Mass Wasting
Mass wasting is movement of loose material under the direct force of gravity, without water, ice or airplaying a major role. (This definition/distinction is rather arbitrary.) It includes mudflows, landslides,rockfalls, and soil creep.
v 0022 of 'Mass Wasting'
by Greg Pouch at 2013-09-23 15:51:38
LastSavedBeforeThis 2011-09-07
09MassWasting.ppt
Mass Wasting
3 Causes
Types of Mass Wasting
4 Types of failure>Creep
5 Types of failure>Catastrophic
6 Failure Diagrams
7 Failure Pictures
8 Signs of Mass Wasting
9 What to do about Mass Wasting: Avoid, flee, go with the flow
Causes
•Ultimate
–Gravity The ultimate cause of mass wasting is gravity.
A better understanding comes from considering when mass wasting doesnot occur
–Slope A solid material can support shear stresses up to some threshold,known as its strength or yield point. When the strength is exceeded, eitherfrom increased stress or changed strength, failure of some sort occurs.
•Immediate
–Jolt from earthquake or other thing hitting it.
–Introduction of fluid (rain, volcanoes melting ice)
•Materials tend to fail more readily when wet. The water in porescounteracts normal stress on grain-grain contacts, effectively loweringthe normal stress, but shear stress remains the same, so failure tends tooccur. Water acts a lot like oil as a lubricant.
•When wet, the material is denser and so exerts more stress.
–Liquefaction some soils/sediments, when saturated, can go from beingsolid to plastic or fluid. If they lose strength in response to jarring, theyare thixotropic. (Common in unconsolidated, recently-depositedsediments.) If they lose strength in response to saturation, we call itsolifluction (common in permafrost zones in the active layer). Quicksandis both. When a material liquefacts, a slope that was stable can becomeunstable.
–Loss of vegetation
–Pre-existing planes of failure Joints, faults and fractures (includingbedding planes) can slip if the shear stress relative to the normal stressexceeds a threshold value.
Types of failure>Creep
Mass wasting is classified in many ways. Asa first cut, it can be divided into gradual andcatastrophic.
•Gradual: Creep along very slowly. Mostcommon.
Mechanism can include flow, slumping, orsliding; but because the movement isslow, you usually don’t form a visiblescarp or other clear indicators of type ofmovement. If the creeping is fast enough,you might see curved tree trunks or mis-aligned features.
Types of failure>Catastrophic
For granular materials, there is a maximum angle at which the particles can be stacked called the angle ofrepose; if particles are stacked steeper than this, they slide, although they may need a trigger. The angle ofrepose depends on the size and shape of the particles and fluid content, but is generally in the neighborhoodof 20 to 30. The slope might move as a coherent unit (slide) or totally lose cohesion (slump). Sandslumps and clay often slides, but can also slump.
For rocks, there is also an angle of repose, that depends on the material and degree of jointing/fracturing. Atsome scale, even “solid” granite can be thought of as very coarse gravel. Usually, rocks have moreinterlocking particles than sediments, and so have higher angles of repose.
When the angle of repose is exceeded, the material may:
–Fall freely through air (or water). Often involves hopping and knocking other chunks into motion.
–Slide along a plane surface. Typical of avalanches. Usually exploits previous plane of weakness.
–Slump along a curved surface. Common in clays and silts.
–Flow like a liquid.
•Debris flow is mostly rock (in big chunks) and mud with some water: plastic or viscous behavior
•Mudflow is mostly water with some rock and mud : viscous behavior
•Earthflow is a slow mudflow. Mostly soil and rock with water.
•Lahar is a debris flow associated with a volcano and involving ice
•Nuee ardent is a debris flow associated with a volcano and involving a gas cushion. Deadly.
•Any slope over 30 in loose materials or 50 in rock should be considered over-steepened and avoided ifpossible. Do not buy/build a house on cliffs, bluffs, or steep hillsides, no matter how cool the view. Ifin doubt, consult an engineering geologist or civil engineer.
Scarp
Hummockytopography
Scarp
Failure Diagrams
Failure Pictures
Signs of Mass Wasting
•Talus and scree
•Hummocky topography instead of normal stream-dominated, smoothly-sloping topography(This also occurs in glacial areas, where it is not a problem.)
•Scarps at the head of slides or slumps
•Curved trees Trees grow vertically. If the soil is moving underneath them, they developcurved trunks (more curved on faster-moving slopes) as they continue to grow vertically.Also, a lot of downed, healthy-looking trees should suggest mass wasting.
•Tilted and mis-aligned artifacts like phone poles, fences, tombstones, buildings. This canalso be due to initial shoddy work.
•Curved-at-the-surface bedding This only happens if you have bedding favorably oriented
What to do about Mass Wasting: Avoid, flee, go with the flow
•Spectacular views from a cliff are dangerous : do not live on steep slopes, or at thefoot of steep slopes
•Evacuate when told to do so
•In a mass-wasting event:
–If running out of its path is an option, run at right angles to the slide (slope-parallel)
–You’re better off on top of the moving debris than beneath/within the debris.
–Protect your head
–If submerged in flow or slide mass, try to swim towards top
–Keep an air pocket and free space near your chest/stomach so you can breathe.Sand cave-ins typically kill by suffocation.
–Spread your weight over as large an area as possible (reduce stress on debris),while keeping your head and chest protected.
–Try to ride on top if it on your butt, feet first (downward) and knees bent. Protectyour head and chest. Use friction to slow down, so you land on top of debrisrather than the debris landing on you. Do not try to stand.
Mass wasting
•Mass wasting is rocks/soil falling/rolling/oozing/flowing downhill inresponse to gravity.
•Mass wasting does not happen when the shear strength of the rockexceeds the shear stresses.
•Mass wasting is pervasive in erosional environments, and rangesfrom slow creep to rock falls to dramatic landslides.
•Mass wasting is recognized by the evidence it leaves behind:hummocky topography, warped trees and artifacts, scarps… Mostare small and easily altered and subject to stream erosion, so theydon't usually show up on maps and airphotos.