chapter 11 mass movement - learning.hccs.edu
TRANSCRIPT
Mass Movement • Mass movement is the downslope
transfer of loose rock from hillsides to low lying areas
-very common occurrence -on continents and ocean floor -all slopes are mobile & constantly changing due to gravity
• Gravity is the force which drives mass-wasting
Factors increase chances of downslope movement
1) Saturation of material with water 2) Vibration from earthquakes 3) Alternating expansion/contraction 4) Undercutting slopes by streams, waves 5) Man made modification of slopes ex.
Undercutting of slopes, tree removal
Types of Mass Movement
• Includes all types of slope failures • Classified by
– Type of motion (slippage planes or not) – Type of material (rock thru clay+/- water) – Rate of movement (slow vs. fast)
Creep • Extremely slow movement of soil and
regolith - 1 to 10 mm/yr • Evidence: Bulges, wavelike swells in soil
– Bending of strata downslope – Tilted poles, deformed roads/fences
• Combination of factors cause creep – Heaving of soil - expansion & contraction
is the primary cause • Wet-dry cycles (Houston) • Freeze-thaw cycles
Debris Flows • Mixtures of water, mud and rock
– Can begin on steep hill as soil slumps – Liquified soil flows downhill
• Up to 50 km/hr; as slow as freshly mixed concrete; as fast as rapid current of water
– Water lubricates mass of soil and rock; is responsible for high velocity
– Large boulders, building etc. may be carried by viscous fluid
Mudflows
• Debris flows consisting of silt and clay sized particles – Usually associated with heavy rain – High water content - up to 30% – Usually follow stream valleys – Also capable of floating houses, large
boulders
Landslides
• Mass movement along well defined slippage or fracture plane
• Landslide block moves as a single or group of units called slumps
• Rock type, orientation and water content influence events
• Matter of seconds or weeks/months
Slump Block
• Spoon shaped slippage plane • Bedding or surfaces are tilted
backward toward the scar • Usually have multiple slippage planes • Traits: scar, tilting of bedding, jumbled
poorly drained small hills from previous slides
Figure 11.10 a,b. Diagram of a slump block landslide
Rockslide
• Rapid movement of large blocks of rock
• Slippage plane usually associated with: – Bedding plane – Joint plane – Structural weakness
• Blocks generally degrade as they move
Rock Falls & Avalanches
• Range from single free-falling rock to a mass of rock particles
• Lack water to lubricate flow • Flows generally do not move far • No slippage plane
Subaqueous Mass Movement
• Active in areas of rapid sediment deposition & steep slopes ex. Deltas, convergent margins; cont. slopes
• Also occur on flanks of volcanic islands & seamounts
• May spread out over large areas • Flow several hundred kilometers
End of Chapter 11
1963 Vaiont Landslide Disaster • Worst dam disaster in history; N. Italy
– Killed ~ 2600 people • 240,000,000 m3 of rock involved in
landslide • Filling of dam caused change in subsurface
hydrogeology • Water lubricated bedding planes; weak
limestone interbedded with clay layers steeply inclined toward reservoir
Fig. 11.3. Vaiont dam disaster