Slope stability I EOSC316 Engineering Geoscience
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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
−+=
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