why do hillslopes move at some times and not at others? forces, resistances and thresholds

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