3. Content
• Landslides
• Slope Stability
• Methods to determine factor of safety
• Retaining Structures
• Subsidence
• Major Disaster in India
4. A DISASTER
• the set of failures that overwhelm the capability
of a community to respond without external
help when three continuums:
1) people,
2) community (i.e., a set of habitats,
livelihoods, and social constructs), and
3) complex events (e.g., landslides, ...)
intersect at a point in space and time.
5. LANDSLIDES
Represent permanent deformation caused
by the downward and outward, down-slope
movements of large volumes of soil and/or
rock under the influence of the force of gravity.
6. PHYSICS OF LAND SLIDE
• Landslides occur naturally on slopes.
• Landslides can be triggered and/or exacerbated
by:
1) water (from precipitation during a tropical
storm, hurricane, or typhoon),
2) vibrations (from ground shaking during an
earthquake).
7. SITING AND BUILDING ON
UNSTABLE SLOPES
LANDSLIDES
SOIL AND ROCK SUCEPTIBLE
TO FALLS
SOIL AND ROCK SUCEPTIBLE
TO TOPPLES
SOIL AND ROCK SUCEPTIBLE
TO SPREADS
SOIL AND ROCK
SUSCEPTIBLE TO FLOWS
PRECIPITATION THAT
TRIGGERS SLOPE FAILURE
SHAKING
GROUND SHAKING THAT
TRIGGERS SLOPE FAILURE
CAUSES
OF
DAMAGE
CASE HISTORIES
11. • Slopes: The most common landforms
• Consists of cliff face (“free face”) and talus
slope or upper convex slope, a straight
slope, and a lower concave slope
• Dynamic evolving feature, depending upon
topography, rock types, climate, vegetation,
water, and time
• Materials constantly moving down the slope
at varied rates
12. LANDSLIDES
Flow: continuous movement of
soil masses where shear
surfaces are short lived.
(Turner and Schuster 1996)
Spread: is the sudden movement of water
bearing rock masses
13. LANDSLIDES
Slide: is the down slope displacement of soil
or rock masses. It includes: rotational,
translational, and debris slide
14. Slope Stability
• Slope Stability Analysis (Abramson et al. 2002)
– understand the development and shape of natural
slopes
– determine the short-term and long term stability
conditions
– evaluate the possibility of failure of natural or
engineering slides
– analyze and understand failure mechanisms
– enable the retrofit of failed slopes
– understand the effect of seismic loading on slope and
embankments
15. Slope Stability
Effect of Water on Soils
• Dry sand grains will form a pile. The slope angle is determined by
the angle of repose (i.e., the steepest angle at which a pile of
unconsolidated grains remains stable - controlled by the frictional
contact between the grains. It usually lies between about 30 and 37
degrees.
(Nelson 2004)
Dry sand
Angle of
repose
Grain-to-grain frictional contact
16. Slope Stability
Effect of Water on Soils
• Slightly wet soil materials exhibit a very high
angle of repose because surface tension
between the water and the grains tends to hold
the grains in place.
(Nelson 2004)
Wet sand
Angle of
repose
Surface tension thin film
17. Slope Stability
Effect of Water on Soils
• When the material becomes saturated, the strength may
reduced to a very small values and it may tends to
flow… water (between the grains) eliminates grain to
grain frictional contact.
(Nelson 2004)
Angle of
repose
Fully saturated
sand
Water surrounds the grain and
prevent grain-to-grain contact
20. • The quantitative determination of the stability of
slopes is necessary in a number of engineering
activities, such as:
(a) the design of earth dams and
embankments,
(b) the analysis of stability of natural slopes,
(c) analysis of the stability of excavated
slopes,
(d) analysis of deep seated failure of
foundations and retaining walls.
21. Slope Types
• Infinite –
– soil is homogeneous
– Failure surface is plane and parallel to
ground
– Boundaries not known
– Ex- Himalaya
22. • Finite –
– soil is non homogeneous
– Failure surface is curved with arc of
rotation
– Boundaries known
– Example- Ooty
23. Slope Stability
• Safety Factor: = Resisting/Driving Forces
If SF >1, then safe or stable slope
If SF <1, then unsafe or unstable slope
• Driving and resisting force variables:
– Slip surface – “plane of weakness”
– Type of Earth materials
– Slope angle and topography
– Climate, vegetation, and water
– Shaking
24. Factor of safety of Infinite slope
• The factor of safety is commonly thought
of as the ratio of the maximum load or
stress that a soil can sustain to the actual
load or stress that is applied.
• Factor of safety F, with respect to strength
25. where ζff is the maximum shear stress that the
soil can sustain at the value of normal stress of
σn is the actual shear stress applied to the soil
27. FINITE SLOPES
Method of Slices
In the method, the sliding mass
above the failure surface is divided
into a number of slices. The forces
acting on each slice are obtained
by considering the mechanical
(force and moment) equilibrium for
the slices. Each slice is considered
on its own and interactions
between slices are neglected
because the resultant forces are
parallel to the base of each slice.
28. • Factor of safety for (Ø=0 Soil)
fully saturated clay soil = Cu R2 θ / W x
• Factor of safety for C – Ø soils
= C’ L + tan θ ΣW cos α / Σ W sin α
29. Modified Bishop’s method
• The Modified Bishop’s method is slightly
different from the ordinary method of slices in
that normal interaction forces between adjacent
slices are assumed to be collinear and the
resultant inter slice shear force is zero.
30. • Assumptions:-
• circular arc, radius R, centre O
• The soil mass above a trial failure surface is
divided into slices by vertical planes. Each slice
is taken as having a straight line base.
• The Factor of Safety of each slice is assumed to
be the same, implying mutual support between
the slices, ie. there must be forces acting
between the slices.
37. • As a whole, retaining structures are not
particularly effective methods of remedy.
• They are very difficult to construct on an already
moving slide.
• One use of them, though, is to ensure complete
stability of an existing (old) landslide, which may
in the future be reactivated.
• The wall provides additional resistance which is
only mobilized by further deformation of the
slope.
• The force then acts along the line of action into
the soil or rock beneath the slope
39. •Often these soil anchors are stressed, and the force
they exert on the slope needs to be considered along
with the other forces.
•The axial load on the anchor increases the effective
stresses at depth, therefore increasing the strength of
the slope.
•A vector component of the force may also act to help
stabilize the slope against destabilizing forces.
40. DRAINAGE METHODS
Drainage is best used as a short-term
stabilizing method, due to the fact that, in the
long-term, the drains need much maintenance
and repair, which is often difficult to perform,
and expensive.
41. • Shallow/Surface Drains:
• The aim of these is to reduce surface water and
thereby reduce the pore water pressures at a
much deeper level.
• These are the easiest drains to repair but get
damaged easily.
• These drains will usually be either -
»lined or unlined ditches
»shallow, gravel-filled trenches
• Commonly a herringbone or chevron shape is
used for the drains on the surface as this is most
effective in catching run-off water.
42. Surface Drainage on a slope on the road into Runswick
Bay, a coastal village in Yorkshire, England
44. • This method is most effective for deep-
seated instability.
• The berm can either be constructed from
material which is removed from the crest
of the slope (involving regarding of the
slope), or from that which is brought to the
site from elsewhere.
45. GRASSING-OVER THE SLOPE
• By covering over a slope with either sand or
grass, we immediately reduce the amount of
water which can infiltrate it.
• This method is often used in conjunction with
more effective and long-term methods. It's
benefits in being inexpensive and simple, whilst
still performing a stabilizing function, make it
worth serious consideration.
47. • Geotextiles are man-made (usually plastic
based) soil reinforcement materials.
• In the area of slope stabilization, geogrids are
used eg. In an embankment fill to reduce the
amount of movement possible, keeping the fill in
place.
• Very often the geogrid is used as an anchor,
providing a reaction against the disturbing
moment.
• They are often also used to repair small slides in
engineering earthworks. They perform this
function well.
49. • This can be done in three ways:-
– Regrading the slope to a flatter angle,
– Reducing the overall slope height, keeping its profile
unchanged,
– Removing some material from the crest and placing it
at the toe.
REGRADING THE SLOPE
50. • Acheiving a flatter slope angle: can be
done by either cutting material away from
the slope, or adding (filling) material. This
method is most effective where there are
predominant shallow forms of instability.
See the case study on Walton's Wood for
a practical example.
51. • Reducing the height of the Slope: In a man-
made (earthworks) slope, this method is not
very practical as the height of the slope usually
is not able to be changed due to design. For a
natural slope though, this may well be
considered.
• However, the reduction in instability achieved
using this method is much lower than that
achieved by say loading the toe, unless the
failure is deep-seated. It does not help much in
the case of shallow failures.
52. WHICH METHOD IS BEST?
METHOD NOTES
1
Regarding the Slope
(includes Loading the
Toe)
Has an immediate effect
Unlikely to become uneffective
with time.
2Drainage
Should be used if regrading is
impractical
Has immediate effect in high-
permeability soils.
Takes more time to take effect in
fine-grained soils.
Surface drainage is very common.
53. METHOD NOTES
3
Incorporating
Structures into
the Slope
(eg. nailing,
geotextiles,
retaining
structures).
These structures can be active
(eg. Stressed nails/anchors), or
passive (eg. walls or sheet
piling.)
Passive schemes only take
effect on further movement of
the slope, which of course may
not be desirable.
54. Subsidence
• Subsurface ground
failure
• Natural or human-
induced
• Slow settling or rapid
collapse
• Causes:
– Withdrawal of fluids
(water, oil and gas,
steam)
– Removal of solid
materials (dissolution,
mining)
55. • Sinkholes
Dissolution of carbonate rocks, limestone,
and dolomite
Affecting most of the conterminous states
Natural or artificial fluctuations in water
table increasing the problem
Triggering other problems: Sinkholes as
waste dumping sites
Removal of Solid Materials
56. • Salt and coal mining
Salt dissolution and pumping
Active coal mines and abandoned coal
mines
Ground failure due to depleted subsurface
pressure
More than 8000 km2 of land subsidence due
to underground coal mining
Removal of Solid Materials
57. Major Disaster in India
Sl.No Name of Event Year State & Area Fatalities
1.
Sikkim
Earthquake
2011
North Eastern India with
epicenter near Nepal Border
and Sikkim
Most recent disaster
2. Cloudburst 2010 Leh, Ladakh in J&K
3. Drought 2009 252 Districts in 10 States -----
4. Floods 2009
Andhra Pradesh, Karnataka,
Orissa, Kerala, Delhi,
Maharashtra
300 people died
5. Kosi Floods 2008 North Bihar
527 deaths, 19,323
livestock perished,
2,23,000 houses
damaged, 3.3 million
persons affected
6. Cyclone Nisha 2008 Tamil Nadu 204 deaths
58. 7.
Maharashtra
Floods
2005 Maharashtra State
1094 deaths
167 injured
54 missing
8. Kashmir 2005
Mostly Pakistan,
Partially Kashmir
1400 deaths in Kashmir
(86,000 deaths in total)
9. Tsunami 2004
Coastline of Tamil Nadu,
Kerala, Andhra Pradesh,
Pondicherry and
Andaman and Nicobar
Islands of India
10,749 deaths
5,640 persons missing
2.79 million people affected
11,827 hectares of crops
damaged
300,000 fisher folk lost their
livelihood
10.
Gujarat
Earthquake
2001
Rapar, Bhuj, Bhachau,
Anjar, Ahmedabad and
Surat in Gujarat State
13,805 deaths
6.3 million people affected
11.
Orissa Super
Cyclone
1999 Orissa Over 10,000 deaths
12. Cyclone 1996 Andhra Pradesh
1,000 people died, 5,80,000
housed destroyed, Rs. 20.26
billion estimated damage
59. 13. Latur Earthquake 1993
Latur, Marathwada
region of
Maharashtra
7,928 people died
30,000 injured
14. Cyclone 1990 Andhra Pradesh
967 people died, 435,000
acres of land affected
15. Cyclone 1990 Andhra Pradesh
967 people died, 435,000
acres of land affected
16. Drought 1987 15 States 300 million people affected
17. Cyclone 1977 Andhra Pradesh
10,000 deaths
hundreds of thousands
homeless
40,000 cattle deaths
18. Drought 1972
Large part of the
country
200 million people affected
