Mass Wasting Flashcards
Define mass movement
a downhill movement of masses of soil or rock under the force of gravity
What are the 3 main factors of mass movement
- The nature of the MATERIALS, are they consolidated or unconsolidated
- The steepness and stability of SLOPES
- The amount of WATER in the materials
Define the types of materials
Unconsolidated = loose and uncemented material. Ex: sand, silt, soil, gravel and rock fragments Consolidated = compacted and cemented material. Ex: jointed rock, deformed rock, massive rock
How does slope impact mass movement
Mass wasting occurs when a slope fails.
A slopes stability is determined by angle and strength of material
A slope failure occurs if angle is increased to a point or if strength of material decreases ( adding water)
Define normal and shear force
Fn or normal force is pushing down into the slope
Fs or shear force is pushing the object down the slope
When Fs exceeds Fn the object slides down under the force of gravity
Define angle of repose
a slope angle of unconsolidated material at gravitational equilibrium (the steepest angle at which the material is stable)
- this range is 35 to 45 degrees
The equilibrium is usually based off the wet or dryness of the material. Dry being stable
How does water effect materials
The amount of water collected by materials is determined by
- Porosity of materials
- amount of exposure to rain or other water
- -rain filling pore space strengthens material and increases the angle of repose up to a point. If to much water fills up the shear strength decreases
What is the role of water in regards to unconsolidated materials
Surface tension of water causes cohesion of grains if only a film is present. This causes an increase of cohesion. Only caused by small amounts of water
If the water content becomes high enough to fill the pores, the grains will float. This causes the grains to push apart causing failure
What can happen if large amounts of water decrease cohesion
liquefaction
When does a failure of a slope occur
When gravity exceeds cohesion
How can you classify mass movement
- A slide or fall
- Coherant mass slides along a discrete surface - Flow
- Flows are a dense fluid mixture of water, rock, sand and mud
What type of classification would you call a Earth Creep
unconsolidated flow
Moves at about 1 cm/year
Is the slowest form of mass movement where unconsolidated material slowly drifts downhill by gravity.
What classification would you give to a debris flow and lahar
Unconsolidated flow
Is a moderate speed flow - 1-3 km/hour
Mainly mud or volcanic ash or melted snow
What are the factors required for debris flows
Especially steep slope Pre-existing loose material Heavy rains Trigger- often a small landslide common in semi-arid settings with a heavy rain. Areas with forest fires are also places this occurs
What classification would a rock slide be
Consolidated
Moderate speed
movement of blocks of rock
