Natural Geo-disaster and its Prevention

Landslide Problems and Remedies

Md. Azizul Moqsud, Ph.D.

Guest Associate Professor

Institute of Lowland and Marine Research

Saga University

moqsud@gmail.com

8th January 2011

BENJapan's Webiner/Tele-conference on Environmental Aspects

What is Landslide??• A landslide (or landslip) is a

geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments.

• Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability.

Damage caused by Landslide

• Block roads; • Damage and destroy

homes; • Locally disrupt water

mains, sewers, and power lines;

• Damage oil- and gas- production facilities.

• Kill many people

Important Concepts• Earth materials are subject to

movement under the force of gravity:

- these are generally termed landslides, slope failure or mass wasting

- when movement is strictly vertical it often called subsidence

- when dominated by water (e.g like a slurry) they will be termed debris flows .

Slope elements with examples from nature

Mechanisms of Landsliding• Landsliding is controlled by the ratio of 1) resisting force to 2) driving forces

Factor of safety (FS) for slope stability is equal to this ratio

• FS greater than 1 means slope considered stable

Driving forces are increased by:- steep slopes- increasing weight on a slope- Resisting forces are weakened

Causes of landslides

• Natural causes of landslides include:• groundwater (porewater) pressure acting to destabilize the slope • Loss or absence of vertical vegetative structure, soil nutrients, and

soil structure (e.g. after a wildfire) • erosion of the toe of a slope by rivers or ocean waves• weakening of a slope through saturation by snowmelt, glaciers

melting, or heavy rains• earthquakes adding loads to barely-stable slopes • earthquake-caused liquefaction destabilizing slopes (see Hope

Slide) • volcanic eruptions

Landslides are caused when the stability of a slope changes from a stable to an unstable condition.

Man made causes• landslides are aggravated by human activities,

Human causes include: deforestation, cultivation and construction, which destabilize the already fragile slopes

• vibrations from machinery or traffic• blasting• earthwork which alters the shape of a slope, or which

imposes new loads on an existing slope • in shallow soils, the removal of deep-rooted

vegetation that binds colluvium to bedrock• Construction, agricultural or forestry activities

(logging) which change the amount of water which infiltrates the soil.

More man made causes

• Excavation • Loading • Drawdown• Land use change • Water management • Mining • Water leakage • Deforestation

Geological causes• Weak materials • Sensitive materials • Weathered materials • Sheared materials • Jointed or fissured materials • Adversely orientated discontinuities • Permeability contrasts • Material contrasts • Rainfall and snow fall

Morphological causes

• Slope angle • Uplift • Rebound • Fluvial erosion • Wave erosion • Glacial erosion • Erosion of lateral margins • Subterranean erosion • Slope loading • Vegetation change

Physical causes• Intense rainfall • Rapid snow melt • Prolonged precipitation • Rapid drawdown • Earthquake • Volcanic eruption • Thawing • Freeze-thaw • Shrink-swell • Ground water changes • Soil pore water pressure • Surface runoff • Seismic activity

Landslide Mitigation WorksLandslide mitigation works are conducted in order to stop or reduce the

landslide movement so that the resulting damages can be minimized.

The landslide mitigation works are broadly classified into two categories:– 1) control works; and– 2) restraint works.

The control works involve modifications of the natural conditions of landslides such as topography, geology, ground water, and other conditions that indirectly control portions of the entire landslide movement.

The restraint works rely directly on the construction of structural elements.

Landslide Control Works• a) Surface Drainage Control Works

– The surface drainage control works are implemented to control the movement of landslides accompanied by infiltration of rain water and spring flows. The surface drainage control works include two major works: drainage collection works and drainage channel works.

– The surface drainage control works are often combined with the subsurface control works

An example of a drainage ditch with interceptor drain

b) Subsurface drainage Control Works

• The purpose of the subsurface drainage control works is to remove the ground water within the landslide mass and to prevent the inflow of ground water into the landslide mass from outside sources.

Interceptor trench drains

A drainage well usnig reinforced concrete segments

Drilling works for horizontal gravity drains Outlet of horizontal drain pipes

A drainage well using steel segments Drainage well works

Landslide MitigationMitigation approaches focus on increasing resisting

forces or reducing driving forces

Technique Decrease driving force

Increase resistance

Drainage control (removal/diversion of water)

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Grading (slope steepness adjustment)

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Slope supports (piles, walls, bolts)

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Landslide prevention for house foundation

Retaining wall slope support

Drainage control above a slope to reduce water pore pressure

Landslide Restraint Worksa) Pile Works– The pile works consist of driving piles as keys to tie together the moving landslide and the

stable ground to restrain the movement. Generally, a thick walled steel pipe is used as the pile, and is then filled with concrete

b) Large Diameter Cast-in-Place Pile Works– The large diameter cast-in-place works function similar to those of the pile works and are

designed to tie the moving landslide and the stable ground together. However it involves much larger diameters. The construction is similar to the drainage well, and generally consists of pile of 1.5 to 6.5m in diameter and filed with reinforced concrete. compared to the piles, the large diameter cast-in-place type are much more resistant to bending stresses

c) Anchor Works– The anchor works utilize the tensile force of anchor bodies embedded through the slide mass

and into stable earth, and are connected to thrust blocks located on the ground surface. The thrust blocks are anchored with a tendon that counteracts the driving forces of the landslide to restrain the slide movement. The advantage is that large restraint forces can be obtained from a relatively small cross sectional tendon

d) Retaining Walls– Retaining walls are constructed to prevent smaller sized and secondary landslides that often

occur along the toe portion of the larger landslides. Because of the large-scale earth- movement and numerous springs that are expected in landslide terrain, crib walls are common instead of conventional reinforced concrete retaining walls .

Pile works Under construction of the large diameter cast-in-place pile works

Anchor works An example of crib walls

U.S. Landslide Hazard Map

Landslide in Yamaguchi 2009

Torrential rains triggered floods and landslides in southern Japan, leaving at least 8 people dead and 10 others missing, including elderly residents at a nursing home。

Recent Landslides in Bangladesh

15th June 2010: 45 people killed by landslide in Cox’s Bazar.

11th June 2007: 125 people killed by landslide in Chittagong.

Thank you so much for your kind attention!!

Lets work together to stop climate change for the next generation..