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

•

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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.

• 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

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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

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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

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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

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• 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

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berms

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

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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

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• 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

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e.g. North Coast Road, Trinidad

Fill Slope

Retaining Wall

“Natural” Slope

Cut Slope

Landslides: Categorisation of Slopes

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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 13:00

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

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

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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

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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

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• 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

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Rain Gauge Network in Hong Kong

Built Up Areas

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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

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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


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• 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)

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•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.

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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

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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

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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.

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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.

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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

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0 100 200 300 400 500 600 700 800


Rai

nfal

l on

Lan

dslip

Day

(m

m)

400

300

200

100

0

0020

16

12

09

04


Even with 24hr day plotting, the plot for 29th May should have been as follows

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0 100 200 300 400 500 600 700 800


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


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!

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09

04

0020

16

12


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

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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

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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

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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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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

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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.


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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.

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

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