Upload
sylvan-anugrah
View
216
Download
0
Tags:
Embed Size (px)
DESCRIPTION
geologi dinamis
Citation preview
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