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

shearfo rce

norm a l fo rce

w e igh t

• = angle of repose (= angle of friction)• Factor of safety = shear force at failure / shear force

Wedge failure in rock

angle of repose,

intersection of joint set

Factor of safety = tan / tan

Toppling failure, Masada, Israel

Effect of cohesive strength on bench height in rock

h

wcos wsin

a

a = h/sin

• Shear stress on sliding plane:

• Failure occurs when:

• Thus

hwC

hw

a

wC

sincos

sinsin

cos

Ch

w cossinsin2

tan2

cossin2

2hw

hh

Area of wedge

cossinsin

tan2

cossinsintan2

2max

22

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

shearfo rce

norm a l fo rce

w e igh t

• = angle of repose (= angle of friction)• Factor of safety = shear force at failure / shear force

Planar slip - analysis

angle of repose,

angular range where failure possible

intersection of joint set

Factor of safety = tan / tan

Planar slip in soils

GW L

Slipsurface

hw

h r

n = h /hw r

• Cohesionless soils (φ=0)

• Soils with cohesion (φ≠0)

tan

tanwnF

tan

tan

cossinw

r

n

h

CF