1. Angle of Repose?

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35. Mass Movements Mass movements include: • Landslides • Rock falls
• Avalanches
• Mud flows
• Debris flows
• Creep

37. Dry mass movement from Slow to fast
Creep >Rock fall

38. Creep

39. Soil Creep

40. Creep

41. Rock fall

42. Rockfall

43. Talus slope

44. Slow-medium velocity
mass movement from dry to wet

46. Earthflow

47. 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

48. Solifluction

49. Fast mass movement processes from dry to wet

51. Rockfall

52. Debris avalanche

53. Mud flow

54. Angle of Repose Varies for Different Materials

55. 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

56. Water decreases rock/soil cohesion

57. Water decreases rock/soil cohesion

58. Water decreases rock/soil cohesion
Water circulating underground can dissolve cements
that hold sedimentary rocks together

60. Internal Causes for Slope Failure
Water (weight & changes with clay minerals)
Decreases rock cohesion
Weak material
Adverse geologic structures

61. The Weight of Water
Sedimentary rocks commonly have porosities of %
If pore spaces fill with water, the weight of the material is increased substantially, creating instability

62. La Conchita, CA
March 1995

63. in exactly the same place…
It happened again in 2004…
in exactly the same place…

65. La Conchita, CA

66. 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.

67. Falls
talus

68. Slides

69. 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.

70. Flows

71. 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.

72. Geology Contributes to Mass movement
Mass movement will exploit geological discontinuities,
or parts of rock/soil that have some preferred weakness.

73. 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.

74. 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

75. 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?