Slope stability I

EOSC316 Engineering

Geoscience

Landslip trigger processes

• Water

• Toe removal

– Natural

– Artificial

• Head loading

• Vibration

Folkestone Warren, Kent

• Toe erosion due to

extension of harbour

wall, 1905

• Since 1915 slide

– mass concrete toe weights

– drainage adits

Mt. Huascaran, Peru, 1970

• earthquake triggered

• hit towns of Yungay

and Ranrahirca, 18

km away, at around

150 km/hr

• Yungay completely

buried, 66,000 dead

Types of landslide

• Rock failure

– failure plane pre-

determined

• Soil failure

– failure plane along line

of max stress

Rock failure

Planar translational slip in rock

φ

shearforce

normal force

weight

• φ = angle of repose (= angle of friction)

• Factor of safety = shear force at failure / shear force

Wedge failure in rock

angle of

repose, φ

intersection of joint set

β

Factor of safety

= tan / tanφ β

Toppling failure, Masada, Israel

Effect of cohesive strength on

bench height in rock

θ

h

wcosθwsinθ

a

a = h/sinθ

• Shear stress on sliding plane:

• Failure occurs when:

• Thus

hwC

hw

a

wC

θθµ

θθτ

θµ

sincos

sinsin

cos

+==

+=

( ) Ch

w≥− θθµθ cossinsin

2

θ

ρ

θθ

tan2

cossin2

2h

w

hh

=∴

⋅=Area of wedge

( )

( )θθµθρ

θ

θθµθθ

ρ

cossinsin

tan2

cossinsintan2

2max

2

2

−=

≥−

Ch

or

Ch

h

Half dome, Yosemite Carrara marble quarries, Italy

Rock failure – remedial measures

Slope failure in ‘unconsolidated’

material

Planar versus rotational slip in

unconsolidated material

• Planar slip

– frictional

– cohesionless

– C = 0

• Rotational slip

– frictional

– cohesional

– C ≠ 0

La Conchita, Santa Barbara, California,

Spring 1995

Planar translational slip

φ

shearforce

normal force

weight

• φ = angle of repose (= angle of friction)

• Factor of safety = shear force at failure / shear force

Planar slip - analysis

angle of

repose, φ

angular range where failure possible

intersection of joint set

β

Factor of safety

= tan / tanφ β

Planar slip in soils

GWL

Slipsurface

β

hw

hr

n = h /hw r

• Cohesionless soils (φ=0)

• Soils with cohesion (φ≠0)

( )βρ

φρρ

tan

tanw

nF

−=

( )βρ

φρρ

ββρ tan

tan

cossin

w

r

n

h

CF

−+=