Landslides - brittle failure Modeling Landslides - 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

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 force Driving 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 FORCE SHEAR 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 TO PASCALS. TILLS HAVE COHESIONS ABOVE 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 )

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

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

Soil Strength Parameters (Selby, 1993)

Strength or shear force

WATER MATTERS SUBTRACTING VECTORS FORCE BALANCE MAY TIP WET NORMAL FORCE BOUYANCY 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 Formula Includes pore pressure

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

CLEARCUTTING AND LANDSLIDES Slide scars

 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