![Page 1: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/1.jpg)
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
![Page 3: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/3.jpg)
.
Slope Model
θ
.W
![Page 4: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/4.jpg)
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 =
![Page 5: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/5.jpg)
Water makes slopes unstable
![Page 6: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/6.jpg)
1. Water adds weight
.
θ
.
W
![Page 7: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/7.jpg)
One gallon of water weighs 8 lbs
![Page 8: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/8.jpg)
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
![Page 9: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/9.jpg)
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
![Page 10: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/10.jpg)
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
![Page 11: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/11.jpg)
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=
![Page 12: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/12.jpg)
.
θ
.W
3. Slopes with high clay contents are more unstablewhen wet.
![Page 13: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/13.jpg)
Scanning electron microscope image of clay
![Page 14: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/14.jpg)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Clay minerals have a “negative” surface charge
![Page 15: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/15.jpg)
water is polar
__
++
![Page 16: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/16.jpg)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
clay minerals attract water molecules to their surfaces
![Page 17: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/17.jpg)
dry, more friction, hence more resistant to sliding wet, less friction, hence lower strength
![Page 18: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/18.jpg)
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
![Page 19: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/19.jpg)
4. Water buoys up the grains which reduces the friction force
.
θ
.
W
![Page 20: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/20.jpg)
.
Slope Model
θ
.W
![Page 21: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/21.jpg)
shear plane
![Page 22: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/22.jpg)
friction force
weight
![Page 23: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/23.jpg)
W.FN = W x cosθ = normal force
θ
θ
cosθ = adjacent
hypotenuse
![Page 24: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/24.jpg)
friction force
weight
friction force = normal force x coefficient of friction
or friction force = FN x μ
FN
![Page 25: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/25.jpg)
friction force
What happens when the grains are immersed in water?
![Page 26: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/26.jpg)
![Page 27: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/27.jpg)
=water surface
P = ρw·g·h = hydrostatic pressure
P = water pressure
ρw = water density
g = acceleration of gravity h
h = depth to the balloon
![Page 28: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/28.jpg)
=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
![Page 29: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/29.jpg)
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
![Page 30: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/30.jpg)
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
![Page 31: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/31.jpg)
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
![Page 32: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/32.jpg)
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
![Page 33: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/33.jpg)
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
![Page 34: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/34.jpg)
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)
![Page 35: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/35.jpg)
Cubic grain immersed in water
FB
The grain weight is reduced by FB
![Page 36: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/36.jpg)
buoyancy force
Wnew = Wdry – buoyancy force
buoyancy force = the weight of the volume of water the grain displaces
![Page 37: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/37.jpg)
friction force
the weight of each grain is reduced by the buoyancy force
The total weight pushing on the shear plane is reduced
![Page 38: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/38.jpg)
friction force
weight
friction force = normal force x coefficient of friction
or friction force = FN x μ
FN
![Page 39: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/39.jpg)
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=
![Page 40: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/40.jpg)
FS = resisting forces
driving forces=
friction + cohesion
Fp
![Page 41: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/41.jpg)
FS = resisting forces
driving forces=
friction + cohesion
Fp
![Page 42: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/42.jpg)
FS = resisting forces
driving forces=
friction + cohesion
Fp
![Page 43: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/43.jpg)
FS = resisting forces
driving forces=
friction + cohesion
Fp
![Page 44: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/44.jpg)
FS = resisting forces
driving forces=
friction + cohesion
Fp
![Page 45: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/45.jpg)
FS = resisting forces
driving forces=
friction + cohesion
Fp
![Page 48: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/48.jpg)
Seattle Area, Washington
Cumulative Precipitation Threshold
![Page 49: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/49.jpg)
Seattle Area, Washington
Precipitation Intensity-Duration Threshold
![Page 50: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/50.jpg)
![Page 51: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/51.jpg)
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
![Page 52: Lecture 17: The Role of Water in Slope Stabilitygeology.wwu.edu/rjmitch/L17_slopes_water.pdf · Lecture 17: The Role of Water in Slope Stability . Key Questions. 1. How does the weight](https://reader035.vdocuments.us/reader035/viewer/2022070822/5f27807b4623cb782238261a/html5/thumbnails/52.jpg)
Dec 11, 2004 - Massive landslide on the Sultan River, CA