mass movements.pptx
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GeologyTRANSCRIPT
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Mass Movements
Engineering Geology
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Landslide on California state route 140, June 2006
http://en.wikipedia.org/wiki/File:Ferguson-slide.jpg
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Introduction
• Mass movements refer to the
movements of superficial earth material
• ‘Mass Wasting’ or ‘Mass movements’ are
known in popular culture, as landslides
• They present baffling situations,
involving heavy losses of life and
property
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Classification
Based on the type of failure, mass
movements are classified into:
o Flowage
o Sliding
o Subsidence
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1. Flowage
• Refers to a downgrade movement of
mass, along no definite surface of failure
• It involves unconsolidated material
• The movement is distributed throughout
the mass, hence it is of an irregular
nature
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Mudflow
resources.teachnet.ie
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Slow and Rapid flowage
• Slow flowage
– failure is not easily perceptible
– ground moves at rates less than a few
centimeters per year
• Rapid flowage
– movement of the failing mass is easily
visible
– the mass may travel a few meters a day
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Soil Creep
• Involves the gradual, imperceptible
down slope transit of soil
• Soil creep, mostly is a surface
phenomenon
• Frost action and gravity play a
prominent role in soil creep
• The rate of downgrade movement varies
from 1mm to several centimeters a year
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Soil creep
Pearson Education 2010
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Effects of creep at Chalk Grasslands, Sussex, UK
© Ian Alexander
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12seattletimes.nwsource.com
Pistol grip trees
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Solifluction
• In solifluction, the soil moves in
saturated conditions
• Solifluction is characteristic of
permafrost regions
• In summer, water melts only in the
upper regions of the soil
• In the presence of a gentle slope, soil
grains will move down, in association
with water 13
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14Solifluction
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Earth and mud flows
• Rapid types of mass movements
• Confined to clays and silts, they take
place after heavy melting of snow and ice
• Wet clay is plastic, slips under its self
weight
• Mud flows have a greater quantity of
water per unit volume of the soil mass
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2. Sliding
• In landslides, superficial mass fails by
moving as a whole, along a definite
surface of failure
• The mass above the failure surface is
generally unstable, while that below it is
relatively stable
• Sliding may involve material of any
composition, shape and of varying
degree of consolidation16
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Types of landslides
Based on the type of movement
involved in the failure, landslides are
subdivided into:
o Translational slides
o Rotational slides
o Rock toppling and falls
o Debris slide/fall
o Slump
o Rock avalanche 17
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Translational Landslide
Highland, USGS & Bobrowsky, GSC
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20Highland, USGS & Bobrowsky, GSC
Rotational Landslide
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21Michael J. Crozier, Encyclopedia of New Zealand
Rotational Landslide in New Zealand
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22Highland, USGS & Bobrowsky, GSC
Rock Toppling and Falls
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Rock toppling at Fort St. John, British Columbia,
Canada
© G Bianchi Fasani
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24Highland, USGS & Bobrowsky, GSC
Debris fall
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25L.M. Smith, Waterways Experiment Station, U.S. Army Corps of Engineers
Debris flow damage, Caraballeda, Venezuela, 1999
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Rock avalancheAmazing rock avalanche! - YouTube.flv
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3. Subsidence
• Defined as the downward sinking or
settling of the ground
• Causes
– Natural
• Solution of subsurface rocks
• Geological constitution
– Artificial
• Mining
• Removal of groundwater 27
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28aegweb.org
Subsidence
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29britfa.gs
Subsidence
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Causes of Subsidence
http://en.wikipedia.org/wiki/File:Wiki_Image_Rev1.svg
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Causes of Mass Movements
• Internal Factors
– Nature of the slope
– Role of water
– Composition of the mass
– Geological structures
• External Factors
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1.1 Nature of the slope
Refers to:
o The type of material (soil or rock),
making up the land mass
o The angle of the slope (angle made with
the horizontal)
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Stability of slope
A slope is subjected to:
o Forces which help to retain its position in
space – shearing resistance
o Forces that tend to induce failure –
gravity
shearing strength,
τ = c + σ tan φ
Angle of repose – the angle of slope, up to
which a material is stable33
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Gravity & angle of slope
http://gomyclass.com/geology10/files/lecture15/html/web_data/file28.htm
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1.2 Role of water
1. Causes uplift/pore pressure, within the
mass
2. Water accumulating in the back of a mass
exerts a pressure parallel to the direction
of flow
3. Frost action
4. Lubricating action along planes of
weakness
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Influence of water on stability
http://gomyclass.com/geology10/files/lecture15/html/web_data/file28.htm
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1.3 Composition of the mass
• Nature of the mass - soil or rock?
• Composition of the soil – sandy, silty or
clayey?
• Type and class of rock
• Texture of the rock – influences porosity
and permeability
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1.4 Geological Structures
• The bedding planes (in sedimentary
rocks)
• The schistosity (in metamorphic rocks)
• The jointing structures, faults and shear
zones in all types of rocks
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1.4.1 The Bedding planes
• The bedding planes may be horizontal or dipping
– The layers are horizontal (dip = 0°)
– The layers are inclined
40http://www.tulane.edu/~sanelson/geol204/slopestability.htm
Slope on the
right – dipping
into the
mountain, stable
Slope on the left
– dipping into
the valley,
unstable
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Influence of Dip, on Mass Wasting
Unstable
Stable
Thompson & Turk
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1.4.2 The Metamorphic structures
• Schistosity, foliation and cleavage
structures of metamorphic rocks are
surfaces of weakness
• Weathering, take place along these
planes, making them vulnerable
• Slips are common, when these planes
are inclined towards the free side of the
slope
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1.4.3 The Jointing structures
• Jointing structures are common in rocks
• They occur in groups, reduce the shear
strength of the rocks
• The geometry, spacing, grouping and
inclination w.r.t the face of the slope is
to be studied
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2. External factors
• Artificial and natural vibrations
• Removal of support at the foot of the
slope
• Loading a critical region of the slope
from above
• Deforestation
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Factors influencing slope stability
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Deforestation and Landslides
epacha.org
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Monitoring and Control of Mass Movements
A. Monitoring
– Detection in advance, the symptoms
indicative of slope failure
– Use of conventional surveying
techniques in combination with
sophisticated instrumentation
– Settlement gauges, extensometers &
piezometers (for pore water pressure)
are used
B. Control 47
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Control Techniques
A detailed geological examination
reveals:
• Composition of the failing mass
• Structural disposition of the mass
• Position of the groundwater table
• Slope of the ground
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Control Techniques (contd.)
• Drainage
• Restraining structures
• Rock reinforcement
• Slope treatment
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1. Drainage
• Involves removal of water from the
mass, prevention of water from reaching
the mass
• May be surface or sub-surface drainage,
or a combination of both
• Ditches, interception drains are provided
for drainage purposes
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Sub-surface drainage to prevent landslides
http://www.horizontaldrill.com/services/hillsidestability.htm
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2. Restraining structures
Retaining walls are successful,
when:
• The ground is neither too fine, nor too
plastic
• The sliding mass is likely to remain dry
• The movement is of shallow nature and
limited extent
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A Gabion Retaining wall that failed
http://stevengivler.blogspot.com/2010_05_01_archive.html
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Failure…
http://stevengivler.blogspot.com/2010_05_01_archive.html
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3. Rock reinforcement
• Rock bolts are used to tie up different
rock blocks together
• Rock anchors are used for larger areas,
such as foundations
• Rock bolts expand within the rock,
thereby stabilizing it
• Rock anchors are made up of structural
elements like cables or bars
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Rock bolt
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4. Slope treatment
• Guniting – applying mortar/concrete,
under pressure
• Afforestation
• Other methods:
– Flattening the slope
– Decreasing the load on the slope
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58galeforce.co.uk
Guniting
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Afforestation
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Reference
• Parbin Singh, Engineering and General
Geology, S K Kataria & Sons
• Chenna Kesavulu, N, Textbook of Engineering
Geology, MacMillan India
• Thompson, G R and J Turk, Introduction to
Physical Geology, Thomson Brooks/Cole
• The Landslide Handbook – A Guide to
Understanding Landslides, Highland, USGS &
Bobrowsky, GSC