Lecture 11 landslides

Landslides
Lecture No. 11
Engr. Muhammad Jawad
Assistant Professor
Civil Engineering Department
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 Most humans consider Earth to be
“terra firma.”
 Earth’s surface, however, is mostly
unstable ground.
 This is a consequence of
weathering and erosion.
 We may be reminded of this
instability without warning.
Landslides

 Landslide (also called mass movement) is defined as
the downslope movement of material under the direct
influence of gravity .
 Most types of mass movement are aided by
weathering and usually involve surficial material.
 The material moves at rates ranging from almost
imperceptible to extremely fast.
 Landslides are an important geologic process that can
occur at any time and almost any place.
 While they are mostly associated with steep and
unstable slopes, they can also occur on near-level and
given the right geological conditions.
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Landslides

 The landslide starts with slow movements
along a slip surface, followed by a more rapid,
movement of the separated portion of earth
mass.
 The slip surface is usually bounded by a crack
 The chief types of landslides are :
(i) Slump,
(ii) Rock slides and
(iii)Rock falls
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Landslides

 When the gravitational force acting on a slope exceeds
its resisting force, slope failure (landslide) occurs.
 The resisting force helping to maintain slope stability
include the slope material’s strength; cohesion and
the amount of internal friction between grains and any
external support of the slope.
 These factors collectively define a slope’s shear
strength.
 Opposing slope’s shear strength is the force of gravity.
Gravity operates vertically but has a component
acting parallel to the slope, thereby causing instability.
 The greater a slope’s angle, the greater the
component of force acting parallel to the slope and the
greater the chance for mass movement.
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Landslides

The. factors which promote landslides are :
i. Water
ii. Slopes
iii.nature of rocks
iv.structure of rocks, and
v. disturbance of equilibrium.
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Causes of Landslides

 The essential conditions which cause landslides are lack
of support in front and lubrication behind.
 Thus water is an important factor in causing landslides.
 It acts in following ways
 Water diminishes the strength of rocks and thus help
in their movement.
 Water seeps into the rock or soil produces lubrication
and also exerts additional force on the grains tending
to displace them along the direction of water
movement.
 It adds weight to the material. Hence many
landslides occur after rains.
 On freezing it exerts an expansive force.
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i. Water

 It has been observed that majority of the earth or rock
failures are confined to slopes.
 This indicates that slopes are directly responsible for
landslides. As a rule, steeper the slope, greater is the
instability of such a mass.
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ii. Slope

 Unconsolidated sediments, such as clay, sand, gravel,
etc. can not stand permanently along slopes greater
than their angle of repose (about 35°) and are likely
to be affected by landslides.
 Where weak and slippery rocks like shale, volcanic
tuff are present, they are not likely to remain stable
on steep slopes for a considerable period of time.
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iii.Nature of Rocks.

 Joints, fractures, shear zones and bedding planes
usually become the slip surface in case of rock slides.
10 iv.Structure of Rocks

1. Rotational slide:
This is a slide in which the
surface of rupture is curved
concavely upward and the
slide movement is roughly
rotational.
Types of Landslides11

Rotational
Slide
12 Types of Landslides

2. Translational slide:
In this type of slide, the
landslide mass moves along a
roughly planar surface with
little rotation or backward
tilting.

3. Block slide:
It 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.

4. Fall:
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.
Falls are strongly influenced by gravity,
mechanical weathering, and the presence
of interstitial water.

5. Earthflow:
This is a variety of
very rapid to
extremely rapid
earth material flow.

6. MUDSLIDES/Debris flow:
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 down slope.
Debris flows include less than 50% fines. 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.

7. MUDSLIDES/Mudflow:
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.
Sometimes, mudflows and debris flows are commonly referred
to as "mudslides”

8. Lateral Spreads:
Lateral spreads are distinctive because they usually occur on very gentle
slopes or flat terrain.
The dominant mode of movement is lateral extension accompanied by
shear or tensile fractures.
The failure is caused by liquefaction, the process whereby saturated,
loose, cohesion less sediments (usually sands and silts) are transformed
from a solid into a liquefied state.
Failure is usually triggered by rapid ground motion, such as that
experienced during an earthquake, but can also be artificially induced.

9. Creep:
Creep is the imperceptibly slow, steady, downward movement of
slope-forming soil or rock.
Movement is caused by shear stress sufficient to produce permanent
deformation, but too small to produce shear failure.
Creep is indicated by curved tree trunks, bent fences or retaining
walls, tilted poles or fences, and small soil ripples or ridges

The methods which are commonly used for prevention of
landslides are follows
1. Slides in the impervious material are prevented by
reducing slopes.
2. Efforts should also be made not to allow the
additional water to enter into the material.
3. To check/limit the surface water to enter into the
unstable ground, it is diverted and made to run off as
rapidly as possible away from that area.
4. In pervious materials, landslides may be prevented
by increasing the internal friction of the mass by
lowering the water content.
28
Prevention of Landslides.

5. Their water may be removed by drain pipes, by
drainage through tunnels or by pumping from wells.
6. In situations where slides may cause loss of life and
property, the loose rock material is prevented from
sliding by constructing retaining walls, concrete piers
or by use of piling.
7. At some places the unstable unconsolidated material
may be consolidated by cement grouting, chemical
means, and artificial freezing.
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Prevention of Landslides.

LANDSLIDES
Landslide Remediation30

LANDSLIDES
Types of Landslides31

LANDSLIDES

Lecture 11 landslides

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