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LOGO Civil Engineering

Geotechnical Engineering 2

Slope Stability

References:

Introduction to Geotechnical Engineering by: Braja M. Das

An Introduction to Geotechnical Engineering by: Robert Holtz, William

D. Kovacs, and Thomas C. Sheanan

Dr. Marcos

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Key Terminology

stability finite slope

slope infinite slope

factor of safety Unrestrained slope

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Classification of Slope Failures

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slope can be natural or constructed.

a component of gravity will cause the soil to

move downward.

if the gravity is large enough, slope failure can

occur.

the driving force overcomes the resistance

from the shear strength of the soil along the

rupture surface.

An exposed ground surface that stands at an angle

with the horizontal is called an unrestrained slope.

Slope Stability Analysis

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Slope failure

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Slope Stability Analysis

civil engineers are expected to make

calculations to check the safety of natural

slopes, slope excavations, and compacted

embankments.

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Factor of Safety

Analyzing slope stability is determining the factor of safety

d

f

sFS

where:

FSs = factor of safety with respect to strength

τf = average shear strength of the soil (resisting forces

or moments)

τd = average shear stress developed along the potential

failure plane (driving forces or moments)

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Factor of Safety

dds

c

cFS

'''

'''

tan

tan

where:

c' = cohesion

ϕ' = drained angle of friction

σ’ = effective normal stress on the potential failure

surface

cd' = effective cohesion develop along the potential

failure surface

ϕd' = drained angle of friction develop along the

potential failure surface

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Factor of Safety

dc

c

cFS

'

'

'

Factor of safety

with respect to

cohesion

d

FS''

''

'tan

tan

Factor of safety

with respect to

friction

ddc

c'

'

'

'

tan

tan

'' FSFSFS cs

If FS=1 the slope is in the state of impending failure.

Generally, a value of 1.5 for the factor of safety with

respect to strength is acceptable for the design of a stable

slope.

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Stability of Infinite Slope

An infinite slope is one in which the depth to failure surface is

much smaller than the slope height.

If FS=1, H=critical depth

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Stability of Infinite Slope

If there is seepage through the soil and the ground water level

coincides with the ground surface.

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Stability of Finite Slope

When the value of Hcr approaches the height of the slope, the

slope is generally considered finite.

Culmann’s Method

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Sample Problems