Landslides and their controls

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Course Title: Earth Science
Paper Title: GEOHAZARDS AND DISASTER
MANAGEMENT
Landslides and their ControlLandslides and their Control
By
Prof.A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore, India

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Table of Contents

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 After attending this lesson, the user would be able
to understand the nature and causative factors of
landslides, their characteristics, classifications,
triggering mechanisms, and effects.
 The methods of controlling the effects of
landslides, and avoiding their menace are also
highlighted.
Objectives
(…Contd)

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Disaster management methods are to be
adopted to mitigate the never ending natural
hazards.
This lesson is an important topic in disaster
management.
Objectives

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 Geologists, engineers, and other earth science
professionals often rely on the unique and
slightly differing definitions of landslides.
This diversity in definitions reflects the complex
nature of the disciplines that are associated with
studying the landslide as a major phenomena.
Introduction
(…Contd)

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 Landslide is a general term used to describe the
downslope movement of soil, rock, and organic
materials under the effects of gravity and also the
landform that results from such movement.
 Varying classifications of landslides are associated
with specific mechanics of slope failure and the
properties and characteristics of failure types.
 It is a major subject of study in disaster
management.
Introduction

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 Earth’s natural calamities include Earthquakes,
Tsunamis, Volcanic eruptions, Cyclones, Floods, and
Landslides.
 Whenever such natural hazards occur, there will be
a severe loss to life and properties.
 Landslides are the regular natural hazards
experienced in the hilly and mountainous areas all
over the world.
Natural Calamities
(…Contd)

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 The occurrences of landslides have been recorded by
many countries. Landslide is a general term used to
describe the downslope movement of soil, rock, and
organic materials under the effects of gravity and also
the landform that results from such movement.
 Varying classifications of landslides have been
proposed in association with specific mechanics of
slope failure, properties and characteristics of failure
types.
Natural Calamities

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 Hence, it is necessary to understand the
following aspects of landslides:
 Landslides as natural hazards
 Causes of landslides
 Classification of landslides
 Landslides and other hazards
 Effects and evaluation of landslides
Understanding the landslides

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 Hills and mountains are typical geomorphic
features.
 They are characterized by gentle or steep slopes.
 Sometimes they may have cliff like vertical slopes.
 They are all called as hill-slopes.
 Hill slopes are typical geomorphic conditions.
Hill slopes
(…Contd)

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 They are unique climatic zones.
 They are characterized by thick soil profiles,
richness with soil moisture and plant growth.
 Hillslopes have a good drainage of rainwater.
 The bed rock configurations are also very unique.
Hill slopes
(…Contd)

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 The Subsurface Conditions of a hilly terrain may
have two distinct zones as overburden and the
basement rock.
 The overburden may be mostly loose soil, may
have a thick weathered zone, adequate natural or
man-made vegetation, good root penetration, and
all are under the direct influence of Climatic
variations, especially rainfall.
Hill slopes

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Landslides as natural hazards
 Landslides normally occur on hillslopes.
 When the slopes are stable there will be no
problem.
 When the slopes become unstable then they may
lead to landslides.
(…Contd)

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Landslides as natural hazards
 Landslides occur when the stability of the slope
changes from a stable to an unstable condition.
 The change in the stability of a slope may happen
due to several factors.
 These factors may be acting alone or acting
together.

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Impacts of landslides
 Occurrence of landslides also change the general
landscape or the geomorphic conditions.
 The topography and drainage get modified due to
these hazards.
 Landslides can destroy the forests and other
resources.
(…Contd)

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Impacts of landslides
 Landslides will also damage the man-made
structures like roads, railways, tunnels and
buildings on the hilly regions.
 The study of landslides helps in identifying the
weak zones, classifying the hill slopes in to
different categories and to minimize the impacts of
landslides.

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 The Bedrock is related to the basement rocks of
the hill. These may be massive or fresh,
structurally weak or deformed.
 All of these may be under the influence of a
tectonic force originating from the deep interior
or a nearby huge structure like a dam.
 The next factor is the degree of slope of the hills.
Slope and bedrocks
(…Contd)

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 Slope is the common factor for a landslide. The
categories of slope is to be understood first.
 The term slope relates to the angle of inclination of
the land surface.
 It varies depending upon the geomorphic conditions,
bedrock, nature of overburden, vegetation and
drainage systems.
Slope and bedrocks

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The following are the categories of slopes:
 Cliff > 80 degrees
 Precipitous 50-80 degrees
 very steep or steep 20-50 degrees
 moderate slope 6-20 degrees
 gentle slope 1-6 degrees
 flat terrain < 1degree.
Categories of slopes

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 There is an engineering classification of slopes as
stable (or) unstable slopes.
 The Hill slopes may be stable or unstable.
 The slope instability comes due to the relations
between overburden and bedrock.
 It also depends on the limiting angles which
control the geomorphic process acting on it.
Classification of slopes
(…Contd)

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 Stable slopes are below the limiting angle for mass
movement. They are normally ranging from 38 to 40
degrees.
 Slope instability comes due to the geological,
geomorphological and hydrological conditions.
 These include hill-slope processes, changes in
vegetation, landaus practices, human activities,
frequency and intensity of precipitation and local
seismicity if any.
Classification of slopes

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 The forces acting on the slopes are of two fold as
Driving Force (or) Resisting Force.
 Landslide occurs when the driving force is greater
than the resisting force.
 All happens under the influence of gravity.
 The increase in driving forces (weight of slope) may
happen due to adding water or adding structures.
Forces acting on slopes
(…Contd)

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 The decrease in resisting forces may happened due
to decrease in weight at the bottom of slope or
over-steepening or by removing the toe of slope or
due to decrease the frictional energy or by adding
water which may lubricate and reduces the
strength of the rock.
Forces acting on slopes

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 Landslides are caused due to several factors. Some
may be natural, and some may be human induced.
 The natural causes of landslides include:
 Change in Hydrologic conditions: The groundwater
(pore water) pressure may be acting to destabilize
the slope.
Natural causes of landslides
(…Contd)

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 Change in vegetation: There will be loss or absence
of vertical vegetative structure, soil nutrients, and
soil structure due to forest fires or mining
 Removal of surface mass: The erosion of the toe of a
slope by running river water or rainwater
 Snowmelt: Weakening of a slope through
saturation by snow melt, glaciers melting, or due to
heavy rains
(…Contd)

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 Seismically induced changes: The occurrence of
micro-seismic waves or tremors.
 This may also include earthquake-induced
liquefaction which may be destabilizing the
slopes.
 Volcanic eruptions- may also cause landslides.

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The geological factors influencing landslides are:
 Continuous rainfall, heavy rainfall
 Slope beyond the limiting slope
 Soil type, thickness of soil-profile
 Bulk-density, permeability and moisture
 soil flowage - increase in soil moisture
 Plasticity index(Atterberg limit=amount of water added to
change a soil from plastic to liquid state of movement).
Geological factors

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Role of vegetation
 Vegetation plays a dominant role in this hazard. It has
both positive and negative impacts on the slopes. The
factors influencing are:
 Density of vegetation- dense or sparse
 Type of vegetation, grass, plants or trees
 Canopy and weight
 Root penetration and its role in infiltration
 Age and life of vegetation
 Rejuvenation pattern of forests

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 The overland flow of water play a distinctive role. In
fact, water plays the major role in the origin of
landslides. The factors are:
 Heavy rainfall will induce more percolation
 Increase in the levels of saturation of subsurface soil
horizons will induce forces drainage patterns, stream
frequency and density.
 When water infiltrates, a slippery face is created
above the bed-rocks.
Role of water

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 There are certain Morphological causes for
landslides. They are:
 Tectonic or volcanic uplift
 Glacial rebound
 Fluvial, wave, or glacial erosion of slope toe or
lateral margins
 Subterranean erosion (solution, piping)
Morphological causes
(…Contd)

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 Deposition loading slope or its crest
 Vegetation removal (by fire, drought)
 Thawing
 Freeze-and-thaw weathering
 Shrink-and-swell weathering.
Morphological causes

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 Although landslides are primarily associated with
mountainous regions, they can also occur in areas
of generally low relief. In low-relief areas,
landslides occur as
 cut-and-fill failures (roadway and building
excavations),
 river bluff failures,

Slides in Low-lying areas
(…Contd)

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 lateral spreading landslides,
 collapse of mine-waste piles (especially coal), and
 a wide variety of slope failures associated with
quarries and open-pit mines.

Slides in Low-lying areas

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Landslides are also caused due to human activities.
The major human causes include:
 Deforestation & Cultivation
 Disturbing the drainage patterns through cut and fill
operations
 Construction, Blasting, Mining and Quarrying
 Vibrations from machinery or heavy traffic and
movement of heavy vehicles.
Human activities
Human activities
(…Contd)

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 Earthwork which alters the shape of a slope and
impose new loads on the existing slopes
 Removal of deep-rooted vegetation
 Bushfires for clearing trees.
Human activities
Human activities

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 Landslides are classified into various types based on
nature of materials involved and type of movement
involved.
 The type of movement describes the actual internal
mechanics of how the landslide mass is displaced
like fall, topple, slide, spread, or flow.
Classification of landslides
(…Contd)

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 Similarly, the material in a landslide mass may be
either a rock or a soil (or both).
 The soil is described as earth if mainly composed of
sand-sized or finer particles and debris if it is
composed of coarser fragments.
 Each type of movement can be further subdivided
according to specific properties and characteristics.
(…Contd)

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 Based on the nature of materials involved,
landslides are classified into Rockslides, Debris
slides, and Earthslides/landslides.
 Based on the type of movement, landslides are
classified into these types:

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Slides
 Although many types of mass movements are
included in the general term "landslide" the more
restrictive use of the term refers only to mass
movements, where there is a distinct zone of
weakness that separates the slide material from
more stable underlying material.
 The two major types of slides are rotational slides
and translational slides.

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 This is a slide in which the surface of rupture is
curved concavely upward and the slide movement
is roughly rotational about an axis that is parallel
to the ground surface and transverse across the
slide.
 The displaced mass may, under certain
circumstances, move as a relatively coherent mass
along the rupture surface with little internal
deformation.
Rotational slides
(…Contd)

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 The rock slump is a rotational slide. It is related to
sliding of a mass of weak rock on a cylindrical or
ellipsoidal rupture surface which is not
structurally-controlled.
 There will be little internal deformation. There
may have a large main scarp and characteristic
back-tilted bench at the head.
 They are usually slow events.
Rotational slides

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 Because rotational slides occur most frequently
in homogeneous materials, they are the most
common landslide occurring in “fill” materials.
 These are associated with slopes ranging from
about 20 to 40 degrees. In soils, the surface of
rupture generally has a depth-to-length ratio
between 0.3 to 0.1.

Occurrence of Rotational slides
(…Contd)

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 The velocity of travel or rate of movement are
extremely slow (less than 0.3 meter or 1 foot
every 5 years) to moderately fast (1.5 meters or 5
feet per month) to rapid.

Occurrence of Rotational slides

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 These triggered by intense and (or) sustained rainfall or
rapid snowmelt. Such events may lead to the saturation of
slopes and increased groundwater levels within the mass.
 They are also caused due to rapid drops in river water
level following flood events, ground-water levels rising as a
result of filling reservoirs, or the rise in level of streams,
lakes, and rivers.
 All these activities can cause erosion at the base of slopes.
These types of slides can also be earthquake-induced.
Triggering mechanism of slides

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 Rotational slides can be extremely damaging to
structures, roads, and lifelines.
 They are not usually life-threatening if movement
is slow. Structures situated on the moving mass
also can be severely damaged as the mass tilts and
deforms.
 The large volume of material that is displaced is
difficult to permanently stabilize. Such failures can
dam rivers and lead to severe flooding.
Effects (direct/indirect) of slides

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 Instrumental monitoring to detect movement and
the rate of movement is a major method to be
implemented.
 The disrupted drainage pathways should be
restored or reengineered to prevent future water
buildup in the slide mass.
Mitigation measures
(…Contd)

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 It is necessary to establish proper grading and
engineering of slopes, wherever possible.
 This will reduce the hazard considerably.
 Construction of retaining walls at the toe may be
effective to slow or deflect the moving soil.
Mitigation measures

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Predictability of Rotational slidesPredictability of Rotational slides
 The historical slides can be reactivated at any
time. So, prediction is necessary.
 The distribution of cracks at the tops (heads) of
the slopes are good indicators of the initiation of
failures.
 Mapping of such regions and controlling their
triggers is necessary.

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 In this type of slide, the landslide mass moves along
a roughly planar surface with little rotation or
backward tilting.
 A block slide is a translational slide in which the
moving mass consists of a single unit or a few closely
related units that move downslope as a relatively
coherent mass.
Translational slides
(…Contd)

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 The translational rockslide is the sliding of a mass of
rock on a planar rupture surface, or a wedge of two
planes with downslope-oriented intersection.
 The rupture surface may be stepped. There will be
no internal deformation.
 The slide head may be separating from stable rock
along a deep, vertical tension crack. These are
usually extremely rapid events.
(…Contd)

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 The material in the slide may range from loose,
unconsolidated soils to extensive slabs of rock, or
both.
 Translational slides commonly fail along geologic
discontinuities such as faults, joints, bedding
surfaces, or the contact between rock and soil.

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 One of the most common types of landslides,
occurring all over the world.
 They are found globally in all types of
environments and conditions.
 Their movement is shallower than rotational
slides.
Occurrence of Translational slides
(…Contd)

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Occurrence of Translational slides
 The surface of rupture has a distance-to-length
ratio of less than 0.1 and can range from small
(residential lot size) failures to very large, regional
landslides that are kilometers wide.
 Their movement may initially be slow (5 feet per
month or 1.5 meters per month) but many are
moderate in velocity (5 feet per day or 1.5 meters
per day) to extremely rapid. With increased
velocity, the landslide mass of translational
failures may disintegrate and develop into a debris
flow.

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 These are triggered primarily due to intense
rainfall, rise in ground water within the slide due to
rainfall, snowmelt, flooding, or other inundation of
water resulting from irrigation, or leakage from
pipes or human-related disturbances such as
undercutting.
 These types of landslides can also be induced by
earthquakes or tremors.
Triggering mechanism and effects
(…Contd)

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 Translational slides may initially be slow,
damaging property and (or) lifelines. In some
cases, they can gain speed and become life-
threatening.
 They also can dam rivers, causing severe flooding.
Triggering mechanism and effects

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 These are to be controlled by adequate drainage
systems. It is necessary to prevent sliding or, in
the case of an existing failure, to prevent a
reactivation of the movement.
 The other common corrective measures include
leveling, proper grading and drainage, and
construction of retaining walls.
Mitigation measures
(…Contd)

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 More sophisticated remedies in rock include anchors,
bolts, and dowels, which in all situations are best
implemented by professionals.
 Translational slides on moderate to steep slopes are
very difficult to stabilize permanently.
 These slides have high probability of occurring
repetitively in areas where they have occurred in the
past, including areas subject to frequent strong
earthquakes. Widening cracks at the head or toe bulge
may be an indicator of imminent failure.
Mitigation measures

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 Rock collapse are also sliding events of a rock mass
on an irregular rupture surface consisting of a
number of randomly-oriented joints.
 Rotational Soil Slumps are sliding of a cohesive soil
mass on a cylindrical or ellipsoidal rupture surface.
They show little internal deformation.
 Debris slide is also a sliding of a landmass of granular
material on a shallow, planar surface parallel with
the ground.
Rock collapse and soil slumps

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Falls
 Falls are abrupt movements of masses of geologic
materials, such as rocks and boulders, that
become detached from steep slopes or cliffs.
 Separation occurs along discontinuities such as
fractures, joints, and bedding planes, and
movement occurs by free-fall, bouncing, and
rolling
(…Contd)

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Falls
 Falls are strongly influenced by gravity,
mechanical weathering, and the presence of
interstitial water.
 A fall begins with the detachment of soil or rock, or
both, from a steep slope along a surface on which
little or no shear displacement has occurred.

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 Falls are abrupt, downward movements of rock or
earth, or both. They get detached from steep
slopes or cliffs.
 The falling material usually strikes the lower slope
at angles less than the angle of the fall, causing a
bouncing action.
 The falling mass may break on impact, may begin
rolling on the steeper slopes, and may continue to
move until reaching a flat terrain.
Rockfalls
(…Contd)

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 These are common, worldwide, on steep or vertical
slopes.
 They are also seen in coastal areas, and along rocky
banks of rivers and streams.
 The volume of material in a fall can vary
substantially, from individual rocks or clumps of soil
to massive blocks thousands of cubic meters in size.
 The velocity may be very rapid to extremely rapid.
Rockfalls
(…Contd)

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 There may be free-fall, bouncing and rolling action of
the detached soil, rock, and boulders.
 The rolling velocity depends on slope steepness.
 The triggering mechanism may be undercutting of
slope by natural processes such as streams and
rivers or differential weathering (such as the
freeze/thaw cycle), human activities such as
excavation during road building and (or)
maintenance, and earthquake shaking or due to
intense vibration, by traffic.
Rockfalls

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Effects of rockfalls (direct/indirect)
 The falling material can be life-threatening.
Rockfalls can damage property beneath the
fall-line of large rocks.
 Boulders can bounce or roll great distances and
damage structures or kill people.
 Damage to roads and railroads is particularly
high: rockfalls can cause deaths in vehicles hit
by rocks and can block highways and railroads.

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 These are controlled by constructing rock
curtains or other slope covers, protective covers
over the roadways, and by constructing retaining
walls to prevent rolling or bouncing.
 The removal of mass in hazardous target areas,
removal of rocks or other materials from
highways and railroads can be used.
Corrective measures/mitigation of rockfalls
(…Contd)

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 Rock bolts or other similar types of anchoring
may be used to stabilize the cliffs. Scaling may
lessen the hazard.
 Warning signs are recommended in hazardous
areas for public awareness.
 Stopping or parking under hazardous cliffs should
be avoided.
Corrective measures/mitigation of rockfalls

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 Mapping of hazardous rockfall areas should be
done.
 Rock-bounce calculations and estimation methods
for delineating the perimeter of rockall zones are
to be attempted.
 These list of information should be widely
published.
Predictability
(…Contd)

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 The indicators of imminent rockfall include
terrain with overhanging rock or fractured or
jointed rock along steep slopes.
 They should be notified.
 The areas that are subjected to frequent freeze-
thaw cycles may be affected regularly.
Predictability

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 A topple is the forward rotation out of a slope of a
mass of soil or rock. It happens around a point or
axis below the centre of gravity of the displaced
mass.
 Toppling is sometimes driven by gravity exerted by
the weight of material upslope from the displaced
mass.
Topples
(…Contd)

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 Toppling failures are distinguished by the
forward rotation of a unit or units about some
pivotal point, below or low in the unit, under the
actions of gravity and forces exerted by adjacent
units or by fluids in cracks. These are known to
occur globally.
Topples
(…Contd)

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 They are often prevalent in columnar-jointed
volcanic terrain, as well as along stream and river
courses where the banks are very steep.
 Their velocity of travel are extremely slow to
extremely rapid, sometimes accelerating
throughout the movement depending on the
distance of travel.
Topples

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Triggering mechanism of topples
 Topples are sometimes driven by gravity exerted by
material located upslope from the displaced mass
and sometimes by water or ice occurring in cracks
within the mass.
 The other reasons are the vibration, undercutting,
differential weathering, excavation, or stream
erosion.
 The effects can be extremely destructive, especially
when failure is sudden and (or) the velocity is rapid.

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 In rocky terrains, there are many options for the
stabilization of topple-prone areas.
 One approach is the reinforcement of these slopes
include rock bolts and mechanical and other types of
anchoring.
 Seepage is also a contributing factor to rock
instability.
 Hence, the drainage should be considered and
addressed as a corrective measure.
Corrective measures/mitigation

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 Topples are not generally mapped for
susceptibility.
 But some inventory of their occurrence should be
done in certain areas. Monitoring of topple-prone
areas is a useful method. The use of tiltmeters is
inevitable.

Predictability of topples
(…Contd)

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 Tiltmeters are used to record changes in slope
inclination near cracks and areas of greatest
vertical movements.
 Warning systems based on movement measured
by tiltmeters could be effective methods of
control.

Predictability of topples

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 A flow is a spatially continuous movement in
which the surfaces of shear are short-lived, closely
spaced, and usually not preserved.
 The component velocities in the displacing mass
of a flow resemble those in a viscous liquid.
 Often, there is a gradation of change from slides
to flows, depending on the water content,
mobility, and evolution of the movement.
Flows
(…Contd)

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 There are five basic categories of flows that differ
from one another in fundamental ways.
 They are debris flows, debris avalanches,
earthflows, mudflows and creeps.
Flows

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 A debris flow is a form of rapid mass movement in
which a combination of loose soil, rock, organic
matter, air, and water mobilize as a slurry that flows
downslope.
 Debris flows include <50% fine-grained mass.
 Debris flows are commonly caused by intense
surface-water flow, due to heavy precipitation or
rapid snowmelt, that erodes and mobilizes loose soil
or rock on steep slopes.
Debris flow
(…Contd)

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 Debris flows occur around the world and are
prevalent in steep gullies and canyons; they can
be intensified when occurring on slopes or in
gullies that have been denuded of vegetation due
to wildfires or forest logging.
 They are common in volcanic areas with weak
soil.
Debris flow

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Lahars (Volcanic Debris Flows)
 The word “lahar” is an Indonesian term. Lahars are
also known as volcanic mudflows.
 These are flows that originate on the slopes of
volcanoes and are a type of debris flow.
 A lahar mobilizes the loose accumulations of tephra
(the airborne solids erupted from the volcano) and
related debris.
(…Contd)

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Lahars (Volcanic Debris Flows)
 They are found in nearly all volcanic areas of the
world. Lahars can be hundreds of square
kilometers or miles in area and can become larger
as they gain speed and accumulate debris as they
travel downslope.
 They can be small in volume and affect limited
areas of the volcano and then dissipate
downslope.

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Debris avalanche
 Debris avalanches are essentially large, extremely
rapid, often open-slope flows formed when an
unstable slope collapses and the resulting
fragmented debris is rapidly transported away
from the slope.
 In some cases, snow and ice will contribute to the
movement if sufficient water is present, and the
flow may become a debris flow and (or) a lahar.
(…Contd)

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Debris avalanche
 They occur worldwide in steep terrain environments.
 They are also common on very steep volcanoes where
they may follow drainage courses.
 There are large avalanches that have been known to
transport material blocks as large as 3 kilometers in
size, several kilometers from their source.
(…Contd)

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Debris avalanche
 Their velocity of travel may be rapid to extremely rapid.
 Debris avalanches can travel close to 100 meters/sec.
 In general, the two types of debris avalanches are those
that are “cold” and those that are “hot.”

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 The debris avalanches may travel several kilometres
before stopping, or they may transform into more
water-rich lahars or debris flows that travel many
tens of kilometres farther downstream.
 Such failures may inundate towns and villages and
impair stream quality.
Effects (direct/indirect) of Debris avalanches
(…Contd)

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 They move very fast and thus may prove deadly
because there is little chance for warning and
response.
 It is necessary to avoid constructions in the valleys
on volcanoes or steep mountain slopes.
 Real-time warning systems may help in lessening the
damages.
(…Contd)

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 However, warning systems may prove difficult due
to the speed at which debris avalanches occur—
there may not be enough time after the initiation of
the event for people to evacuate.
 Debris avalanches cannot be stopped or prevented
by engineering means because the associated
triggering mechanisms are not preventable.

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 Earthflows have a characteristic "hourglass" shape.
 The slope material liquefies and runs out, forming
a bowl or depression at the head.
 The flow itself is elongate and usually occurs in
fine-grained materials or clay-bearing rocks on
moderate slopes and under saturated conditions.
 However, dry flows of granular material are also
possible.
Earthflows
(…Contd)

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 Earthflows occur worldwide in regions underlain
by fine-grained soil or very weathered bedrock.
 The flows can range from small events of 100
square meters in size to large events
encompassing several square kilometres in area.
 Their velocity of travel may be slow to very
rapid.
Earthflows

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 The triggers include saturation of soil due to
prolonged or intense rainfall or snowmelt,
sudden lowering of adjacent water surfaces
causing rapid drawdown of the ground-water
table, stream erosion at the bottom of a slope,
excavation and construction activities, excessive
loading on a slope, earthquakes, or human-
induced vibration
Triggers and effects of earthflows
(…Contd)

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 The effects may be rapid.
 They may run out for considerable distances,
potentially resulting in human fatalities,
destruction of buildings and linear infrastructure,
and damming of rivers with resultant flooding
upstream and water siltation problems
downstream.
(…Contd)

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 Slower earthflows may damage properties and sever
linear infrastructure.
 Grading of slopes and protecting the base of the
slope from erosion or excavation is needed.

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 A mudflow is an earthflow consisting of material
that is wet enough to flow rapidly and that
contains at least 50 percent sand-, silt-, and clay-
sized particles.
 In some instances, for example in many
newspaper reports, mudflows and debris flows
are commonly referred to as "mudslides."
Mudflow

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 Creep is the informal name for a slow earthflow
and consists of the imperceptibly slow, steady
downward movement of slope-forming soil or
rock.
 Movement is caused by internal shear stress
sufficient to cause deformation but insufficient to
cause failure.
 Generally, the three types of creep are:
Creep
(…Contd)

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1. seasonal, where movement is within the depth of
soil affected by seasonal changes in soil moisture
and temperature;
2. continuous, where shear stress continuously
exceeds the strength of the material; and
3. progressive, where slopes are reaching the point
of failure for other types of mass movements.
Creep

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Spreads
 Spreads are an extension of a cohesive soil or rock
mass movement combined with the general
subsidence of the fractured mass of cohesive
material into softer underlying material.
 The spreads may result from liquefaction or flow
(and extrusion) of the softer underlying material.
 The types of spreads include block spreads,
liquefaction spreads, and lateral spreads.

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 Lateral spreads usually occur on very gentle slopes
or essentially flat terrain, especially where a
stronger upper layer of rock or soil undergoes
extension and moves above an underlying softer,
weaker layer.
 Such failures are normally accompanied by some
general subsidence into the weaker underlying
unit.
Lateral Spreads
(…Contd)

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 In earth spreads, the upper stable layer extends
along a weaker underlying unit that has flowed
following liquefaction or plastic deformation.
 If the weaker unit is relatively thick, the overriding
fractured blocks may subside into it, translate,
rotate, disintegrate, liquefy, or even flow.
Lateral Spreads

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 The spreads move slowly with moderate speed.
 Their movement may sometimes be rapid after
certain triggering mechanisms, such as an
earthquake are activated.
 The ground may then slowly spread over time
from a few millimetres per day to tens of square
meters per day
Velocity and triggers of spreads
(…Contd)

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 The Triggers for spreads may be the presence of a
weak layer.
 Liquefaction of lower weak layer by earthquake
shaking is a major trigger.
 Natural or anthropogenic overloading of the
ground above an unstable slope is yet another
reason.
(…Contd)

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 Saturation of underlying weaker layer due to
precipitation, snowmelt, and ground-water
changes may also induce spreads.
 Plastic deformation of unstable material at depth
(for example, salt) is another reason.

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 Spreads can cause extensive property damage to
buildings, roads, railroads, and lifelines.
 They can spread slowly or quickly, depending on
the extent of water saturation of the various soil
layers.
 Lateral spreads may be a precursor to
earthflows.
Effects (direct/indirect) of spreads

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 Preparation of liquefaction-potential maps is necessary.
 Areas with potentially liquefiable soils can be avoided as
construction sites, particularly in regions that are
known to experience frequent earthquakes.
 If high ground-water levels are involved, sites can be
drained or other water-diversion efforts can be added.
Mitigation measures
(…Contd)

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 Spreads may have high probability of recurring in
areas that have experienced previous problems.
 Most prevalent in areas that have an extreme
earthquake hazard as well as liquefiable soils.
 Lateral spreads are also associated with
susceptible marine clays.
Mitigation measures

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FloodingFlooding
 Landslides are much related to the influence of
water.
 Oversaturation of soil mass with water on the slope
areas is a primary cause of landslides.
 This effect can occur in the form of intense rainfall,
snowmelt, changes in ground-water levels, and
water-level changes along coastlines, earth dams,
and the banks of lakes, reservoirs, canals, and rivers.
Overall triggers of landslides
(…Contd)

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FloodingFlooding
 Landsliding and flooding are closely allied because
both are related to precipitation, runoff, and the
saturation of ground by water.
 In addition, debris flows and mudflows usually
occur in small, steep stream channels and often are
mistaken for floods; in fact, these two events often
occur simultaneously in the same area.

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 Landslides are also related to Seismic Activity.
 Many mountainous areas that are vulnerable to
landslides have also experienced at least
moderate rates of earthquake occurrence in
recorded times.
Seismic activitySeismic activity

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 Landslides are also related to Volcanic Activity.
 Landslides due to volcanic activity are some of the
most devastating types.
 Volcanic lava may melt snow at a rapid rate,
causing a deluge of rock, soil, ash, and water that
accelerates rapidly on the steep slopes of
volcanoes, devastating anything in its path.
Volcanic activityVolcanic activity

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 Landslides affect manmade structures whether they are
directly on or near a landslide. Residential dwellings
built on unstable slopes may experience partial damage
to complete destruction as landslides destabilize or
destroy foundations, walls, surrounding property, and
above-ground and underground utilities.
 One of the greatest potential consequences from
landslides is to the transportation industry, and this
commonly affects a large number of people all around
the world.
Effects of Landslides on the Built Environment

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 Landslides have effects on the natural
environment. They affect the morphology of the
Earth’s surface—mountain and valley systems,
both on the continents and beneath the oceans.
 The mountain and valley morphologies are most
significantly affected by downslope movement of
large landslide masses.

Effects of Landslides on the Natural Environment

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 The forests and grasslands that cover much of the
continents and the native wildlife that exists on the
Earth’s surface and in its rivers, lakes, and seas,
will be severely affected.
 Forest, grasslands, and wildlife often are
negatively affected by landslides.
 The forest and fish habitats are most easily
damaged.

Effects of Landslides on the Natural Environment

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 How to Reduce the Effects of Landslides is a major
step in disaster management.
 Vulnerability to landslide hazards is a function of
location, type of human activity, use and frequency
of landslide events.
Landslide Mitigation methods
(…Contd)

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 The effects of landslides on people and structures
can be lessened by total avoidance of landslide
hazard areas or by restricting, prohibiting, or
imposing conditions on hazard-zone activity.
 The hazard from landslides can be reduced by
avoiding construction on steep slopes and
existing landslides, or by stabilizing the slopes.
Landslide Mitigation methods

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Technological Tools for Evaluation of Landslides
 Mapping, Remote Sensing, and Monitoring are the
major techniques for analysing and evaluating the
landslides.
 One of the guiding principles of geology is that the
present is the key to the past and past will help to
know about the future.
(…Contd)

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Technological Tools for Evaluation of Landslides
 In evaluating the landslide hazards, the future slope
failures could occur as a result of the same geologic,
geomorphic, and hydrologic situations that led to
the past and are leading the present failures.
 Based on this assumption, it is possible to estimate
the types, frequency of occurrence, extent, and
consequences of slope failures that may occur in the
future.

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 Human-induced conditions, such as changes in the
natural topography or hydrologic conditions, can
create or increase an area’s susceptibility to slope
failure.
 Useful conclusions concerning increased
probability of landsliding can be drawn by
combining geological analyses with knowledge of
short- and long-term meteorological conditions.
Monitoring is a must

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 Map analysis is usually one of the first steps in a
landslide investigation.
 Necessary maps include bedrock and surficial
geology, topography, soils, and if available,
geomorphology maps.
 Using knowledge of geologic materials and
processes, a trained person can obtain a general
idea of landslide susceptibility from such maps.
Map Analysis

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 Analysis of aerial photography is a quick and
valuable technique for identifying landslides,
because it provides a three-dimensional overview
of the terrain and indicates human activities as well
as much geologic information to a trained person.
 In addition, the availability of many types of aerial
imagery or satellite images makes aerial
reconnaissance very versatile and cost-effective.
Aerial surveys

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 Many of the more subtle signs of slope movement
cannot be identified on maps or aerial
photographs.
 Indeed, if an area is heavily forested or if it has
been fully urbanized, even major features may
not be evident. Furthermore, landslide features
change over time on an active slide.
Field ReconnaissanceField Reconnaissance
(…Contd)

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 Field reconnaissance is always mandatory to
verify or detect landslide features.
 It is also needed to critically evaluate the potential
instability of vulnerable slopes.
 It identifies the areas of past landslides by using
field mapping and laboratory testing of terrain
through the sampling of soil and rock.
Field ReconnaissanceField Reconnaissance

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 Geophysical techniques are suitable to determine
some subsurface characteristics such as the depth
to bedrock, stratigraphic layers, zones of
saturation, and sometimes the ground-water table.
 They are also useful to determine texture, porosity,
and degree of consolidation of subsurface
materials and the geometry of the units involved.
Geophysical StudiesGeophysical Studies

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ConclusionConclusion
 In India, landslides occur frequently in the regions
of Himalayas, Western Ghats, Eastern Ghats and
other hill ranges like Vindhyas.
 The vulnerability to landslide hazards is a function
of a site’s location (topography, geology,
drainage), type of activity, and frequency of past
landslides.
(…Contd)

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 The effects of landslides on people and structures
can be lessened by total avoidance of landslide
hazard areas or by restricting, prohibiting, or
imposing conditions on hazard-zone activity.
 The Disaster Management is an emerging subject of
earth science.
 We have to minimize the impacts of natural
hazards, like landslides.
(…Contd)

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 Our ultimate aim is to protect the life and all
properties.
 For proper understanding of their impacts and
causative factors, the landslide hazard zonation maps
are to be prepared.
 The knowledge of geology of the region is essential for
mitigating the effects of landslides.
 The impacts of landslides can me minimized through
several mitigation methods that are available.

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Thank YouThank You

Landslides and their controls

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