1. Modeling Landscapes - Force Balance
Landslides - brittle failure
Debris Flows- plastic materials
Rockslide spawns debris flow in British Columbia, GSC

2. FAILURE OCCURS WHEN THE STRESS ON 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
~ shear strength
FRICTION AND COHESION

3. 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

4. FACTOR OF SAFETY Resisting force Driving force >1, slope is stable
<1 slope fails
Failure driven by transient event, superimposed on geology

5. LANDSLIDES OCCUR ON A FAILURE SURFACE
LANDSLIDES OCCUR ON A FAILURE SURFACE. ALL STRENGTH ANALYSES ARE MADE IN REFERENCE TO THE FAILURE SURFACE.
Translation - flat plane
rotation - circular arc

6. SHEAR FORCE 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

7. GREATER THE NORMAL FORCE THE GREATER THE RESISTING FORCE
FRICTIONAL STRENGTH
GREATER THE NORMAL FORCE THE GREATER THE RESISTING FORCE
What is normal anyhow?
NORMAL FORCE

8. NORMAL FORCE () 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 ()

9. "c" COHESION IS THE TENDENCY OF A MATERIAL TO "STICK" TO ITSELF
"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 TO PASCALS. TILLS HAVE COHESIONS ABOVE 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)

10. How does one find parameters?
Shear stress or strength
Normal Force

11. What does each parameter mean?
Hint..this is the equation of a straight line!

12. Soil Strength Parameters (Selby, 1993)

13. Strength or shear force
Normal stress (kN/m2)

14. WATER MATTERS SUBTRACTING VECTORS
FORCE BALANCE MAY TIP
BOUYANCY FORCE
WET
NORMAL FORCE
High water table (pore pressure) reduces effective normal force

15. 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).

16. Factor of Safety Calculating Formula Includes pore pressure

17. Effective Cohesion -- Vegetation Effects
from Selby, 1993
Orgeon slide in clearcut

18. CLEARCUTTING AND LANDSLIDES
Slide scars
CLEARCUTTING AND LANDSLIDES

19. 100 Till 10 Sand 1 0.1 4 8 12 Factor of Safety
100
Till o
(c = 150 kpa, 
= 33.5 ) 10
Sand
Factor of Safety o
(c = 1 kpa, 
= 37 )
25o slope 1
35o slope
saturated
0.75 m slab
0.1 4 8 12
Effective Root Cohesion (kPa)

20. Bishop method of slices
Rotational Failures

21. http://www. uky. edu/AS/Geology/howell/goodies/elearning/module11swf

22. What is the shear strength of debris flow material in Death Valley?
Rise/run
60’/1200’= 0.02
What is the shear strength of debris flow
material in Death Valley?
2000 kg m-3 * 9.8 m s-2 * 3 m * sin 
 g h sin 
2000 kg m-3 * 9.8 m s-2 * 3 m * 0.02
(h) Measured thickness = 3 m
(measured)= 2000 kg m-3
g (given) = 9.8 m s-2
(from map) = tan -1 (0.02) = 1.15o
= 1176 m-1 s-2 (pascals)
= 1.2 kp