Angle of Repose?
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016
26th June 2016 Around 30 minutes
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016
30th June 2016 Around 1 hour
Mass Movements Mass movements include: • Landslides • Rock falls • Avalanches • Mud flows • Debris flows • Creep
Dry mass movement from Slow to fast Creep------->Rock fall
Creep
Soil Creep
Creep
Rock fall
Rockfall
Talus slope
Slow-medium velocity mass movement from dry to wet
Earthflow
The earthflow began 700 years ago. Slumgullion Earthflow, CO 6 m/yr Highway 149 The earthflow began 700 years ago. About 350 years ago, a second earthflow when, lubricated by heavy rains, the weak volcanic tuff and breccia on the southern flank of Mesa Seco slumped several miles down the
Solifluction
Fast mass movement processes from dry to wet
Rockfall
Debris avalanche
Mud flow
Angle of Repose Varies for Different Materials
Angle of Repose Dry sand cannot support an angle of >35o from horizontal: this is termed the angle of repose. Coarser grains (and more angular materials) support steeper angles of repose. 35o
Water decreases rock/soil cohesion
Water decreases rock/soil cohesion
Water decreases rock/soil cohesion Water circulating underground can dissolve cements that hold sedimentary rocks together
Internal Causes for Slope Failure Water (weight & changes with clay minerals) Decreases rock cohesion Weak material Adverse geologic structures
The Weight of Water Sedimentary rocks commonly have porosities of 10 - 30% If pore spaces fill with water, the weight of the material is increased substantially, creating instability
La Conchita, CA March 1995
in exactly the same place… It happened again in 2004… in exactly the same place…
La Conchita, CA
Mass movement Terminology A fall is a mass movement where singular or multiple blocks of rock plunge from a height. A slide is a mass movement that occurs as one unit. A flow is a mass movement that is internally chaotic and turbulent. What begins as a slide may become flow as it moves downslope and disaggregates. Slides and flows begin in areas of steep slope, but after accelerating may continue to spill over land of low slope at moderate speed for substantial distance. Their specific names derive from the material they carry: rock, debris, earth, and mud being the most common. The term avalanche refers to a debris flow made up in large part of snow and ice. A lahar is a mudflow or debris flow consisting mainly of volcanic material.
Falls talus
Slides
Slides This is a typical landslide. Note that materials hold together in more-or-less singular blocks. As is common, when the slide blocks reach a lower slope, they break up (becoming a debris flow in this case). The La Conchita landslide, near Santa Barbara, CA, Spring, 1995.
Flows
Optimal Conditions for Falls, Slides, Flows: Occur near the bases or tops of slopes. Rock falls require fractured rock exposures. Slides begin along contacts between layers of rock, soil. Where surface vegetation is destroyed by fires, followed by rainfall Roadcuts or other areas of altered or excavated slopes Volcanoes are common targets for such mass movement because of the fine, loose sediment. All of these are made more prevalent by earthquakes that trigger.
Geology Contributes to Mass movement Mass movement will exploit geological discontinuities, or parts of rock/soil that have some preferred weakness.
areas where limited friction exists between geological units. Gros Ventre River, WY, 1925 We see geology playing the starring role in the potential for mass movement: areas where limited friction exists between geological units.
370 190 400 Intact rock – stronger / fewer joints Water affected slope and fine grained material 400 Larger blocky loose material – drained – will probably move -> 35o with time
Angle of repose https://www.youtube.com/watch?v=a-6YbkZJ5UY When granular material is heaped, there exists a limit for the inclination of the surface. Beyond that angle, the grains start rolling down. This limiting angle up to which the grains repose (sleep) is called the angle of repose of the granular material. What can affect this? https://www.youtube.com/watch?v=a-6YbkZJ5UY