INFINITE SLOPE ANALYSIS
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Transcript of INFINITE SLOPE ANALYSIS
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