Land%20Slides

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Land Slides Causes and Protective Measures Prof.
Dr. Attaullah Shah
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Landslide refers to the downward sliding of huge
quantities of land mass, which occur along steep
slopes of hills or mountains and may be sudden or
slow Geological Phenomena involving downward
movement of large quantities of material such as
rocks, earth, sand and combination The
Movement may be slow from few millimeters per
year to few centimeter per year. In flow type
land slides, it may be 15km/h in some cases The
collapse of masses may also be suddent as in case
of Avalanche on the steep slope. In recent days,
the earthquake at Nepal led to huge avalanche at
the bases camp of Mount Everest which caused huge
human losses.
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Classification of Earth Movements

• All movement of land masses are referred as
  landslides, but differ in many respects,
  therefore all types of landslides are categorized
  as Earth Movements.
• These are classified as

Landslides
Subsidence
Earth Flow
Debris slide or slump
Plastic flow
Solifluction
Rock slides
Creep
collapse
Rock falls
Rapid flows
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SOLIFUCTION

• Solifuction is a downward movement of wet soil
  along the slopes under the influence of gravity.

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SOIL CREEP

• Creep is extremely slow downward movement of dry
  surficial matter.
• Movement of the soil occurs in regions which are
  subjected to freeze-thaw conditions. The freeze
  lifts the particles of soil and rocks and when
  there is a thaw, the particles are set back down,
  but not in the same place as before.
• It is very important for CEs to know the rate of
  movement

• RAPID FLOWS Rapid flow is similar to the creep,
  but differ in terms of speed and depth. It is
  faster.
• Creep is involved upto shallow depth (app. 1-2
  m), whereas the rapid flow is involved to greater
  depth (app. upto 5 m or more)

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Landslides

• If a mass of earth moves along a definite plane
  or surface the failure is termed as Landslide
• Large block known as a slump block moves during
  the landslide.
• The scar above a landslide is easily visible.
• They can occur along a slope where the internal
  resistance of the rocks are reduced or they loose
  their holding capacity.
• Common after earthquakes or after removal of part
  of the slope due to construction, particularly
  for construction of roads.

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• During the movement landslide can result into the
  Debris slides - are failure of unconsolidated
  material on a surface Rock slide or Rock Fall
  where movement of large rock block rolls
• They are also common along the steep banks of
  rivers, lakes etc.
• Pore Water Pressure is the key to monitoring
  landslides. Shear strength (a resisting force)
  decreases and the weight (a driving force
  increases).

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• Talus accumulation formed by the coarser rock
  fragments resulted from the mechanical weathering
  along a slope under influence of gravity

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Subsidence

• It represents the downward movement of the
  surface
• It may occur due to plastic outflow of the
  underlying strata or due to the compaction of the
  underlying material
• (1) Subsidence due to Plastic outflow It may
  occur when a plastic layer like clay bed is
  squeezed outward due to overlying heavy load
• (2) Subsidence due to collapse It occur due to
  extensive pull out of large volume of underground
  water or due to subsurface solution activity in
  limestone terrain.

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• The Leaning Tower of Pisa, Italy, the tilting of
  which accelerated as groundwater was withdrawn
  from aquifers to supply the growing city.

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CAUSES OF LANDSLIDES

• LANDSLIDES OCCUR DUE OF VARIOUS REASONS
• Internal Causes
• Influence of slope- Provides favorable condition
  for landslides steeper slope are prone to
  slippage of land. It is known that most of the
  materials are stable upto certain angle-
  Critical angle or angle of repose it varies
  from 300 for unconsolidated sediments to 900 for
  massive rocks and 600-900 for partially jointed
  rocks.
• Ground water or associated water- Main factor
  responsible for slippage. Suppose the hard or
  massive rocks are underlaid by softer rocks
  (shale or clay bed)
• When rain water percolates through some fractures
  or joints the clayey beds becomes very plastic
  and acts as slippery base, which enhance the
  chances of loose overburden to slip downward.
• Water is the most powerful solvent, which not
  only causes decomposition of minerals but also
  leaches out the soluble matter of the rock and
  reduces the strength.

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• Lithology- rock which are rich in clay
  (montmorillonite, bentonite), mica, calcite,
  gypsum etc are prone to landslide because these
  minerals are prone to weathering.
• Geological structures- Occurrence of inclined
  bedding planes, joints, fault or shear zone are
  the planes of weakness, which create conditions
  of instability.
• Human Influence- undercutting along the hill
  slopes for laying roads or rail tracks can result
  into instability.
• Deforestation in the uplands, result into more
  erosion during the rainy season.

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• External factors
• Most common is the vibration resulted due to
  earthquakes blasting to explosives volcanic
  eruption etc.
• Earthquakes often initiate mass failures on large
  scale e.g. 1897 Assam quake produced gigantic
  landslide ever recorded in the region.

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Geological process causing Landslides

• Erosion
• Cause steepening of slopes
• Remove cementing material
• Weathering of Rocks
• Freeze and Thaw actions ( Swelling and expansion)
• Shearing, jointing and cracks etc
• Leaching of limes and earth

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Human actions causing landslides

• Construction of human settlements at vulnerable
  areas near the critical slopes.
• Blasting and mining
• Vibrations of machines and earth moving equipment
  
• Dumping of Rocks and debris causes lateral
  pressure
• Vegetation and tree roots bind the slopes, but
  its cutting can cause slides.
• Overgrazing in unconsolidated soils
• Water leakages from utilities

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Natural Causes

• Heavy rainfalls leads to saturation of soils
• Erosion and undercutting of slopes by rivers
• Earthquakes and ground movements
• Excessive water filtration in ground
• Volcanic eruption
• Ocean waves may also cause coastal slides
• Freeze and Thaw actions
• Action of thunder and storms

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PREVENTIVE MEASURES

• The main factors which contribute to landslides
  are Slope, water content, geological structure,
  unconsolidated or loose sediments, lithology and
  human interference.
• Slope Retaining wall may be constructed against
  the slopes, which can prevents rolling down of
  material. Terracing of the slope is an effective
  measure.
• Effect of water Make proper drainage network for
  quick removal of percolating moisture or rain
  water by constructing ditches and water ways
  along the slope
• Geological structures Weak planes or zones may
  covered or grouted to prevent percolation of
  water, this increases the compaction of loose
  material.

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• LANDSLIDES AND MUDFLOWS
• Plant ground cover on slopes and build retaining
  walls.
• In mudflow areas, build channels or deflection
  walls to direct the flow around buildings.
• Install flexible pipe fittings to avoid gas or
  water leaks.

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Landslide Mitigation measures

• Afforestation
• Local suitable plants that can withstand the
  existing hydrological conditions
• Modification of Slope geometry
• Drainage arrangement for Ground water management
• Slope Reinforcement
• Retaining Structures
• Other methods
• Electro Thermo Osmosis
• Use of Geogrids and Geotextiles
• Use of steel wire meshes and Gabions
• Soil nailing

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Landslides Introduction
Consequences of Landslides

• Injury
• Death
• Economic Loss
• Disruption to Transport Links

Stability Assessment
Landslide Preventive Measures
Landslide
Design Cost Build
Safe at the moment
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Landslides Removing the Consequence
Main Manchester Sheffield Road (A625)
Alternative route only suitable for light
vehicles gradient of 1 in 4
1 km
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Landslides Removing the Consequence
Landslides in Kowloon East 28th - 31st May 1982
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Landslides Engineering Modelling Methods
Mans Influence (Agriculture /Development)
Geology
Stability Assessment
Landslide Preventive Measures
Landslide
Design Cost Build
But only for specific slopes
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Landslides Engineering Modelling Methods

• Applicable to very specific locations only
• Can have moderate to good accuracy for spatial
  predictions where information exists
• Moderate accuracy for temporal predictions (good
  if accurate ground water temporal variations are
  available)
• Poor for overall spatial coverage
• Is costly to implement.

But one must not be complacent
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berms

• Landslide in man made Cut Slope at km 365 west of
  Sao Paolo - August 2002

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Landslides GIS Modelling Methods
General Slope (and aspect)
Land Use
Soil Type
Geology
Hydrology
Cataloguing slopes and landslides
Classification into potential Areas of Risk
Database of existing Landslides
General Planning Guidelines of Landslide Risk
Identification of areas for detailed Engineering
Study
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Landslides GIS Modelling Methods

• Good spatial (geographic) coverage of likelihood
  of landslides
• Poor to moderate prediction of precise locations
  of landslides
• Effective use of resources
• Poor accuracy for temporal predictions
• i.e. precisely when landslides occur

Accuracy is dependant on existence of a good
unbiassed database of landslides and slopes
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Landslides Categorisation of Slopes
e.g. North Coast Road, Trinidad
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Landslide at Maracas December 2002
December 2004 note the slide is much more
extensive
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December 9th Landslide 3 km beyond Las Cuevas as
seen on TV half of road blocked
Landslide 11th December 2004 at approximately
1300 1 km before Las Cuevas half of road blocked
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Slope before failure at Couva
Slope after Landslide
Slide by Derek Gay, UWI
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LANDSLIDE HAZARD ALASKA

• Slope failure was induced by ground shaking of
  Quick Clay.

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LANDSLIDE HAZARD ECUADOR
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LANDSLIDE HAZARD JAPAN
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LANDSLIDE HAZARD WASHINGTON STATE
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LATERAL SPREADING JAPAN
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LATERAL SPREAD SAN FRANCISCO
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Landslides GIS Modelling Methods
Requirements for the future
Landslides triggered by anthropogenic activity

• Cut Slopes
• Fill Slopes
• Retaining Walls
• Hybrids Cut/Retaining Wall / Fill/Retaining
  Wall
• Natural Slopes - is there a better word?
• slopes where there has been no anthropogenic
  activity, or where there is such activity it
  causes small changes to the geometry of the slope
  so that the Factor of Safety is largely
  unaffected.

Deep seated landslide unaffected by anthropogenic
activity
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Landslides Statistical Methods
Rainfall Data
Historical Database of Landslide Occurrence
Research to correlate Rainfall with Landslide
Incidence
Antecedent Rainfall
Current/ Predicted Rainfall
Prediction of exactly when landslides are likely
to occur
Mobilise Emergency Teams
Issue warnings to affected people
Aim to minimise injury and loss of life
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Landslides Statistical MethodsLandslide
Warning System

• Poor prediction for spatial location of
  Landslides.
• Potentially effective use of resources to
  minimise death and injury.
• Moderate ability to predict when landslides are
  likely to occur.
• Requires automatic recording of rainfall over
  short periods of time (e.g. 5 15 minute
  intervals).
• Requires a robust historic database of landslides
  and associated rainfall.

Method aims to alert people to impending danger
so they can seek safety during critical periods
it will not prevent landslides
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Rain Gauge Network in Hong Kong
Built Up Areas
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Landslides Landslide Warning System
Requirements

• It should
• 1) provide sufficient warning of an event
• to alert general public
• to mobilise Emergency Services
• to open temporary Shelters
• 2) predict IN ADVANCE all serious EVENTS
• 3) minimise number of false alarms

• Three criteria can be in conflict
• How long should warning be?
• Longer the time, the less accurate will be
  prediction
• more false alarms

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Landslides Landslide Warning System
Background to Warning System

• Two Approaches
• Detailed Warning - e.g. 1. Conduit Road
• Warning based solely on Rainfall

automatic piezometer gives warning when ground
water level gets above a critical level as
determined by Slope Stability Analysis Aim to
give warning when a significant
number of landslides are likely to occur.
(gt10)
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Landslides Statistical MethodsLandslide
Warning System (continued)

• Research needed to correlate incidence of
  landslides with rainfall
• antecedent
• current
• predicted
• Hong Kong scheme mid 1980s
• Research needed to adapt ideas to local
  conditions in Trinidad and Tobago.
• Emergency Services need clear guidelines on how
  to react.
• Reporting system needed to notify public (via
  radio/ television)

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ANTECEDENT CONDITIONS.

• Are Slopes more susceptible to failure if there
  has been prolonged rainfall on preceding days?
• How should Antecedent rainfall Conditions be
  incorporated.
• Lumb (1975) - 15-day antecedent conditions.
• charts for Warning Purposes based both on
  Rainfall on Day AND Antecedent conditions.
• Most simple model uses simple cumulative 15-day
  antecedent rainfall.
• Could use a weighted system with days more
  distant weighted less.
• Lumb favoured simple approach.

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Basis of Lumbs Predictor
24 hour criteria
Cummulative Rainfall
Cumulative Rainfall over previous 15 days
2 3 4 5 6 7 8 9
10 11 12 13 14 15 Day
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Rainfall Profile and Onset of Landslides
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First Landslide Warning System (1977 - 1979)
AMBER and RED Warnings issued when predicted 24
hour rainfall would plot above relevant line.
A Problem Difficult to use without direct
access to Chart.
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Landslide Warning System 2 (1980 - mid 1983)
Advantage Much easier to identify whether
WARNING should be called - even when chart is not
to hand.
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Landslide Event 28 - 29th May 1982
400 300 200 100 0
Landslide Warning 1/82 Issued at 0900 on
29/05/82 Landslides reported Total
223 Squatters 107
Rainfall on Landslip Day (mm)
0 100 200
300 400
500 600 700
800 Antecedent Rainfall in previous 15 days (mm)
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Landslide Event 28 - 29th May 1982
Even with 24hr day plotting, the plot for 29th
May should have been as follows
400 300 200 100 0
Landslide Warning 1/82 Issued at 0900 on
29/05/82 Landslides reported Total
223 Squatters 107
Rainfall on Landslip Day (mm)
0 100 200
300 400
500 600 700
800 Antecedent Rainfall in previous 15 days (mm)
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Landslide Event 28 - 29th May 1982
Situation with running 24 hr criterion
400 300 200 100 0
Criterion was reached at approx 0300 BUT 1st
Landslide was reported at 0200 when rainfall was
about 220mm
Rainfall on Landslip Day (mm)
Even if Warning procedure has been operated
correctly, warning would have been 1 hour too
late!
0 100 200
300 400
500 600 700
800 Antecedent Rainfall in previous 15 days (mm)
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All Landslide Warning Incidents in 1982
400 300 200 100 0
09 04 00 20 16 12
Landslide Warning 1/82 Issued at 0900 on
29/05/82 Landslides reported Total
223 Squatters 107
20 16 12 08 04 00
20 16 12
16 12 08 04
LW 5/82 0550 16/08/82 Total 98 Sq 32
LW 2/82 0615 31/05/82 Total 91/ Sq 40
LW 7/82 2352 16/09/82 Total 3 Sq 3
00
LW 4/82 1100 03/08/82 Total 9 Sq 5
LW 6/82 0635 18/08/82 Total 8 Sq 2
16 06
LW 3/82 1100 02/06/82 Total 28/Sq 12
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0 100 200 300
400 500 600 700
800 Antecedent Rainfall in previous 15 days (mm)
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Performance of All LandSlip Warnings 1982 - 1983
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All Rainstorm Events Daily Rainfall vs
Antecedent Rainfall
Disastrous gt 50 reported Landslides Severe 10
- 50 Landslides Minor lt 10 Landslides
Null Event No reported Landslides

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Landslide Warnings The Problems
1. Antecedent Condition leads to confusion -
(Incident 1/82) 2. Must use rolling 24 hour
scheme 3. Previous Analysis (e.g. Lumb) has been
based on 24 hr day basis 4. Total Rainfall in day
will not generally be a good correlator as
final cumulative 24 hr rainfall (whether day or
rolling) will occur AFTER Landslides have
occurred. 5. Some Landslides Events will occur
after very low Antecedent Rainfall 6. Some
Landslides Events occur after short periods of
very intense rainfall. 7. It is difficult
to predict with accuracy future rainfall.
Is it sensible to continue with Antecedent
Rainfall Condition??
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Severe and Disastrous Landslide Events with 1984
Scheme
Existing Criteria Line - in use mid 1982 - mid
1984 Warning and Landslide Lines in use from mid
1984
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Landslides Landslide Warning System
Landslide Warnings The Final (1984) Approach
1. Abolish Antecedent Criteria - base solely on
Rolling 24hr approach. 2. When Rainfall exceeds
100 mm in a period of 24 hours and is expected to
exceed 175 mm (total) within 4 hours CONSIDER
issuing a LANDSLIDE WARNING. If weather
conditions suggest that Rainfall will cease
shortly then issue could be delayed. 3. If
Rainfall exceeds 175 mm then Landslides are
likely and Warning should now be issued
regardless of whether rain is likely to cease
shortly 4. Landslide Warning should be issued
regardless of above if rainfall in any one hour
exceeds 70 mm in any one hour in Urban Area.
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Landslides Landslide Warning System
The 1984 Warning Scheme

• Simple to understand
• On average 0 - 7 Warnings in a Year
• up to one third are false alarms
• identifies all serious/disastrous events
• about one third of warnings classified as
  minor
• (i.e. less than 10 landslides).

Further Improvements were introduced in 1999
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Landslides The Way Forward

• the Engineering Approach is justified in a few
  cases
• New developments / highways etc
• GIS methods are powerful and cost effective
• BUT
• Requires development of a robust Database
• Catalogue of Slope Types (whether failed on not)
• Catalogue of Landslides
• Trinidad and Tobago (Carribean) can build on an
  improve on the scheme developed in Hong Kong.
• Research needed to enhance GIS prediction of
  landslides
• Incorporate Geotechnical information

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Landslides Conclusions

• Interdisciplinary Research incorporating all
  three approaches is important for effective
  management of slopes and mitigation of adverse
  effects of landslides.
• Proactive Management of slope hazards will be
  more cost effective in the long term.
• Hong Kong woke up to the seriousness of the
  issues following disastrous landslides in 1972.
  Caribbean Countries should learn from their
  experience.
• Important to begin and resource fully the
  research needed to achieve these aims.