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

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 forceDriving 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 FORCESHEAR 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 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)

How does one find parameters?

Shear

stre

ss o

r st

ren

gth

Normal Force

What does each parameter mean?

Hint..this is the equation of a straight line!

Soil Strength Parameters

(Selby, 1993)

Str

engt

h or

she

ar f

orce

WATER MATTERSSUBTRACTING VECTORS

FORCE BALANCE MAY TIP

WET

NORMAL FORCEBOUYANCY 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 FormulaIncludes pore pressure

Effective Cohesion -- Vegetation Effects

from Selby, 1993 Orgeon slide in clearcut

CLEARCUTTING AND LANDSLIDES

Slide scars

0.1

1

10

100

0 4 8 12

Fa

cto

r o

f S

afe

ty

Effective Root Cohesion (kPa)

Till

Sand (c = 1 kpa, = 37

o)

(c = 150 kpa, = 33.5o)

saturated0.75 m slab

35o slope25o slope