Landslides - brittle failure

1
Modeling Landscapes - Force Balance

1. Landslides - brittle failure

Rockslide spawns debris flow in British Columbia,
GSC
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FAILURE OCCURS WHEN THE STRESS ON A MATERIAL
EXCEEDS THE STRENGTH OF THAT MATERIAL.
A deposit at equilibrium can tell you about
material strength since you can calculate driving
force
GRAVITY shear stress
FRICTION AND COHESION shear strength
3
Simple Physicsvectors

• DRIVING force (SHEARING STRESS) is gravity,
• RESISTING force is soil/rock strength/FRICTION
• Steep slopes give greater Driving Force (STRESS)

Vectors lengthmagnitude
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FACTOR OF SAFETY
Resisting force Driving force
gt1, slope is stable lt1 slope fails
Failure driven by transient event, superimposed
on geology
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LANDSLIDES OCCUR ON A FAILURE SURFACE. ALL
STRENGTH ANALYSES ARE MADE IN REFERENCE TO THE
FAILURE SURFACE.
Translation - flat plane
rotation - circular arc
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h IS THE THICKNESS OF THE SLAB MEASURED
PERPENDICULAR TO THE SURFACE, meters ??IS THE
DENSITY OF THE SLAB MATERIAL, kg m-3 ? IS THE
SLOPE ANGLE OF THE FAILURE SURFACE g IS THE
ACCELERATION OF GRAVITY, 9.8 m s-2
SHEAR FORCE
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FRICTIONAL STRENGTH GREATER THE NORMAL FORCE THE
GREATER THE RESISTING FORCE
What is normal anyhow?
NORMAL FORCE
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h IS THE THICKNESS OF THE SLAB MEASURED
PERPENDICULAR TO THE SURFACE, meters ??IS THE
DENSITY OF THE SLAB MATERIAL, kg m-3 ? IS THE
SLOPE ANGLE OF THE FAILURE SURFACE g IS THE
ACCELERATION OF GRAVITY, 9.8 m s-2
NORMAL FORCE (?)
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"c" COHESION IS THE TENDENCY OF A MATERIAL TO
"STICK" TO ITSELF. CLAYS ARE COHESIVE AS ARE
ROCKS OR SOILS WHICH HAVE UNDERGONE SOME
CEMENTATION. COHESION IS EXPRESSED IN PASCALS
(kg m-1 s-2). CLAYS HAVE COHESIONS RANGING FROM
10000 TO 150000 PASCALS. TILLS HAVE COHESIONS
ABOVE 150000 PASCALS. ????OR THE ANGLE OF
INTERNAL FRICTION IS A PROPERTY OF THE MATERIAL
AND RANGES FROM OVER 50o FOR COARSE GRAVEL TO
LESS THAN 30o FOR FINE SAND. ? IS THE NORMAL
FORCE EXCERTED ON THE FAILURE SURFACE BY THE
MATERIAL ABOVE IT.. THE NORMAL STRESS CAN BE
EXPRESSED IN PASCALS (kg m-1 s-2)
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How does one find parameters?
Shear stress or strength
Normal Force
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• What does each parameter mean?

Hint..this is the equation of a straight line!
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Soil Strength Parameters(Selby, 1993)
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Strength or shear force
Normal stress (kN/m2)
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WATER MATTERSSUBTRACTING VECTORS
FORCE BALANCE MAY TIP
BOUYANCY FORCE
WET NORMAL FORCE
High water table (pore pressure) reduces
effective normal force
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• u IS THE PRESSURE OF THE WATER AT THE FAILURE
  PLAIN WHICH CAN BE EXPRESSED IN PASCALS (kg m-1
  s-2) USING THE EQUATION BELOW. h' IS THE
  ELEVATION OF THE WATER TABLE ABOVE THE FAILURE
  SURFACE (meters).

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• Factor of Safety
• Calculating Formula
• Includes pore pressure

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Effective Cohesion -- Vegetation Effects
from Selby, 1993
Orgeon slide in clearcut
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CLEARCUTTING AND LANDSLIDES
Slide scars
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?
100
Till
o

(c 150 kpa,
??
33.5
)
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Sand
Factor of Safety
o

(c 1 kpa,
??
37
)
25o slope
1
35o slope
saturated
0.75 m slab
0.1
0
4
8
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Effective Root Cohesion (kPa)