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

2. 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

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. ALL STRENGTH ANALYSES ARE MADE IN REFERENCE TO THE FAILURE SURFACE. Translation - flat plane rotation - circular arc

6. 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 SHEAR FORCE

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

8. 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 (  )

9. "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 50 o FOR COARSE GRAVEL TO LESS THAN 30 o 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. Normal stress (kN/m 2 ) Strength or shear force

14. WATER MATTERS SUBTRACTING VECTORS FORCE BALANCE MAY TIP WET NORMAL FORCE BOUYANCY 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

19.  0.1 1 10 100 04812 Factor of Safety Effective Root Cohesion (kPa) Till Sand (c = 1 kpa,  = 37 o ) (c = 150 kpa,  = 33.5 o ) saturated 0.75 m slab 35 o slope 25 o slope