Mass Wasting

one process that moves regolith, sediment and soils about the Earth’s

surface is

Mass Wasting

Mass Wastingdownslope movement of rock, regolith, and soil under direct influence of gravity

movement when force or gravity exceeds resisting force (slope stability)

earth materialsbehave as solids or viscous massesmay be:

consolidated (compacted and cemented) unconsolidated (loose and uncemented)

Mass Wasting

slope stability determined by:

1) strength and cohesiveness of slope material(s)

2) internal friction between grains3) any external support of slope

Mass Wasting

Factors Involved in Mass Wasting1) gravity, vertical force split into vectors

parallel to (tangential) and perpendicular to surface (normal)

Mass Wasting

2) friction on surface or between grains

3) shear strength - material strength and cohesion; force necessary to disrupt material

Factors Involved in Mass Wasting

Mass Wasting

instability depends on: angle of repose

of material:" steepest angle that slope maintains without failing" typically 25˚-40˚ for unconsolidated materials

slope can be oversteepened by: natural causes - stream

and wave erosion human activity (e.g.,

roadcuts, hillside construction)

What Causes Mass Wasting1) slope gradient

most important factormovement wherever slope steeper than natural angle of repose

Mass Wasting

2) weathering and climateweathering disaggregates and disintegrates bedrockclimate controls type and rate of weathering

Mass Wasting

What Causes Mass Wasting

3) water contentincreased water decreases slope stability because it:

adds weightreduces cohesion displacing air and destroying surface tensionreduces cohesion as pore pressure pushes grains apartenhances movement by lubricating surfaces

Mass Wasting

What Causes Mass Wasting

4) defoliationremoval of vegetation destabilizes slopes because:

plants adsorb water decreasing water saturation limit of slope materialplants bind together soil particlesplants hold soil to bedrock with roots

Mass Wasting

What Causes Mass Wasting

5) Overloadinginvolves increase in weight which may:

increase tangential force on planeincrease water pressure and decrease friction causing failure

almost always result of human activity - weight of buildings, dumping, filling, or piling up material

Mass Wasting

What Causes Mass Wasting

Mass Wasting

What Causes Mass Wasting6) bedrock geologya) bedding planes

and fractures - zones of weakness for weathering and movement

b) rocks inclined (dipping) in same direction as slope more prone to mass wasting

7) triggering mechanisms

some event: excessive rainearthquakes and volcanic eruptionsvibrations and noise

Mass Wasting

What Causes Mass Wasting

Mass Wasting

several types of mass wasting recognized and involve:

one type of movementor

combination of movement types

Types of Mass Wasting

mass movements classified by dominant behavior:

1. type of motion (e.g., falling, sliding, flowing)

2. rate of motion (velocity)3. type of material involved (elastic

solid, a plastic substance, or as a liquid)

Mass Wasting

Types of Mass Wasting

free descent of material - no contact with any surface except to bounce

Mass Movement - Falls

Mass Wasting

failure along:joints (subparallel fractures throughout rock)bedding planes in solid rock

talus - accumulations of loose rock at base of steep slopes

slopes rarely exceed 40°

Mass Movement - Falls

Mass Wasting

movement of coherent blocks of material along one or more well-defined surface of failureslow or rapiddescribed as:

SlumpGlide

Mass Movement - Slides

Mass Wasting

downward movement along curved failure surfacerotation of block abovescarp (small cliff) up slopeinitiated by slope oversteepeningaided by water infiltration

Mass Movement - Slumps

Mass Wasting

Mass Movement - Slumps

Mass Wasting

Mass Movement - Slumps

Mass Wasting

movement (sliding) along nearly planar surface (usually bedding plane)millions of tons transported downslope

Mass Movement - Glide

Mass Wasting

factors contributing to sudden movement include:

1. clay-rich units underlying more resistant rocks,

2. steeply dipping rock strata3. fractures4. undercutting5. heavy rains6. earthquakes

Mass Movement - Glide

Mass Wasting

speeds can exceed 100 km/hrincreased by momentum transfer (momentum of material at back transferred forward through collisions)

Mass Movement - Glide

Mass Wasting

Mass Movement - Glide

Mass Wasting

29 April 1903 Frank, Alberta

Mass Movement - Glide

Mass Wasting

29 April 1903 Frank, Alberta

movement as viscous fluid or plasticextremely slow to extremely rapidprincipal types:

mudflows debris flows earthflows

Mass Movement - Flows

Mass Wasting

most common in arid or semi-arid areas following

rare heavy rains

well-mixed mass of water (up to 30%) and earth (at least 50% clay- and silt-sized particles)moves as fluid

Mass Movement - Mudflows

Mass Wasting

Mass Wasting - Mudflows

Mass Wasting

Mass Wasting

Mass Movement - Debris Flows

typically in semi-arid mountainous areas after heavy rainfallscoarser than mudflowscontain less water

Mass Wasting

tongue-shaped mass with hummocky surface and lobed ends

Mass Wasting - Earthflows

Mass Wasting

Mass Wasting - Earthflows

commonly associated with grass-

covered slopes in

humid areas

Mass Wasting

Mass Wasting - Earthflows

solifluction - slow downward movement of water-saturated sedimentcold climates with freeze-thaw of permafrost

Mass Wasting

special case earthflow silt/clay-sized

particles produced by glacial grinding and deposited in a marine environment

stable in salt water, but unstable and liquefy exposed to fresh water

Mass Movement - Quick Clay

Mass Movement - Quick Clay

Mass Wasting

Anchorage, 1964 Alaska earthquake

Mass Movement - Creep

Mass Wasting

slow downward movement of surface materialcaused by: wetting/drying increased

burrowing by animals

decaying roots loading

Mass Wasting

Mass Movement - Creep

Mass Wasting

Mass Movement - Creepmost widespread type of flowmost common in humid regionstypified by:

tilted telephone polesfence postsbent tree trunks

Complex Mass Movement

Mass Wastingcombination of styles with none dominant

Mass Wasting

slump followed by slide or flow down mountainside

Complex Mass Movement - Debris Avalanche

Mass Wasting

more viscous and slower than mudflows

can carry large objects

hummocky surface and lobed ends

grade into debris flows with water saturation

Complex Mass Movement - Debris Slides

.... how do we deal with the

effects of mass wasting?

Minimizing Disaster

Mass Wasting

Identification of High Risk Areasareas prone to mass wasting often readily identifiedsite hazard assessment studies involve:

1.identification of former landslide areas

2.geologic assessment of rock/soil material and structure

3.preparation of slope stability mapsvarious engineering methods used to minimize danger and damage

Minimizing Disaster

Mass Wasting

slope dewatering: surface and subsurface

drains installed to remove excess water

reduces weight and increases shear strength of slope material

slope reduction: a) Cut-and-fill - material removed from upper

part and used to fill bottomb) Benching - steps cut into slope (commonly

with drains)

Minimizing Disaster

Mass Wasting

stabilizing structures: a) retaining walls support base of slope

Minimizing Disaster

Mass Wasting

b) Rock bolts - fasten potentially unstable rock masses to stable bedrock (used with drains and chain-link fencing)

Minimizing Disaster

Mass Wasting