Mass wasting Flashcards
why mass wasting
slope fails when it is too steep and unstable for existing materials and conditions.
Slope stability is determined by two principal factors: the slope angle and the strength of the underlying material.
+ Force of gravity
The gravitational force acting on a slope: divided into two components
- the shear or driving force pushing the block down the slope
- the normal or resisting force pushing into the slope, which produces friction.
The relationship between shear force and normal force is called shear strength. When the normal force, i.e., friction, is greater than the shear force, then the block does not move downslope.
However, if the slope angle becomes steeper or if the earth material is weakened, shear force exceeds normal force, compromising shear strength, and downslope movement occurs.
cause landslide
rapid snowmelt, intense rainfall, earthquake shaking, volcanic eruption, storm waves, rapid-stream erosion, or human activities, such as grading a new road.
Increased water content within the slope is the most common mass-wasting trigger.
prevent and mitigate landslides
Slope reinforcement
steel bolts: drilled a few meters into a rock face, can secure loose pieces of material that could pose a hazard
Buttressing a slide by adding weight at the toe of the slide and removing weight from the head of the slide, can stabilize a landslide.
Terracing, which creates a stairstep topography, can be applied to help with slope stabilization, but it must be applied at the proper scale to be effective.
shield, catch, and divert the runout material.
= divert and slow the falling material.
-Special stretchable fencing: in areas where rockfall is common to protect pedestrians and vehicles.
-Runout channels, diversion structures, and check dams can be used to slow debris flows and divert them around structures.
-highways have special tunnels that divert landslides over the highway
= the shielding has to be engineered to a scale that is greater than the slide, or catastrophic loss in property and life could result.
classification
by type of movement and type of material
most common mass-wasting types:
falls, slides (rotational and translational), flows, and creep.
fall
abrupt rock movements that detach from steep slopes or cliffs.
Rocks separate along existing natural breaks such as fractures or bedding planes.
Movement occurs as free-falling, bouncing, and rolling.
Falls are strongly influenced by gravity, mechanical weathering, and water.
slide
Rotational slides commonly show slow movement along a curved rupture surface.
Translational slides often are rapid movements along a plane of distinct weakness between the overlying slide material and more stable underlying material.
subdivided into rock slides, debris slides, or earth slides depending on the type of the material involved
flow
rapidly moving mass-wasting events in which the loose material is typically mixed with abundant water, creating long runouts at the slope base.
-debris flow (coarse material)
- earthflow (fine material)
= depending on the type of material involved and the amount of water.
Some of the largest and fastest flows on land are called sturzstroms, or long runout landslides = known to travel for long distances, even in places without significant atmospheres like the Moon.
creep
slowest form of mass movement
= downward movement of soil and uppermost bedrock
— caused by a cycle of nighttime freezing followed by daytime thawing in unconsolidated material (soil)
During the freeze, expansion of ice pushes soil particles out away from the slope, while the next day following the thaw, gravity pulls them directly downward.
=== net effect is a gradual movement of surface soil particles downhill.
= expansion with freezing leads to frost heaving
= subsidence occurs as frost melt
A special type of soil creep is solifluction, which is the slow movement of soil lobes on low-angle slopes due to soil seasonally freezing and thawing in high-latitude, typically sub-Arctic, Arctic, and Antarctic locations.
impact: buildings crack; poles and fences lean; trees grow with curved trunks; roads crack
= curving growth pattern
= downslope breaking of rock outcrop
what is mass wasting
all forms of downslope movement of earth materials.
naturally occurring processes but they have been affected by human activities
= have created situations of slope instability:
- urbanization
- irrigation
- timber harvesting in weak, relatively unstable areas
- artificial modification of landscape
- dam construction
- mining
gravity
constant force on everything on Earth – over
the length of geologic time, all slopes can fail.
However, some of that failure is very slow, whereas at other times it can be catastrophic.
= The steeper the slope, the greater potential for gravity to pull objects down
the only force that prevents gravity from doing its job is FRICTION
If the friction on a rock is stronger than gravity for a particular slope, the rock material will likely stay. But if gravity is stronger, the slope will fail.
The steeper the slope, the stronger the friction or rock strength must be to resist downslope motion.
friction
resistance to motion of a body that
keeps it from moving over another body
inertia
tendency of another body to remain
at rest until an external force is applied
We see that the ground surface does not sit still. Why?
Because the volume of soil swells and shrinks.
a. Soil can have void space – aka porous or porosity.
b. Soil can contain expandable materials such as clay.
c. Heating of the soil by the Sun also causes an increase in volume
chemical weathering
most rapid in humid and hot environment
1) hydrolysis
2) oxidation
3) dissolution
mechanical weathering
= physical desintegration
// rocks break into pieces
1) frost action
2) plant growth
3) salt crystalization
4) thermal cycling
