Module 2b Landslides , Types, causes & effects

LANDSLIDES – Module 2 02-04-2019

Compiled by Shimi Lawrence, SCTCE 1

Module 2b Landslides , Types, causes &

effects

CE 488



DefinitionLandslides – the movement of a mass of rock, debris or earth downthe slope, when the shear stress exceeds the shear strength of thematerial

Earth movements – all movement of land masses are referred aslandslides. All types of landslides are categorized as earth movements.(eg : Earth flow, landslides, subsidence)

A process by mainly gravitational forces – Mass movement of rock,debris or earth down the slope.



Landslides

The occurrence of slope movements is the consequence of a complex field offorces (stress is a force per unit area) which is active on a mass of rock or soilon the slope.

The consequence of these forces in conjunction to the slope morphology andthe geotechnical parameters of the material define together the specific typeof landslide which might occur.

Landslides -Increase in shear strength (causes landslides)

➢ Removal of lateral and underlying support (erosion, previousslides, road cuts and quarries)

➢ Increase of load (weight of rain/snow, fills, vegetation)

➢ Increase of lateral pressures (hydraulic pressures, roots,crystallisation, swelling of clay)

➢ Transitory stresses (earthquakes, vibration of trucks, machinery,blasting)

➢ Regional tilting (geological movements)



Causes - Natural1. Ground water pressure acting to destabilize the slope

2. Loss or absence of vertical vegetative structure, soil nutrients, andsoil structure

3. Erosion of the toe of a slope by rivers or ocean waves

4. Weakening of a slope through saturation of snow melt, glaciersmelting or heavy rains

5. Earthquakes adding loads to barely stable slopes

6. Earthquake caused liquefaction destabilizing slopes

7. Volcanic eruptions

Causes - human1. Vibrations from machinery or traffic

2. Blasting

3. Earthwork which alters the shape of the slope or which imposesnew loads on an existing slope

4. In shallow soils, the removal of deep rooted vegetation that bindscolluvium to bed rock.

5. Construction , agricultural or forestry activities which change theamount of water which infiltrates into the soil



Landslides – on earth movements



Causes of landslides /landslipsGeological Morphological Physical Human

Weathered material due to Heavy rainfall

Slope angle Intense rainfall Excavation

Shearing of soil Uplift Rapid snow melt Loading

Jointed or fissured materials

Rebound Prolonged precipitation Draw- down

Adversely oriented discontinuities

Fluvial erosion Earthquake Land use pattern change (road, house)

Permeability contrasts Wave erosion Volcanic eruption Water management

Material contrasts Glacial erosion Thawing Mining

Rainfall. Snow fall Erosion of lateral margins

Freeze – thaw Quarrying

Working of machinery Subterranean erosion

Ground water changes Vibration

Earthquakes Slope loading Soil pore water pressure Water leakage

Vegetation change Surface run off Deforestation

erosion Seismic activity Land use pattern

Soil erosion Pollution

Classification of Earth Movements – Earth FlowsAre down the slope, viscous flows of saturated, fine-grained materials, which move at any speed from slow to fast.

Reasons – heavy rains, volcanic eruptions, earthquakes



Munnar 2018 – earth flow

Photograph from aerial survey showing the upper parts of the landslidethat occurred in northwest Washington on March 22, 2014.



Mud flow & LaharMudflow (mudslide) Lahar

A mudflow, when the soil becomes so saturated with water that it speeds down the hill in a muddy river carrying debris

Lahar is the volcanic debris flow.

Fastest and wettest type of flow Rapidly flowing – contains rock debris and water

SOLIFUCTIONSolifuction is a downward movement of wet soil along the slopes under the influence of gravity



SOIL CREEP (depth 1-2m)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 whenthere is a thaw, the particles are set back down, but not in the sameplace as before. It is very important for Civil Engineers to know the rateof movement (few centimetres per day)

Can be slowed down by afforestation







Snow meltCold countries

Sudden increase in temperature leads to rapid melting of snow pack.This water try to infiltrate to ground, which may have impermeablelayers below the surface (still frozen soil, rock water)

Leading to increase in pore water pressure and landslides.

A serious phenomenon

When warmer weather is followed by precipitation, (precipitation andmelting of water – add water pressure), accelerating thawing.

RiversUndercutting of slopes, during floods in rivers.

Increases gradient of slope, reducing stability, and toe removal –accelerating instability of slopes.



SeismicityAs a result of seismic shaking and pore water pressure generation

Seismic shaking

The passage of the earthquake waves through the rock and soil produces a complex set of accelerations that try to change the gravitational load on the slope.

Vertical accelerations successively increases the or decreases the normal load on the slope,

Horizontal accelerations induce shearing force due to inertia of the landmass.

LiquefactionThe passage of the earthquake waves through a granular material such as soil induce liquefaction.

Pores inside the soil reduces and density of soil increases.

Sometimes, the material can generate FLOW SLODES that can be rapodand creating damages.

The increase in the pore pressure can reduce the normal stress in the slope allowing the activation of translational and rotational failures



IS recommended remedial measures

1. Restraining structures

2. Easing of pressure by excavation

3. Reconstruction of slope using reinforced earth

4. Rock reinforcement

Are the direct methods

Indirectly – erosion control measures, and improving in surface and sub-surface drainage can reduce land slides



Restraining Structures (ht <=4m)

Dry, banded and mortar masonry walls

Gabions/ sausage walls

Concrete retaining walls

Restraining piles

Excavation methodsTo increase stability of the soil mass beneath a slope.

The main methods◦ Removal of unstable materials

◦ Flattening of slopes

◦ Benching of slopes,

◦ Change of line or grade and

◦ Alteration of slope geomety



Reconstruction of slope using reinforced earthReinforced earth using metallic stripes

Geofabric reinforcement structures on slopes

Slope reinforcement by soil nailing

Micro-piles, root piles for slope stabilisation

Erosion control measuresGrass plantation and shrubs to restore vegetative cover

Jute / coir netting



DrainageSurface drainage : Control of surface water flow

• Collection of runoff at the uphill boundary of any unstable area

• Maximixing runoff from the unstable area and controlling and collecting the run off.• Catch water or interceptor drains

• Road side drains

• Cross drains

Subsurface drainage : more effective for deep seated landslide, because it leads to a decrease in pore water pressure directly at the failure plane and tends to produce a more stable condition of slide area

1. Horizontal drains

2. Deep trench drains.



Landslide warning signs1. Sticking or jamming of doors or windows

2. Appearance of cracks in plaster, tile, brick or foundations

3. Pulling away from the building of outside walls or stairs

4. Slow development of widening cracks on the ground or an paved areas such as streets

5. Breakage of underground utility lines

6. Appearance of bulging ground at the base of a slope

7. Emergence of flowing ground water in new sites

8. Sudden decrease in creek water levels through rain is still falling or just recentlystopped

9. Tilting or moving of fences, retaining walls, utility poles or trees

10. Faint rumbling sound that increases in volume as the landslide nears. The groundslopes downward in one specific direction and may begin shifting in that directionunder his/her feet



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Landslides Prediction

The development of a landslide is a temporal process◦ takes as long as a year to develop

◦ Movement speed, several cm per month

Landslides are spatial in nature.◦ Position

◦ Movement direction

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System Architecture

Deployment◦ A network of sensor columns

◦ Placed in vertical holes drilled over the hill surface

Using sensor columns to detect movements



What was the biggest landslide in the world?

The world's biggest historic landslide occurred during the 1980 eruptionof Mount St. Helens, a volcano in the Cascade Mountain Range in theState of Washington, USA. The volume of material was 2.8 cubickilometers (km).

The world's biggest prehistoric landslide (discovered so far and on land),is in southwestern Iran, and is named the Saidmarreh landslide. Thelandslide is located on the Kabir Kuh anticline in Southwest Iran at 33degrees north latitude, 47.65 degrees east longitude. The landslide hasa volume of about 20 cubic kilometers, a depth of 300 m, a traveldistance of 14 km and a width of 5 km. This means that about 50 billiontons of rock moved in this single event!

Effects &

Consequences of Landslides



Effects & consequences of landslides

Effect Type Such as

Direct & indirect

Direct Effects just after the event Death, damage

Indirect Events after the event Psycho-social impacts, bereavement and evacuation

Tangible & intangible

Tangible Possible to assign reasonable reliable monetary values

Replacement of damaged property

Intangible Cannot be satisfactorily assessed in monetary terms

Loss of human life (difficult to assess financially)

Built & natural env

Built Agricultural land, infrastructure

Water supplies, fisheries, sewage disposal systems, dam

Natural Forest

Effects on landslides on Built environment

Residentialbuildings onunstableslopes

Partial damage tocompletedestruction

Landslides destabilize or destroyfoundations, walls, floors,surrounding property, utilities suchas water pipelines, power lines,communication lines, transportroutes.

Commercialstructures

Access routesdamaged.

Disruption in business



Impact of Landslides (1/2)Effect of Landslides on

Buildings

Development of cracks

Collapse of buildings, or complete damage/ burial. ( Slowmoving/ creep)

Burial of buildings (Fast moving landslides)

Transportation system (roads & railways)

Blockages of highways , temporary diversion of traffic or evenclosure of road traffic or a major excavation.

State and National highways affected.

Risk to vehicles and passengers on road side.

Essential services (power, communication, water supply)

Rigid pipelines and structures more affected when comparedwith flexible/elastic pipelines and structures.

Health and education sectors Patients, pregnant ladies, school children, very small childrenand elderly persons.

Natural environment Morphology of earth system – mountain valley system,continents, beneath oceansForest and grass land cover, wild life, rivers.Dams

Impact of Landslides (2/2)Effect of Landslides on

Cultural heritage Damage to the heritage structures (Utharakhand), pine trees (>300 trees) uprooted in 2014

National/ Border security Blockage of roads in international border areas.Movement of army vehicles and troupes on road.

Economic effect Cost of repair, loss of property value, distruption oftransportation routes, medical costs (injury), wateravailiability(non) , quality and quantityIllhealth

Interrelationship of landslides and other hazards (THE MULTIPLE HAZARD EFFECT)

Floods, EQ, Volcanic eruptions can cause landslides. One cantrigger other.

EQ, triggered landslide in ocean can cause Tsunami. (if largeqty of soil displaced)

Volcanic eruption induced landslides, can block river way andflooding upstream.

Landslide in dams – can cause flooding downstreamLandslide in dam, if dam fails, flooding downstream. Riverbank damage, coastal erosion,



Fast moving landslides –

Rockfalls/topples,debris flow

Occur without precursors and warnings, move too quickly –for any safety measures to be taken, with high momentumdue to its mass and velocity, that can destroy anything thatcomes in its way.

Slow movinglandslides

Slight damage and its slow pace may allow implementationof mitigation measures as well. Can destroy structures overtime.

Nature of landslidemovement

Can continue after days, weeks, or months preclude(preventing from happening) rebuilding on the affectedarea, unless mitigation measures are taken appropriately.

Increase of Population

People tend to move to new lands, which deemed to be toohazardous in the past.

Poor / non – existent land use policies allow buildingconstructions to take place on the land that might be betterfor agriculture, parks.

Unsafe building practices and may not have legitimate oradequate means/ expertise to do so.

Documents

Module 2b Landslides , Types, causes & effects