Landslides

From GeoClasses

Landslides

• Mass wasting: any downslope movement of material (gravity driven)

1. driving force = gravity (= weight, including building and vegetation on top)

2. resisting force = strength of slope material along slip planes (surfaces of weakness: foliation planes, bedding planes, faults)

Slope shapes

• 1. cliff - vertical (if hard rock)

Cliffs of Moher, Ireland, also known as the "Cliffs of Insanity" in Princess Bride. Example of cliff formed by the sea.

Angel Falls, Venezuela: the tallest free falling waterfall in the world. Example of cliffs formed by rivers.

tallus slope in NW US

• 2. tallus slope - debris accumulated at base of cliff (if hard rock)

• 3. S curve (convex - straight - concave)

example of the hill Heath Mynd, England, showing an S curve shape

S curve shape of an ant hill

• • if weaker rock (schist, sedimentary)
  • has soil layer on top

Role of materials

• if strong rock on top of weak rock - weak rock erodes out from under = rock falls
  • Differential weathering
• if shales or pyroclastics (easily crumbled) = creep or mudflows if saturated
• if well cemented (some sandstones, limestone, granite) = very resistant
  • Spheroidal Weathering

role of slope angle

• slope = angle from horizontal (how steep in the hill?)
• steeper slope = greater risk
  • Angle of repose

role of climate

• determines amount of water at which time of year and what type of vegetation

role of vegetation

role of vegetation on slope stability

• 1. water - more soaks in (negative) but less erosion (positive)
• 2. roots stengthen slope material (positive)
• 3. adds weight to slope (negative)
  • slope stability

role of water

• 1. shallow soil slips - created during storms when ground is saturated
• 2. slumps - created when water is deep in slope - longer time frames after storm

Colorado River running through the Grand Canyon

• 3. water at base of slope (such as river) can erode and decrease stability

• 4. water in clay-rich sediment can cause liquefaction (clays behave as liquid and flow)
• 5. water increases chemical weathering (H2O + CO2 = H2CO3) especially in limestone

cartoon showing the formation of sinkholes in karst

photo taken after sinkhole collapse

humans as geologic agents

• changes we make
  • 1. timber harvesting

timber harvesting in New Zealand, as seen from space. Tan areas are cleared forest, little tan dots are clearings used to stock timber before shipping

• • • removes roots from unstable weak slopes = more likely to slide
    • access roads affect the drainage and weight distribution (bigger problem)

• • 2. urbanization on hills and slopes

urbanization of slopes in L.A.

• • • includes clearing vegetation, building roads, dumping fill from excavation - all adds weight to slope
    • then when rains the slope is unstable

preventing? and minimizing landslides

man-made drainage on slope

• 1. drainage - object is to divert water from running across the slope of infiltrating the slope (but then no plants on slope?!)

• 2. grading - moving material from top to bottom of slope so the grade is less
  • or build benches or steps with drains in them

terraced rice fields in Yunnan Province, China

terraced rice fields in the Phillipines

• 3. slope supports - retaining walls (but does it work?)

stone retaining wall

subsidence = a related problem

• caused either by withdrawal of fluids from subsurface (water or oil), or by collapse into a void
  • 1. withdrawal of fluids - fluids support ground above
    • oil and gas
    • groundwater
    • steam and water for geothermal power

oil well in Texas

irrigated crops in Kansas (seen from space) using goundwater from underlying Ogallala aquifer

geothermal power plant in Iceland

• • 2. sinkholes

Dead Sea sinkhole, formed by dissolution of salt deposits by incoming freshwater, due to sea level drop

• • • natural voids are created by subterranean removal of material (caves) by chemical weathering
    • probably triggered by changes in water table level
      • high water table = buoyancy
      • how water table = empty space

examples of humans as agents:

• 1. salt mining

rock salt mine near Mount Morris, NY

• • solution methods used (water injected into salt underground, salt dissolves in water, salty water is pumped out and salt extracted from water)

• 2. coal mining (subsurface)

"long wall" underground coal mining

• • where mining was close to surface OR insufficient support was left standing
  • usually 50% of coal is removed and rest is left to support ground above
    • but coal is very weak and crumbly and over time the rest of the coal collapses