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Modeling Landslides - Force Balance

This article explores the force balance and modeling of landslides in British Columbia, focusing on the factors that contribute to the failure of slopes and the strength of materials. Topics covered include brittle failure, debris flow, shear stress, friction, and cohesion.

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Modeling Landslides - Force Balance

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  1. Modeling Landslides - 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

  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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