Lecture 17: The Role of Water in Slope Stability

Key Questions1. How does the weight of water influence the

factor of safety?

2. What does too much water do to the cohesion strength of the sediment?

3. How does water interact with clay minerals in the sediment?

4. How does water decrease the “friction force” and hence the factor of safety?

La Conchita, CA

http://kula.geol.wwu.edu/rjmitch/214www.html

.

Slope Model

θ

.W

The slope fails if FS is less than “1”

FS  = resisting forces

driving forces=

W x cosθ xμ + c

W x sinθ= 

friction + cohesion

Fp

θ

.WFNFf

cohesion = c

FpθFriction =

Water makes slopes unstable

1. Water adds weight

.

θ

.

W

One gallon of water weighs 8 lbs

If a 100 by 200 foot slope soaks up 2 inches of rain, the slope weight increases by 200,000 lbs

Burien, WA near Shorewood Drive SW and 131st Street

The slope fails if FS is less than “1”

FS  = resisting forces

driving forces= 

friction + cohesion

Fp

θ

.WFNFf

cohesion = c

FpθFriction =

W x cosθ xμ + c

W x sinθ=

water increases the weight of the slope which increases the driving force Fp

2. Too much water in the voids spaces reduces or eliminates the cohesion force holding the grains together.

Remember, the cohesion force requires air and water in the pores spaces.

air

grain

water

The slope fails if FS is less than “1”

FS  = resisting forces

driving forces= 

friction + cohesion

Fp

θ

.WFNFf

cohesion = c

FpθFriction =

too much water decreases cohesion

W x cosθ xμ + c

W x sinθ=

.

θ

.W

3. Slopes with high clay contents are more unstablewhen wet.

Scanning electron microscope image of clay

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Clay minerals have a “negative” surface charge

water is polar

__

++

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

clay minerals attract water molecules to their surfaces

dry, more friction, hence more resistant to sliding wet, less friction, hence lower strength

The slope fails if FS is less than “1”

FS  = resisting forces

driving forces= 

friction + cohesion

Fp

θ

.WFNFf

cohesion = c

FpθFriction =

Fp

wet clay reduces friction

=W x cosθ xμ + c

W x sinθ

4. Water buoys up the grains which reduces the friction force

.

θ

.

W

.

Slope Model

θ

.W

shear plane

friction force

weight

W.FN = W x cosθ = normal force

θ

θ

cosθ = adjacent

hypotenuse

friction force

weight

friction force = normal force x coefficient of friction

or friction force = FN x μ

FN

friction force

What happens when the grains are immersed in water?

=water surface

P = ρw·g·h = hydrostatic pressure

P = water pressure

ρw = water density

g = acceleration of gravity h

h = depth to the balloon

=water surface

P2 = ρw·g·h2

P = water pressure

ρw = water density

g = acceleration of gravity h1

h = depth to the balloon

h2

P1 = ρw·g·h1

Cubic grain immersed in water (side length “L”)

Fluid pressure at the top = P = ρgh

ρ = water density

g = acceleration of gravity

h = depth below the water surface

Fluid pressure at the bottom = P = ρg(h +L)

L

Cubic grain immersed in water (side length “L”)

Fluid pressure at the top = P = ρgh

ρ = water density

g = acceleration of gravity

h = depth below the water surface

Fluid pressure at the bottom = P = ρg(h +L)

ΔP = ρgL = change in pressure

L

Cubic grain immersed in water (side length “L”)

Fluid pressure at the top = P = ρgh

ρ = water density

g = acceleration of gravity

h = depth below the water surface

Fluid pressure at the bottom = P = ρg(h +L)

ΔP = ρgL = change in pressure

Area = L2

Force = pressure x area

L

Cubic grain immersed in water (side length “L”)

Fluid pressure at the top = P = ρgh

ρ = water density

g = acceleration of gravity

h = depth below the water surface

Fluid pressure at the bottom = P = ρg(h +L)

ΔP = ρgL = change in pressure

Area = L2

Force = ΔP x L2

L

Cubic grain immersed in water (side length “L”)

Fluid pressure at the top = P = ρgh

Fluid pressure at the bottom = P = ρg(h +L)

ΔP = ρgL = change in pressure

Area = L2

Force = ρgL3 = ρgV = buaoyancy force = FB

L

Cubic grain immersed in water

FB = ρgV = the magnitude of buoyancy force is equal to the weight of the volume of fluid the grain displaces (Archimedes’ Principle)

Cubic grain immersed in water

FB

The grain weight is reduced by FB

buoyancy force

Wnew = Wdry – buoyancy force

buoyancy force = the weight of the volume of water the grain displaces

friction force

the weight of each grain is reduced by the buoyancy force

The total weight pushing on the shear plane is reduced

friction force

weight

friction force = normal force x coefficient of friction

or friction force = FN x μ

FN

The slope fails if FS is less than “1”

FS  = resisting forces

driving forces= 

friction + cohesion

Fp

θ

.WFNFf

cohesion = c

FpθFriction =

water decreases friction and cohesionand increases Fp all of which reduce FS

W x cosθ xμ + c

W x sinθ=

FS  = resisting forces

driving forces= 

friction + cohesion

Fp

FS  = resisting forces

driving forces= 

friction + cohesion

Fp

FS  = resisting forces

driving forces= 

friction + cohesion

Fp

FS  = resisting forces

driving forces= 

friction + cohesion

Fp

FS  = resisting forces

driving forces= 

friction + cohesion

Fp

FS  = resisting forces

driving forces= 

friction + cohesion

Fp

USGS Experimental Debris –Flow Flume

http://www.ecy.wa.gov/programs/sea/landslides/show/seattle.html

Seattle Area, Washington

Cumulative Precipitation Threshold

Seattle Area, Washington

Precipitation Intensity-Duration Threshold

The Pe Ell landslide occurred on December 3rd, 2007, blocking State Highway 6 and destroying three structures

http://www.dnr.wa.gov/ResearchScience/Topics/GeologicHazardsMapping/Pages/landslides_dec07storm.aspx

Dec 11, 2004 - Massive landslide on the Sultan River, CA