocean.
Capturing debris in constructed basins,
permitting excess water to continue.
Objective: Protect residents and traffic
while ensuring highway and railway
safety.