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Landslides

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CAUSESCAUSES OFOF LANDSLIDESLANDSLIDES

g=gp

t= 0

angle of repose

gpgp = gt= gt gt >gt > gpgp

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Water contents and normal stress Water contents and normal stress 

 V

gt

Water table 

= gp -

gp Buoyancy (B)

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- - 

SH = C + . tan φ

Where: SH = total shear strength (τ)

C = cohesion

 

= effective normal stress

 

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Strength of material on slope

•• Shear StressShear Stress

• mass of the material

•• Shear StrengthShear Strength

• cohesion• slope angle

• Shear Stress (S) = Fg x Sin θ

• internal friction

• SH = C + σ tan φ

• g = x g• If the weight =

ou om - erazag eq.• SH – total shear strength

500 kg and θ = 30

• Fg = 500 x 9.8 = 4900 N

• – co es on

•   σ - effective normal stress• ear tress =

4900 x Sin (30) = 2450 N

• tan φ - coefficient of

internal friction

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Good Good Friday Friday earthquake earthquake on on March March 27 27,  , 1964 1964,  , Alaska  Alaska 

• This area was built on sands andgravels overlying a marine clay

• The upper clay layers were

relatively stiff, but the lower layers

• The slide moved about 610 m,

a series of blocks.

of the slide, eventually sealed off the sensitive clay layer preventing

further extrusion.

• Movements along blocks resultedinto formation of pull-apart basins

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Rotational slide

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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

. .

 Alaskan earthquake.

•

extensive pull out of large volume of underground water 

or due to subsurface solution activity in limestone terrain.

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•removal of ground water 

deplete the water tablean ncreases e areaof    Vadose zone   (theunsaturated zone above

the water table)

• Because of this theempty space between

collapse after pumping

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