why do hillslopes move at some times and not at others? forces, resistances and thresholds
TRANSCRIPT
WHY DO HILLSLOPES MOVE AT SOME TIMES AND NOT AT
OTHERS?
Forces, Resistances and Thresholds
FORCES RESISTANCES
IF FORCE GREATER THAN RESISTANCE, HILLSIDE MOVES OR “FAILS”.
IF RESISTANCE GREATER THAN FORCE, HILLSIDE IS STABLE.
FORCE = MASS X ACCELERATION
FORCE = MASS X ACCELERATION
1. MASS IS THE MASS OF THE HILLSIDE MATERIAL!
FORCE = MASS X ACCELERATION
1. MASS IS THE MASS OF THE HILLSIDE MATERIAL!
2. ACCELERATION? ……….
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.
ACCELERATION DUE TO GRAVITY ACTS PERPENDICULARLY.
INTRODUCE A HORIZONTALSLOPE, 0°.
0°
W
PLACE BLOCK OF EARTH MATERIAL,
WEIGHT, W, ON THE SLOPE
0°
W 0°
ARBITRARILY DEFINE “DOWNSLOPE” AS TO
THE LEFT.
W 0°
NONE OF THE FORCEOF GRAVITY IS ACTING IN THE DOWNSLOPE
DIRECTION
W 0°
INCLINE THE PLANE TO 30°
W30°
W 0°
W30°
DOWNSLOPE DIRECTION NOW
CLOSER TO DIRECTION OF GRAVITY
W 0°
INCLINE THE PLANE TO 60°
W30°
W
60°
W 0°
W30°
W
60°
DOWNSLOPE DIRECTION EVEN
CLOSER TO DIRECTION OF GRAVITY
W 0°
W30°
W
60°
W90°
INCLINE PLANE TO THE VERTICAL, 90°
W 0°
W30°
W
60°
W90°
ALL OF GRAVITY NOW ACTING IN THE DOWNSLOPE DIRECTION.
W 0°
W30°
W
60°
W90°
PROPORTION OF ACCELERATION DUE TO GRAVITY ACTING IN THE
DOWNSLOPE DIRECTION IS INCREASING.
W 0°
W30°
W
60°
W90°
SIN(0°) = 0
0 X W.g
W 0°
W30°
W
60°
W90°
SIN(0°) = 0
0 X W.g
SIN(30°) = 0.50.5 X W.g
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
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
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.
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
WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?
WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?
1. Changes in Weight 2. Changes in Slope
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.
• http://www.ibtimes.co.uk/floods-chile-landslides-peru-after-heaviest-rain-80-years-1493792
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
WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?
2. Changes in Slope• Erosion of bottom of slope.
Devon, England Colorado, US
WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?
2. Changes in Slope• Erosion of bottom of slope.• Road/rail cuttings.
Thailand Thailand
Virginia
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.
W 0°
WHAT IS NORMAL STRESS ?
Downslope
NormalStress
90°
W30°
Downslope Component:Approaches gravity as slope increases.
Normal Stress:Moves away from gravity (decreases) as slope increases.
W30°
Downslope Component:Approaches gravity as slope increases.
Normal Stress:Moves away from gravity (decreases) as slope increases.
N.S = Cos (Slope) . W. g.
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.
Angle of internal friction (repose)
Angle of internal friction (repose)
GENERALLY:
Big grains High friction
Small grains Low friction
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.
Cohesion
GENERALLY:
Clays High cohesion
Large grains Low cohesion
FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
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.
HYGROSCOPICWATER
FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
HYGROSCOPICNo Role
Vs.
CAPILLARYWATER
FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
CAPILLARYResistance
Negative Pore Water Pressures
Vs.
FREE or GRAVITATIONALWATER
FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
GRAVITATIONALForce
Positive Pore Water Pressures
Vs.
Mass Movement Experiment for Floridians
Mass Movement Experiment for Floridians
Sand Castle – Steep Slopes
Mass Movement Experiment for Floridians
Move up the beach
Mass Movement Experiment for Floridians
Move down the beach
Mass Movement Experiment for Floridians
Same sand, same bucket, same slope angle!
Slope holds!Slope collapses! Slope collapses!
Mass Movement Experiment for Floridians
The Difference – Water Content!
Slope holds!Slope collapses! Slope collapses!
Capillary WaterResistance!
Hygroscopic WaterNeutral!
Free WaterForce!
FORCES RESISTANCES
1. Weight
2. Slope
1. Normal Stress
2. Friction
3. Cohesion
Vs.
Extrinsic Intrinsic
Time
Forc
e an
d Re
sist
ance
Time
Forc
e an
d Re
sist
ance Water is universal solvent,
ultimately breaking down strength of rocks along with other processes
Time
Forc
e an
d Re
sist
ance Forces often fluctuate with
presence of water (rain storms or seasonal snowmelt).
Time
Forc
e an
d Re
sist
ance
Resistance> Force
Stable Slope
Time
Forc
e an
d Re
sist
ance
Resistance> Force
Stable Slope
Time
Forc
e an
d Re
sist
ance
Resistance> Force
Stable SlopeForce>Resistance
SLOPEFAILURE!
Time
Forc
e an
d Re
sist
ance
Resistance> Force
Stable Slope
Resistances?
1. Downslope movement decreases elevation of uplands, increases elevation of valley – decreases slope, increases normal stress (resistance).
2. Downslope movement exposes unweathered, “strong” rock, with greater friction and cohesion - increases resistance.
Time
Forc
e an
d Re
sist
ance
Resistance> Force
Stable Slope
Resistance> Force
StableSlope