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Landslides

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Landslides are life-threatening hazards in many parts of the world including Hong Kong (HK). Extreme rainfall events have recently resulted in a notable increase in the number, volume and runout distance of landslides worldwide. Records from the HK Observatory show that the maximum hourly rainfall has  increased substantially over the past few decades. An unexpected rainstorm in May 2008 bombarded Lantau Island and resulted in 2,400 landslides. In August 2009, Typhoon Morakot caused landslides in Taiwan claiming more than 600 lives.

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Landslide Types *

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Landslide - Topple
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Topple

Key Features: Detach from the source due to bending force exerted followed by forward rotation and overturning
Trigger Mechanisms: Bending force/moment, loss in rotational stability at the base, increase in water pressure in tension crack
Velocity: Extremely rapid
Subdivision:

Based on constituent material

Block: Loss in rotational stability at the base, water pressure in tension crack, yielding of weak foundation and/or earthquake acceleration

Flexural: No well-defined basal joints, rotation must be facilitated by bending, slow and ductile process, tends to self-stabilization

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Landslide - Fall
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Fall

Key Features: Detachment from the source and strike to ground surface as independent rigid bodies under free fall
Trigger Mechanisms: Detachment and free fall
Velocity: Rapid to extremely rapid
Subdivision:

Based on constituent material

 

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Lanslide - Slides
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Slides

Key Features: Prominent scrap, shear band at failure plane, back-tilted bench at head and intact failure mass
Trigger Mechanisms: Shear failure, may fail in tension
Velocity: Wide range depending upon the constituent material and underlying failure mechanism
Subdivision:

Based on underlying failure mechanism and constituent material

Rotational: Spoon-like, cylindrical or other rotational rupture surface

Planar: Plane rupture surface, generally joint or bedding planes

Wedge: Two rupture surface intersecting each other forming wedge

Irregular: Irregular rupture surface with randomly oriented joints

Compound: Consists of numerous failure planes

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Landslide - Spreading
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Spreading

Key Features: Stretching of land mass horizontally
Trigger Mechanisms: Deformation of underlying material
Velocity: Rock (slow), liquefaction (extremely rapid)
Subdivision: Constituent material characteristics
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Flow like landslides
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Flow like Landslides

Key Features: High mobility with fluid like behaviors
Trigger Mechanisms: Presence of steep slope, degraded materials
Velocity: Rapid to extremely rapid
Subdivision:

Flow characteristics and constituent materials

Avalanche: non-channelized flow (very rapid to extremely rapid)

Flow: channelized flow with entrainment (rapid to extremely rapid surging flow)

Flowslide: focused on initiation (very rapid to extremely rapid)

Flood: high water/solid ratio compared to flow (very rapid)

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Flow like Landslides
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Slope Deformation

Key Features:

Climatic volume change or gravity

Trigger Mechanisms: Extremely slow or unmeasurable movement rates to very slow
Velocity: Rapid to extremely rapid
Subdivision:

Slope Deformation: Deep seated

Creep: Surficial soil layers move, resulting from change in climatic conditions.

Solifluction: freezing and thrawing of surficial soil overlying impervious layer, forms solifluction lobes

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*Modified from “The Varnes Classification of landslide types, an update” Hungr et al. (2014)