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1. Landslides - brittle failure
Modeling Landscapes - Force Balance
Rockslide spawns debris flow in British Columbia, GSC
GRAVITY ~ shear stress
FRICTION AND COHESION
~ shear strength
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
A deposit at equilibrium can tell you about material strength since you can calculate driving force
Simple Physics……vectors• DRIVING force (SHEARING STRESS) is gravity, • RESISTING force is soil/rock strength/FRICTION• Steep slopes give greater Driving Force (STRESS)
Vectors: length=magnitude
FACTOR OF SAFETY
Resisting forceDriving force
>1, slope is stable
<1 slope fails
Failure driven by transient event, superimposed on geology
LANDSLIDES OCCUR ON A FAILURE SURFACE.
ALL STRENGTH ANALYSES ARE MADE IN
REFERENCE TO THE FAILURE SURFACE.
Translation - flat plane rotation - circular arc
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 FORCESHEAR FORCE
NORMAL FORCE
FRICTIONAL STRENGTH
GREATER THE NORMAL FORCE THE GREATER THE RESISTING FORCE
What is normal anyhow?
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 ()
NORMAL FORCE ()
"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)
How does one find parameters?
Shear
stre
ss o
r st
ren
gth
Normal Force
What does each parameter mean?
Hint..this is the equation of a straight line!
Soil Strength Parameters
(Selby, 1993)
Normal stress (kN/m2)
Str
engt
h or
she
ar f
orce
WATER MATTERSSUBTRACTING VECTORS
FORCE BALANCE MAY TIP
WET
NORMAL FORCEBOUYANCY FORCE
High water table (pore pressure) reduces effective normal force
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).
Factor of Safety
Calculating FormulaIncludes pore pressure
Effective Cohesion -- Vegetation Effects
from Selby, 1993 Orgeon slide in clearcut
CLEARCUTTING AND LANDSLIDES
Slide scars
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, = 33.5o)
saturated0.75 m slab
35o slope25o slope