INFINITE SLOPE ANALYSIS

THE SIMPLEST WAY TO MODEL

LANDSLIDES!

GRAVITY

FRICTION AND

COHESION

NORMAL FORCE

GREATER THE NORMAL FORCE THE GREATER THE RESISTING FORCE

What is normal anyhow?

AN EXAMPLE WHERE THE ASSUMPTIONS OF A PLANAR SLOPE ARE WELL MET

Driving Force -- Gravity

Driving Force

τ =ρslab ghsinα

h/cos

MOHR-COULOMB FAILURE CRITERIA

Normal stress (kN/m2)

Resisting Force -- Soil Strength

s=c+ρghcos tanφ

Resisting Force

h/cos

Factor of Safety

F =RESISTINGDRIVING

=c+(Δρ gh) tanφΔρghsin

F ≤ 1Slope Instability

F ≥ 1Slope Stability

CLEARCUTTING AND LANDSLIDES

Slide scars

Soil Strength Parameters(Selby, 1993)

Material C (kpa) φ

Till 150 to 250 32o to 35o

Sand(mixed size)

0 34o to 40o

Clay(Glacial)

30 to 70 27o to 32o

Effective Cohesion -- Vegetation Effects

Plant C (Kpa)Conifer 0.9 to 4.4

Alder 2 to 12

Birch 1.5 to 9

Clover 0.1 to 2.0

from Selby, 1993

0.1

1

10

100

0 4 8 12

Fa

cto

r o

f S

afe

ty

Effective Root Cohesion (kPa)

Till

Sand (c = 1 kpa, = 37

o)

(c = 150 kpa, = 33.5 o)

saturated0.75 m slab

35o slope25o slope

Mechanics of Deforestation-Induced Erosion

• Removal of trees; suppression of vegetation by grazing

• Roots root or stumps are pulled

• Cohesion 50 in two to five years

• Greatest effect on steep, sandy slopes

• Consistent with geologic observations