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Modeling Landscapes - Force Balance

Modeling Landscapes - Force Balance. Landslides - brittle failure. Rockslide spawns debris flow in British Columbia, GSC. FAILURE OCCURS WHEN THE STRESS ON A MATERIAL EXCEEDS THE STRENGTH OF THAT MATERIAL.

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Modeling Landscapes - Force Balance

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

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

  8. hIS 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. 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)

  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 MATTERSSUBTRACTING 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. Slide scars CLEARCUTTING AND LANDSLIDES

  19. 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 0 4 8 12 Effective Root Cohesion (kPa)

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