Geology Ch.11 Mass Wasting
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Mass Wasting (mass movement):
- the downslope movement of material under the direct influence of gravity.
- happens when the gravitational force acting on a slope exceeds its resisting force.
What are some factors that influence mass wasting?
- a slopes shear strengthSlope angle
- Weathering and climate
- Water content
- Slope stability
What forces maintain slipe stability?
- Slopes material strength and cohesion
- the amount of internal friction between grains
- external support
A slopes material strength and cohesion, the amount of internal friction between grains, and external support.
What is the angle of repose?
The steepest angle that a slope can maintain without collapsing.
All slopes are in this state, which means that they are constantly adjusting to new conditions.
The major cause of mass wasting.The steeper the slope, the less stable it is. Steep slopes are more likely to experience mass wasting than gentle ones.
What processes oversteepen a slope?
- undercutting by stream or wave action, process removes the slopes base, increases the slopes angle, and increases gravitational force.
- Excavations for road cuts and hillside building sites.
Weathering and Climate:
- mass wasting is more likely to occur in loose or poorly consolidated slope material.
- As soon as rock is exposed at Earth's surface, weathering begins to disintegrate and decompose it, reducing its shear strength and increasing its susceptibility to mass wasting.
- Weathering happens mostly in high temperatures with a lot of rainfall.
- the amount of water in rock or soil influences slope stability.
- The additional weight from water can be enough to cause mass movement.
- Effects slope stability a few ways :
- 1.vegetation decreases water saturation
- 2.root systems stabilize a slope by binding soil particles together and holding the soil to bedrock.
- Removal of vegetation causes mass wasting:
- 1. forrest and brush fires remove vegetation
- 2. fall rainstorms saturate the ground
- almost always the result of human activity, typically results from dumping, filling, or piling up of material.
- The overloading increases the water pressure in a slope, and decreases its shear stength.
Geology and Slope Stability:
If the rocks underlying a slope dip in the same direction as the slope, mass wasting is more likely to occur if the rocks are horizontal or dip in the opposite direction.
Rapid Mass Movement:
- involve a visible movement of material.
- Usually occur on steep slopes and can involve rock, soil, or debris.
Slow Mass Movement:
advance at an imperceptible rate and are usually detectable only by the effects of their movement (tilted trees and power poles or cracked foundations)
- common mass movement when rocks of any size fall through the air.
- Occur along steep canyons, cliffs, and road cuts and build up accumulations of loose rocks and rock fragments at their base called tallus.
How do rockfalls occur?
- failure along joints or bedding planes in bedrock
- undercutting of slopes
- frost wedging in cold climates
- water percolating through fissures in carbonate rocks
- involves movement of material along one or more surfaces of failure.
- Type of material may be soil, rock, or a combination of the two.
What are two types of slides?
- 1. slumps or rotational slides, in which movement occurs along a curved surface.
- 2. rock or block sides, which move along a more or less planar surface.
involves the downward movement of material along a curved surface of rupture and is characterized by backward rotation of the slump block.
Main cause: erosion along the base of a slope, which removes support for overlying material.
Rock or Block Slides:
- occurs when rocks move downslope along a more or less planar surface.
- Take place becayse the local slopes and rock layers dip in the same direction. They can also occur along fractures parallel to a slope.
mass movement in which material flows as a viscous fluid or displays plastic movement. In many cases mass movements begin as falls, slumps, or slides and change into flows farther downslope.
- most fluid and move most rapidly.
- common in arid and semi-arid environments
- composed of larger particles than mud flows and do not contain much water.
- More viscous than mud flows.
- move more slowly than mudflows or debris flows. Slumps from the upper part of a hillside, leaving a scarp, and flows slowly downslope as a thick, viscous, tongue-shaped mass of wet regolith.
- Most commonly occur in humid climates on grassy, soil-covered slopes following heavy rains.
- clays that spontaneously liquefy and flow like water when they are disturbed.
- Composed of fine silt and clay particles made by the grinding action of glaciers.
- the slow downslope movement of water saturate surface sediment.
- Can occur in any environment where the ground is saturated with water, but is most common in areas of permafrost.
- ground that remains permanently frozen.
- During the warmer season when the upper portion of the permafrost thaws, water and surface sediment form a soggy mass that flows by solifluction and produces a characteristic lobate topography.
- the slowest type of flow, the most widespread and significant mass wasting process in terms of the total amount of material moved downslipe.
- Involves extremely slow downhill movement of soil or rock.
- Most effective in humid regions.
- when several types of mass movements are involved.
- Most common complex movement is the slide flow, in which there is sliding at the head and then some type of flowage farther along its course.
a complex movement that often occurs in very steep mountain ranges. Start out as rockfalls when large quantities of rocks, ice, and snow are dislodged from a mountainside. The material then slides or flows down the mountainside, picking up additional surface material and increasing in speed.
What are some features that indicate former landslides?
Scarps, open fissures, displaced or tilted ogjects, a hummocky surface, and sudden changes in vegetation
these maps allow planners and developers to make decisions about where to site roads, utlity lines, and hoising or industrial developments based on the relative stability or instability of a particular location.
How can slopes be stabalized?
- 1. Drain water off the slope into ditches
- 2. Reduce the slope. Cut and fill method, material is removed from the upper part of the slope and used as fill at the base. Benching involves cutting a series of benches or steps into a hillside.
- 3.Rock Bolts are used to fasted potentially unstable rock masses into the underlying stable bedrock.
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