lecture 17: the role of water in slope stabilitygeology.wwu.edu/rjmitch/l17_slopes_water.pdf ·...
TRANSCRIPT
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
.
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
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