Slide 1

WHY DO HILLSLOPES MOVE AT SOME TIMES AND NOT AT OTHERS?Forces, Resistances and ThresholdsFORCES RESISTANCES IF FORCE GREATER THAN RESISTANCE, HILLSIDE MOVES OR FAILS.IF RESISTANCE GREATER THAN FORCE, HILLSIDE IS STABLE.FORCE = MASS X ACCELERATIONFORCE = MASS X ACCELERATION1. MASS IS THE MASS OF THE HILLSIDE MATERIAL!FORCE = MASS X ACCELERATION1. MASS IS THE MASS OF THE HILLSIDE MATERIAL!2. ACCELERATION? .FORCE = MASS X ACCELERATION1. 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, 0.0WPLACE BLOCK OF EARTH MATERIAL, WEIGHT, W, ON THE SLOPE0W0ARBITRARILY DEFINE DOWNSLOPE AS TO THE LEFT.W0NONE OF THE FORCEOF GRAVITY IS ACTING IN THE DOWNSLOPEDIRECTIONW0INCLINE THE PLANE TO 30W30W0W30DOWNSLOPE DIRECTION NOW CLOSER TO DIRECTION OF GRAVITYW0INCLINE THE PLANE TO 60W30W60W0W30W60DOWNSLOPE DIRECTION EVEN CLOSER TO DIRECTION OF GRAVITYW0W30W60W90INCLINE PLANE TO THE VERTICAL, 90W0W30W60W90ALL OF GRAVITY NOW ACTING IN THE DOWNSLOPE DIRECTION.W0W30W60W90PROPORTION OF ACCELERATION DUE TO GRAVITY ACTING IN THE DOWNSLOPE DIRECTION IS INCREASING.W0W30W60W90SIN(0) = 00 X W.gW0W30W60W90SIN(0) = 00 X W.gSIN(30) = 0.50.5 X W.gW0W30W60W90SIN(0) = 00 X W.gSIN(30) = 0.50.5 X W.g0.87 X W.gSIN(60) = 0.87W0W30W60W90SIN(0) = 0.00.0 X W.gSIN(30) = 0.50.5 X W.g0.87 X W.gSIN(60) = 0.87SIN(90) = 1.01.0 X W.g

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 Weight2. Changes in SlopeWHY WOULD FORCES ON A HILLSIDE FLUCTUATE?1. Changes in Weight Add water to slope fill pore spaces62 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-1493792WHY WOULD FORCES ON A HILLSIDE FLUCTUATE?1. Changes in Weight Add water to slope fill pore spaces Build on it!

Bournemouth, EnglandSaskatoon, CanadaWHY WOULD FORCES ON A HILLSIDE FLUCTUATE? 2. Changes in Slope Erosion of bottom of slope.

Devon, EnglandColorado, USWHY WOULD FORCES ON A HILLSIDE FLUCTUATE?2. Changes in Slope Erosion of bottom of slope. Road/rail cuttings.

ThailandThailand

VirginiaWHAT ARE THE RESISTANCES TO MOTION ON A HILLSIDE?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.W0WHAT IS NORMAL STRESS ?DownslopeNormalStress90W30Downslope Component:Approaches gravity as slope increases.Normal Stress:Moves away from gravity (decreases) as slope increases.35W30Downslope Component:Approaches gravity as slope increases.Normal Stress:Moves away from gravity (decreases) as slope increases.N.S = Cos (Slope) . W. g.36WHAT ARE THE RESISTANCES TO MOTION ON A HILLSIDE?Normal Stress: Cosine Slope.

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 grainsLow frictionWHAT ARE THE RESISTANCES TO MOTION ON A HILLSIDE?Normal Stress: Cosine Slope.

Friction: High for big grains, low for small.

Cohesion: The force that holds together molecules or like particles within a soil.

CohesionGENERALLY:

ClaysHigh cohesion

Large grainsLow cohesionFORCES RESISTANCES Weight

SlopeNormal Stress

Friction

Cohesion

FORCES RESISTANCES Weight

SlopeNormal Stress

Friction

CohesionWATER HAS DIFFERING ROLES IN THE BALANCEDEPENDING UPON THE QUANTITY OF WATERPRESENT.HYGROSCOPICWATERFORCES RESISTANCES Weight

SlopeNormal Stress

Friction

CohesionHYGROSCOPICNo RoleVs.CAPILLARYWATERFORCES RESISTANCES Weight

SlopeNormal Stress

Friction

CohesionCAPILLARYResistanceNegative Pore Water PressuresVs.FREE or GRAVITATIONALWATERFORCES RESISTANCES Weight

SlopeNormal Stress

Friction

CohesionGRAVITATIONALForcePositive Pore Water PressuresVs.Mass Movement Experiment for Floridians

Mass Movement Experiment for Floridians

Sand Castle Steep SlopesMass 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 Weight

SlopeNormal Stress

Friction

CohesionVs.ExtrinsicIntrinsicTimeForce and ResistanceTimeForce and ResistanceWater is universal solvent, ultimately breaking down strength of rocks along with other processesTimeForce and ResistanceForces often fluctuate with presence of water (rain storms or seasonal snowmelt).TimeForce and ResistanceResistance> ForceStable SlopeTimeForce and ResistanceResistance> ForceStable SlopeTimeForce and ResistanceResistance> ForceStable Slope

Force>Resistance

SLOPEFAILURE!TimeForce and ResistanceResistance> ForceStable 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.TimeForce and ResistanceResistance> ForceStable Slope

Resistance> ForceStableSlope