1 Land Slides Causes and Protective Measures Prof. Dr. Attaullah Shah
1
Land SlidesCauses and Protective
Measures
Prof. Dr. Attaullah Shah
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 casesThe 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.
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 casesThe 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.
• 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
Classification of Earth Movements
Earth Flow Landslides Subsidence
Solifluction
Creep
Rapid flows
Rock slides
Rock falls
Debris slide or
slump
collapse
Plastic flow
SOLIFUCTION
• Solifuction is a downward movement of wet soil along the slopes under the influence of gravity.
• 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
SOIL CREEP
• 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)
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.
• 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).
• Talus – accumulation formed by the coarser rock fragments resulted from the mechanical weathering along a slope under influence of gravity
• 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.
Subsidence
• The Leaning Tower of Pisa, Italy, the tilting of which accelerated as groundwater was withdrawn from aquifers to supply the growing city.
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.
• 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.
• 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.
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
•
15
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
16
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
17
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.
• 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.
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
20
21
Landslide
Remedial MeasuresSafe at the moment
LandslidePreventive Measures
Landslides: Introduction
DesignCostBuild Consequence
Remove Consequence
Stability Assessment
TemporarilySafe
Landslide Warning
•Injury•Death•Economic Loss•Disruption to Transport Links
Consequences of Landslides
22
Main Manchester –
Sheffield Road (A625)
Alternative route – only suitable for light vehicles – gradient of 1 in 4
Landslides: Removing the Consequence
Manchester
1 km
23Landslides in Kowloon East 28th - 31st May 1982
Landslides: Removing the Consequence
24
Landslide
Remedial Measures
LandslidePreventive Measures
Man’s Influence (Agriculture /Development)
Landslides: Engineering Modelling Methods
Geology
DesignCostBuild Consequence
Remove Consequence
Stability Assessment
TemporarilySafe
Landslide Warning
HydrologyMaterial Properties
(Shear Strength)
Slope AngleLoading
But only for specific slopes
Safe at the moment
25
• 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.
Landslides: Engineering Modelling Methods
But one must not be complacent
26
berms
Landslide in man made Cut Slope at km 365 west of Sao Paolo - August 2002
27
General Planning Guidelines of Landslide Risk
Classification into potential Areas of Risk
Soil Type
Landslides: GIS Modelling Methods
GeologyHydrologyGeneral Slope (and aspect) Land Use
Database of existing Landslides
Identification of areas for detailed Engineering Study
Cataloguing slopes and landslides
28
• 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
Landslides: GIS Modelling Methods
Accuracy is dependant on existence of a good unbiassed database of landslides and slopes
29
e.g. North Coast Road, Trinidad
Fill Slope
Retaining Wall
“Natural” Slope
Cut Slope
Landslides: Categorisation of Slopes
30
Landslide at Maracas
December 2002
December 2004 – note the slide is much more
extensive
31
December 9th
Landslide
3 km beyond Las Cuevas as seen on TV
half of road blocked
Landslide 11th December 2004 at
approximately 13:00
1 km before Las Cuevas
half of road blocked
32
Slope before failure at Couva
Slope after Landslide
Slide by Derek Gay, UWI
LANDSLIDE HAZARD: ALASKALANDSLIDE HAZARD: ALASKALANDSLIDE HAZARD: ALASKALANDSLIDE HAZARD: ALASKA
• Slope failure was Slope failure was induced by ground induced by ground shaking of “Quick shaking of “Quick Clay.” Clay.”
• Slope failure was Slope failure was induced by ground induced by ground shaking of “Quick shaking of “Quick Clay.” Clay.”
LANDSLIDE HAZARD: ECUADOR
LANDSLIDE HAZARD: JAPANLANDSLIDE HAZARD: JAPAN
LANDSLIDE HAZARD: WASHINGTON STATE LANDSLIDE HAZARD: WASHINGTON STATE
LATERAL SPREADING: JAPAN
LATERAL SPREADING: JAPAN
LATERAL SPREAD: SAN FRANCISCO
45
Landslides: GIS Modelling Methods: Requirements for the future
• 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.
Landslides triggered by
anthropogenic activity
Deep seated landslide unaffected
by anthropogenic activity
46
Landslides: Statistical Methods
Prediction of exactly when landslides are likely to occur
Issue warnings to affected people
Mobilise Emergency Teams
Historical Database of Landslide Occurrence
Rainfall Data
Research to correlate Rainfall with Landslide
Incidence
Antecedent Rainfall
Current/ Predicted Rainfall
Aim: to minimise injury and loss of life
47
• 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.
Landslides: Statistical MethodsLandslide Warning System
Method aims to alert people to impending danger so they can seek safety during critical periods – it will not
prevent landslides
48
Rain Gauge Network in Hong Kong
Built Up Areas
49
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
Landslides: Landslide Warning System
50
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.
(>10)
Background to Warning System
Landslides: Landslide Warning System
51
• 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)
Landslides: Statistical MethodsLandslide Warning System (continued)
52
•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.
ANTECEDENT CONDITIONS.
53
2 3 4 5 6 7 8 9 10 11 12 13 14 15
Day
24 – hour criteria
Cumulative Rainfall over previous 15 days
Cu
mm
ula
tiv e
Ra i
nfa
ll
Basis of Lumb’s Predictor
54
20 hours4 hours
LandslipPrediction Criteria(LPC)
Warning Time (WT) (Rainfall predicted to reach LPC in 4 hours)
Cumulative Rainfall
Actual Cumulative Rainfall Predicted Cumulative Rainfall
Landslip Time (LT) (The time when first landslip is reported to FSD).
Criteria Time (CT)
The time when LPC are actually reached.
Rainfall Profile and Onset of Landslides
55
0
50
100
150
200
250
300
350
400
450
0 100 200 300 400 500 600 700 800
Antecedent Rainfall (mm)
Rain
fall
on
Da
y (
mm
)
Minor Incident Severe Incident
Disastrous Incident RED Warning (1977 - 1979)
Amber Warning (1977 - 1979)
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.
56
0
50
100
150
200
250
300
350
400
450
0 100 200 300 400 500 600 700 800
Minor Incident Severe Incident
Disastrous Incident RED Warning
AMBER Warning
Landslide Warning System 2: (1980 - mid 1983)
Advantage: Much easier to identify whether WARNING should be called - even when chart is not to hand.
57
Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total: 223Squatters: 107
0 100 200 300 400 500 600 700 800
Antecedent Rainfall in previous 15 days (mm)
Rai
nfal
l on
Lan
dslip
Day
(m
m)
400
300
200
100
0
0020
16
12
09
04
Landslide Event 28 - 29th May 1982
58
Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total: 223Squatters: 107
0 100 200 300 400 500 600 700 800
Antecedent Rainfall in previous 15 days (mm)
Rai
nfal
l on
Lan
dslip
Day
(m
m)
400
300
200
100
0
0020
16
12
09
04
Landslide Event 28 - 29th May 1982
Even with 24hr day plotting, the plot for 29th May should have been as follows
59
Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total: 223Squatters: 107
0 100 200 300 400 500 600 700 800
Antecedent Rainfall in previous 15 days (mm)
Rai
nfal
l on
Lan
dslip
Day
(m
m)
400
300
200
100
0
09
04
0020
16
12
Situation with running 24 hr criterion
Landslide Event 28 - 29th May 1982
Criterion was reached at approx 03:00
BUT
1st Landslide was reported at 02:00 when rainfall was about 220mm
Even if Warning procedure has been operated correctly, warning would have been 1 hour too late!
60
09
04
0020
16
12
Landslide Warning: 1/82Issued at 09:00 on 29/05/82Landslides reported:Total: 223Squatters: 107
20
16
12
20
16
12
08
0400
00
16
12
08
04
16
0616
LW 2/8206:15 – 31/05/82Total: 91/ Sq: 40LW 4/82
11:00 – 03/08/82Total: 9Sq: 5
LW 6/8206:35 – 18/08/82*Total: 8Sq: 2
LW 3/8211:00 – 02/06/82*Total: 28/Sq: 12
LW 5/8205:50 – 16/08/82Total: 98Sq: 32
LW 7/8223:52 – 16/09/82Total: 3Sq: 3
0 100 200 300 400 500 600 700 800Antecedent Rainfall in previous 15 days (mm)
400
300
200
100
0
All Landslide Warning Incidents in 1982
61
Warning Criteria
Time
Warning Forecast
Gain
First
Landslip
No. Date (CT) Time (WT) (N) Time
1/82 29.05.82 0300 - 0400 0900 (-)b 0123
2/82 31.05.82a 0600 - 0700 0615 0 1351
3/82 02.06.82a not reached 1140 NA NR
4/82 03.08.82 1300 — 1400 1100 2 NR
5/82 16.08.82 0500 - 0600 0550 0 1009
6/82 18.08.82a not reached 0635 NA NR
7/82 16.09.82 not reached 2352 NA NR
1/83 27.03.83 2300 — 2400 2355 0 0011
2/83 08.04.83 not reached 1102 NA NR
3/83 17. 06.83 0800 - 0900 0745 1 0840
Performance of All LandSlip Warnings 1982 - 1983
Red Landslides with No Warning!
Green Landslide Warnings with Several Hours Warning
Blue Landslide Warnings with 1 Hour Warning
62
0
50
100
150
200
250
300
350
400
450
0 100 200 300 400 500 600 700 800
Antecedent 15-day Rainfall (mm)
24-h
ou
r R
ain
fall
du
rin
g E
vent
(m
m) Disastrous
Severe
Minor
Null Event
New 1983 Criteria
All Rainstorm Events: Daily Rainfall vs Antecedent Rainfall
Disastrous > 50 reported Landslides: Severe 10 - 50 LandslidesMinor < 10 Landslides : Null Event: No reported Landslides
Criteria for low antecedent rainfall reduced to conform to actual 1st landslide in Event 1/82
63
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??
64
0
50
100
150
200
250
300
350
400
450
0 100 200 300 400 500 600 700 800
Antecedent 15-day Rainfall (mm)
24-h
ou
r R
ain
fall
du
rin
g E
ven
t (m
m)
Disastrous: > 50 LandslidesSevere: 10 - 50 LandslidesExisting CriteriaWarning LineLandslide Line
Existing Criteria Line - in use mid 1982 - mid 1984
Warning and Landslide Lines in use from mid 1984
Severe and Disastrous Landslide Events: with 1984 Scheme
65
Landslide Warnings: The Final (1984) Approach1. 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.
Landslides: Landslide Warning System
66
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
Landslides: Landslide Warning System
67
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
68
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.