mass movements.pptx
DESCRIPTION
GeologyTRANSCRIPT
Mass Movements
Engineering Geology
2
Landslide on California state route 140, June 2006
http://en.wikipedia.org/wiki/File:Ferguson-slide.jpg
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
3
Classification
Based on the type of failure, mass
movements are classified into:
o Flowage
o Sliding
o Subsidence
4
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
5
6
Mudflow
resources.teachnet.ie
7
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
8
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
9
10
Soil creep
Pearson Education 2010
11
Effects of creep at Chalk Grasslands, Sussex, UK
© Ian Alexander
12seattletimes.nwsource.com
Pistol grip trees
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
14Solifluction
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
15
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
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
18
Translational Landslide
Highland, USGS & Bobrowsky, GSC
19
20Highland, USGS & Bobrowsky, GSC
Rotational Landslide
21Michael J. Crozier, Encyclopedia of New Zealand
Rotational Landslide in New Zealand
22Highland, USGS & Bobrowsky, GSC
Rock Toppling and Falls
23
Rock toppling at Fort St. John, British Columbia,
Canada
© G Bianchi Fasani
24Highland, USGS & Bobrowsky, GSC
Debris fall
25L.M. Smith, Waterways Experiment Station, U.S. Army Corps of Engineers
Debris flow damage, Caraballeda, Venezuela, 1999
26
Rock avalancheAmazing rock avalanche! - YouTube.flv
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
28aegweb.org
Subsidence
29britfa.gs
Subsidence
30
Causes of Subsidence
http://en.wikipedia.org/wiki/File:Wiki_Image_Rev1.svg
Causes of Mass Movements
• Internal Factors
– Nature of the slope
– Role of water
– Composition of the mass
– Geological structures
• External Factors
31
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)
32
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
34
Gravity & angle of slope
http://gomyclass.com/geology10/files/lecture15/html/web_data/file28.htm
35
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
36
37
Influence of water on stability
http://gomyclass.com/geology10/files/lecture15/html/web_data/file28.htm
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
38
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
39
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
41
Influence of Dip, on Mass Wasting
Unstable
Stable
Thompson & Turk
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
42
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
43
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
44
Factors influencing slope stability
45
46
Deforestation and Landslides
epacha.org
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
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
48
Control Techniques (contd.)
• Drainage
• Restraining structures
• Rock reinforcement
• Slope treatment
49
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
50
51
Sub-surface drainage to prevent landslides
http://www.horizontaldrill.com/services/hillsidestability.htm
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
52
53
A Gabion Retaining wall that failed
http://stevengivler.blogspot.com/2010_05_01_archive.html
54
Failure…
http://stevengivler.blogspot.com/2010_05_01_archive.html
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
55
56
Rock bolt
4. Slope treatment
• Guniting – applying mortar/concrete,
under pressure
• Afforestation
• Other methods:
– Flattening the slope
– Decreasing the load on the slope
57
58galeforce.co.uk
Guniting
59
Afforestation
60
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