WHY DO HILLSLOPES MOVE AT SOME TIMES AND NOT AT OTHERS? Forces, Resistances and Thresholds.
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Transcript of WHY DO HILLSLOPES MOVE AT SOME TIMES AND NOT AT OTHERS? Forces, Resistances and Thresholds.
![Page 1: WHY DO HILLSLOPES MOVE AT SOME TIMES AND NOT AT OTHERS? Forces, Resistances and Thresholds.](https://reader035.fdocuments.in/reader035/viewer/2022062805/5697c01b1a28abf838ccf795/html5/thumbnails/1.jpg)
WHY DO HILLSLOPES MOVE AT SOME TIMES AND NOT AT
OTHERS?
Forces, Resistances and Thresholds
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FORCES RESISTANCES
IF FORCE GREATER THAN RESISTANCE, HILLSIDE MOVES OR “FAILS”.
IF RESISTANCE GREATER THAN FORCE, HILLSIDE IS STABLE.
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FORCE = MASS X ACCELERATION
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FORCE = MASS X ACCELERATION
1. MASS IS THE MASS OF THE HILLSIDE MATERIAL!
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FORCE = MASS X ACCELERATION
1. MASS IS THE MASS OF THE HILLSIDE MATERIAL!
2. ACCELERATION? ……….
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FORCE = MASS X ACCELERATION
1. MASS IS THE MASS OF THE HILLSIDE MATERIAL!
2. ACCELERATION? ……… DOWNSLOPE MOVEMENT UNDER THE INFLUENCE OF GRAVITY …… ACCELERATION DUE TO GRAVITY.
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ACCELERATION DUE TO GRAVITY ACTS PERPENDICULARLY.
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INTRODUCE A HORIZONTALSLOPE, 0°.
0°
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W
PLACE BLOCK OF EARTH MATERIAL,
WEIGHT, W, ON THE SLOPE
0°
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W 0°
ARBITRARILY DEFINE “DOWNSLOPE” AS TO
THE LEFT.
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W 0°
NONE OF THE FORCEOF GRAVITY IS ACTING IN THE DOWNSLOPE
DIRECTION
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W 0°
INCLINE THE PLANE TO 30°
W30°
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W 0°
W30°
DOWNSLOPE DIRECTION NOW
CLOSER TO DIRECTION OF GRAVITY
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W 0°
INCLINE THE PLANE TO 60°
W30°
W
60°
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W 0°
W30°
W
60°
DOWNSLOPE DIRECTION EVEN
CLOSER TO DIRECTION OF GRAVITY
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W 0°
W30°
W
60°
W90°
INCLINE PLANE TO THE VERTICAL, 90°
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W 0°
W30°
W
60°
W90°
ALL OF GRAVITY NOW ACTING IN THE DOWNSLOPE DIRECTION.
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W 0°
W30°
W
60°
W90°
PROPORTION OF ACCELERATION DUE TO GRAVITY ACTING IN THE
DOWNSLOPE DIRECTION IS INCREASING.
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W 0°
W30°
W
60°
W90°
SIN(0°) = 0
0 X W.g
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W 0°
W30°
W
60°
W90°
SIN(0°) = 0
0 X W.g
SIN(30°) = 0.50.5 X W.g
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W 0°
W30°
W
60°
W90°
SIN(0°) = 0
0 X W.g
SIN(30°) = 0.50.5 X W.g
0.87 X W.g
SIN(60°) = 0.87
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W 0°
W30°
W
60°
W90°
SIN(0°) = 0.0
0.0 X W.g
SIN(30°) = 0.50.5 X W.g
0.87 X W.g
SIN(60°) = 0.87
SIN(90°) = 1.01.
0 X
W.g
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RELATIVE ABUNDANCE OF SLOPESOF VARIOUS ANGLES
Slope Angle (degrees)
20 25 30 35
Per
cent
age
of S
lope
s
0
5
10
15
0
5
10
15Laramie England
sources: Carson and Petley, 1970. Carson, 1971.
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Mean slope-angle frequency distribution and size frequency distribution of landslide masses in Higashikubiki area, Japan.
Junko Iwahashi, Shiaki Watanabe and Takahiko Furuya
GeomorphologyVolume 50, Issue 4, 1 March 2003, Pages 349-364
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WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?
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WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?
1. Changes in Weight 2. Changes in Slope
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WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?
1. Changes in Weight• Add water to slope – fill pore spaces
62 lbs per cu ft.8 lbs per gallon
Nevado del Ruiz, Colombia.
Sichuan Province, China.
Armero, Colombia.
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• http://www.ibtimes.co.uk/floods-chile-landslides-peru-after-heaviest-rain-80-years-1493792
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WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?
1. Changes in Weight• Add water to slope – fill pore spaces• Build on it!
Bournemouth, England
Saskatoon, Canada
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WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?
2. Changes in Slope• Erosion of bottom of slope.
Devon, England Colorado, US
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WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?
2. Changes in Slope• Erosion of bottom of slope.• Road/rail cuttings.
Thailand Thailand
Virginia
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WHAT ARE THE RESISTANCES TO MOTION ON A HILLSIDE?
1. Normal Stress:
“Normal” meaning at right angles to (the slope).
Stress implies the component of the acceleration dueto gravity, NOT, acting in a downslope direction, but actually holding the hillside in place.
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W 0°
WHAT IS NORMAL STRESS ?
Downslope
NormalStress
90°
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W30°
Downslope Component:Approaches gravity as slope increases.
Normal Stress:Moves away from gravity (decreases) as slope increases.
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W30°
Downslope Component:Approaches gravity as slope increases.
Normal Stress:Moves away from gravity (decreases) as slope increases.
N.S = Cos (Slope) . W. g.
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WHAT ARE THE RESISTANCES TO MOTION ON A HILLSIDE?
1. Normal Stress: Cosine Slope.
2. Friction: Degree to which particles lock together and resist movement.
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Angle of internal friction (repose)
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Angle of internal friction (repose)
GENERALLY:
Big grains High friction
Small grains Low friction
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WHAT ARE THE RESISTANCES TO MOTION ON A HILLSIDE?
1. Normal Stress: Cosine Slope.
2. Friction: High for big grains, low for small.
3. Cohesion: The force that holds together molecules or like particles within a soil.
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Cohesion
GENERALLY:
Clays High cohesion
Large grains Low cohesion
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FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
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FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
WATER HAS DIFFERING ROLES IN THE BALANCEDEPENDING UPON THE QUANTITY OF WATER
PRESENT.
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HYGROSCOPICWATER
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FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
HYGROSCOPICNo Role
Vs.
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CAPILLARYWATER
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FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
CAPILLARYResistance
Negative Pore Water Pressures
Vs.
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FREE or GRAVITATIONALWATER
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FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
GRAVITATIONALForce
Positive Pore Water Pressures
Vs.
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Mass Movement Experiment for Floridians
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Mass Movement Experiment for Floridians
Sand Castle – Steep Slopes
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Mass Movement Experiment for Floridians
Move up the beach
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Mass Movement Experiment for Floridians
Move down the beach
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Mass Movement Experiment for Floridians
Same sand, same bucket, same slope angle!
Slope holds!Slope collapses! Slope collapses!
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Mass Movement Experiment for Floridians
The Difference – Water Content!
Slope holds!Slope collapses! Slope collapses!
Capillary WaterResistance!
Hygroscopic WaterNeutral!
Free WaterForce!
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FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
Vs.
Extrinsic Intrinsic
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Time
Forc
e an
d Re
sist
ance
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Time
Forc
e an
d Re
sist
ance Water is universal solvent,
ultimately breaking down strength of rocks along with other processes
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Time
Forc
e an
d Re
sist
ance Forces often fluctuate with
presence of water (rain storms or seasonal snowmelt).
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Time
Forc
e an
d Re
sist
ance
Resistance> Force
Stable Slope
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Time
Forc
e an
d Re
sist
ance
Resistance> Force
Stable Slope
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Time
Forc
e an
d Re
sist
ance
Resistance> Force
Stable SlopeForce>Resistance
SLOPEFAILURE!
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Time
Forc
e an
d Re
sist
ance
Resistance> Force
Stable Slope
Resistances?
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1. Downslope movement decreases elevation of uplands, increases elevation of valley – decreases slope, increases normal stress (resistance).
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2. Downslope movement exposes unweathered, “strong” rock, with greater friction and cohesion - increases resistance.
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Time
Forc
e an
d Re
sist
ance
Resistance> Force
Stable Slope
Resistance> Force
StableSlope