1. Causes and Protective Measures
Land Slides
Causes and Protective Measures
Prof. Dr. Attaullah Shah

2. 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.

3. 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

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

5. 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)

6. 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.

7. 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).

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

9. 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.

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 The Leaning Tower of Pisa, Italy, the tilting of which accelerated as groundwater was withdrawn from aquifers to supply the growing city.

12. LANDSLIDES OCCUR DUE OF VARIOUS REASONS
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 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.

13. 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.

14. 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 Assam quake produced gigantic landslide ever recorded in the region.

15. 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

16. 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

17. 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

18. 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.

19. 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.

20. 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

21. Landslides: Introduction
Consequences of Landslides
Injury
Death
Economic Loss
Disruption to Transport Links
Stability Assessment
Landslide
Preventive Measures
Temporarily
Safe
Landslide Warning
Landslide
Design
Cost
Build
Consequence
Remedial Measures
Remove Consequence
Safe at the moment

22. Landslides: Removing the Consequence
Manchester
Main Manchester – Sheffield Road (A625)
Alternative route – only suitable for light vehicles – gradient of 1 in 4
1 km

23. Landslides: Removing the Consequence
Landslides in Kowloon East 28th - 31st May 1982

24. Landslide Landslides: Engineering Modelling Methods Geology
Man’s Influence (Agriculture /Development)
Geology
Hydrology
Material Properties (Shear Strength)
Slope Angle
Loading
Stability Assessment
Landslide
Preventive Measures
Temporarily
Safe
Landslide Warning
Landslide
Design
Cost
Build
Consequence
But only for specific slopes
Remedial Measures
Remove Consequence
Safe at the moment

25. 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

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

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

28. 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

29. Landslides: Categorisation of Slopes e.g. North Coast Road, Trinidad
“Natural” Slope
Cut Slope
Fill Slope
Retaining Wall

30. December 2004 – note the slide is much more extensive
Landslide at Maracas
December 2002
December 2004 – note the slide is much more extensive

31. 3 km beyond Las Cuevas as seen on TV half of road blocked
December 9th
Landslide
3 km beyond Las Cuevas as seen on TV
half of road blocked
Landslide 11th December 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

33. LANDSLIDE HAZARD: ALASKA
Slope failure was induced by ground shaking of “Quick Clay.”

37. LANDSLIDE HAZARD: ECUADOR

40. LANDSLIDE HAZARD: JAPAN

41. LANDSLIDE HAZARD: WASHINGTON STATE

42. LATERAL SPREADING: JAPAN

44. LATERAL SPREAD: SAN FRANCISCO

45. 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

46. Landslides: Statistical Methods
Historical Database of Landslide Occurrence
Rainfall Data
Research to correlate Rainfall with Landslide Incidence
Antecedent Rainfall
Current/ Predicted Rainfall
Prediction of exactly when landslides are likely to occur
Issue warnings to affected people
Mobilise Emergency Teams
Aim: to minimise injury and loss of life

47. Landslides: Statistical Methods Landslide 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

48. Rain Gauge Network in Hong Kong
Built Up Areas

49. 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

50. 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.
(>10)

51. Landslides: Statistical Methods Landslide 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)

52. 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.

53. Basis of Lumb’s Predictor
24 – hour criteria
Cummulative Rainfall
Cumulative Rainfall over previous 15 days
Day

54. Rainfall Profile and Onset of Landslides
Cumulative Rainfall
4 hours
20 hours
Landslip
Prediction Criteria
(LPC)
Landslip Time (LT) (The time when first landslip is reported to FSD).
Warning Time (WT) (Rainfall predicted to reach LPC in 4 hours)
Criteria Time (CT)
The time when LPC are actually reached.
Actual Cumulative Rainfall Predicted Cumulative Rainfall

55. 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. 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 Event 28 - 29th May 1982
400
300
200
100 09 04
Landslide Warning: 1/82
Issued at 09:00 on 29/05/82
Landslides reported:
Total:
Squatters:
Rainfall on Landslip Day (mm) 00 20 16 12
Antecedent Rainfall in previous 15 days (mm)

58. 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
Landslide Warning: 1/82
Issued at 09:00 on 29/05/82
Landslides reported:
Total:
Squatters: 09 04
Rainfall on Landslip Day (mm) 00 20 16 12
Antecedent Rainfall in previous 15 days (mm)

59. Landslide Event 28 - 29th May 1982
Situation with running 24 hr criterion
400
300
200
100 09 04 00 20 16 12
Landslide Warning: 1/82
Issued at 09:00 on 29/05/82
Landslides reported:
Total:
Squatters:
Criterion was reached at approx 03:00
BUT
1st Landslide was reported at 02:00 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!
Antecedent Rainfall in previous 15 days (mm)

60. Antecedent Rainfall in previous 15 days (mm)
All Landslide Warning Incidents in 1982
400
300
200
100 09 04 00 20 16 12
Landslide Warning: 1/82
Issued at 09:00 on 29/05/82
Landslides reported:
Total:
Squatters: 20 16 12 08 04 00 20 16 12 16 12 08 04
LW 5/82
05:50 – 16/08/82
Total: 98
Sq: 32
LW 2/82
06:15 – 31/05/82
Total: 91/ Sq: 40
LW 7/82
23:52 – 16/09/82
Total: 3
Sq: 3 00
LW 4/82
11:00 – 03/08/82
Total: 9
Sq: 5
LW 6/82
06:35 – 18/08/82*
Total: 8
Sq: 2 16 06
LW 3/82
11:00 – 02/06/82*
Total: 28/Sq: 12 16
Antecedent Rainfall in previous 15 days (mm)

61. 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. All Rainstorm Events: Daily Rainfall vs Antecedent Rainfall
Criteria for low antecedent rainfall reduced to conform to actual 1st landslide in Event 1/82
Disastrous > 50 reported Landslides: Severe Landslides
Minor < 10 Landslides : Null Event: No reported Landslides

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. Severe and Disastrous Landslide Events: with 1984 Scheme
Existing Criteria Line - in use mid mid 1984
Warning and Landslide Lines in use from mid 1984

65. 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.

66. Landslides: Landslide Warning System
The 1984 Warning Scheme
Simple to understand
On average ~ 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

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 Caribbean Countries should learn from their experience.
Important to begin and resource fully the research needed to achieve these aims.