Mass Wasting

1. Mass Wasting From http://www.bbc.co.uk/news/world-latin-america-24191716 Mass wasting is movement of loose material under the direct force of gravity, without water, ice or air playing 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

2. 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

3. Causes • Ultimate • Gravity The ultimate cause of mass wasting is gravity. A better understanding comes from considering when mass wasting does not 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, either from 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 pores counteracts normal stress on grain-grain contacts, effectively lowering the normal stress, but shear stress remains the same, so failure tends to occur. 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 being solid to plastic or fluid. If they lose strength in response to jarring, they are thixotropic. (Common in unconsolidated, recently-deposited sediments.) If they lose strength in response to saturation, we call it solifluction (common in permafrost zones in the active layer). Quicksand is both. When a material liquefacts, a slope that was stable can become unstable. • Loss of vegetation • Pre-existing planes of failure Joints, faults and fractures (including bedding planes) can slip if the shear stress relative to the normal stress exceeds a threshold value.

4. Types of failure>Creep Mass wasting is classified in many ways. As a first cut, it can be divided into gradual and catastrophic. • Gradual: Creep along very slowly. Most common. Mechanism can include flow, slumping, or sliding; but because the movement is slow, you usually don’t form a visible scarp or other clear indicators of type of movement. If the creeping is fast enough, you might see curved tree trunks or mis-aligned features.

5. Types of failure>Catastrophic For granular materials, there is a maximum angle at which the particles can be stacked called the angle of repose; if particles are stacked steeper than this, they slide, although they may need a trigger. The angle of repose depends on the size and shape of the particles and fluid content, but is generally in the neighborhood of 20 to 30. The slope might move as a coherent unit (slide) or totally lose cohesion (slump). Sand slumps 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. At some scale, even “solid” granite can be thought of as very coarse gravel. Usually, rocks have more interlocking 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 if possible. Do not buy/build a house on cliffs, bluffs, or steep hillsides, no matter how cool the view. If in doubt, consult an engineering geologist or civil engineer.

6. Failure Diagrams Scarp Scarp Hummocky topography

7. Failure Pictures

8. 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 develop curved 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 can also be due to initial shoddy work. • Curved-at-the-surface bedding This only happens if you have bedding favorably oriented

9. 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 the foot 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. Protect your head and chest. Use friction to slow down, so you land on top of debris rather than the debris landing on you. Do not try to stand.

10. Mass wasting • Mass wasting is rocks/soil falling/rolling/oozing/flowing downhill in response to gravity. • Mass wasting does not happen when the shear strength of the rock exceeds the shear stresses. • Mass wasting is pervasive in erosional environments, and ranges from 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… Most are small and easily altered and subject to stream erosion, so they don't usually show up on maps and airphotos.