Mass Wasting. Revisit: Erosion, Transportation, and Deposition

Transcription

1 Mass Wasting Revisit: Erosion, Transportation, and Deposition While landslides are a normal part of erosion and surface processes, they can be very destructive to life and property! - Mass wasting: downslope movement of material under the direct influence of gravity. The rate of movement varies greatly.!1

2 Factors that control Mass Wasting: Mass wasting occurs when the gravitational force acting on a slope exceeds the resisting force.!2

3 - 1) Steepness: slope angle - Excavation for road and hillside construction is a common and similar problem 1)! 2)! Highway excavations disturb the equilibrium of a slope by removing a portion of its support, as well as oversteepening it at the point of excavation, which can result in... 3)! landslides along the highway. - Stream undercutting or wave erosion can oversteepen slopes 1)! 2)!!3

4 -2) Type and orientation of rock: - Revisit some concepts: shear strength and angle of repose A slope s shear strength dependson the slope material s strength and cohesion, the amount of internal friction between grains, and any external support of the slope. These factors promote slope stability. The force of gravity operates vertically, but it also has a component of force acting parallel to the slope. When this force, which promotes instability, exceeds a slope s shear strength, slope failure occurs.!4

5 The angle of repose is a function of shear strength. Dry sand has an angle of repose of about 30 degrees. With wet sand, the shear strength is increased, and much steeper angles of repose are possible, such as this pile that is nearly vertical. - type: granite; strong shear strength loose regolith; low shear strength. ex. clay rich gravel, poorly lithified shale!5

6 - orientation: if rock layers dip in the same direction as the slope, upper layers are more prone to slide.!!!6

7 - 3) Nature of Unconsolidated material: - angular and irregular material is less likely to slide than rounded material - angle of repose of material - 4) Weathering and Climate: - Mass wasting is more common in loose or poorly consolidated material than in bedrock - In areas of high temperature and precipitation, weathering is deeper and produces tens of meters of unconsolidated material - In arid regions soils are thinner, but heavy localized rainfall may result in mudflows - 5) Water and Vegetation: - Water content - Water may increase the weight of a slope enough to induce failure - Water reduces the amount of friction between particles. Becomes a lubricant - Because clay particles are platy and slide easily past one another when wet, clay beds are often the layers on which overlying rock units slide - May make things stick together. Increase angle of repose!7

8 - Vegetation - Decreases water content of slope materials - Root systems stabilize by binding soil and holding the soil to bedrock - Removal of vegetation can lead to mass wasting - 6) Earthquakes and Volcanoes: Most rapid mass movements are triggered by strong vibrations from earthquakes and/or excessive amounts of water - EQ trigger mass wasting by shaking the slope - Volcanic activity may melt snow and ice or blow the sides of mountains off.!8

9 Things we do to Initiate Mass Wasting - Removal of supporting materials at the base of slopes - ex. excavating during road construction, terraced housing developments! - Overloading: putting to much weight on top of hill - ex. homes, building, excess earth - Reducing friction: due to water - ex. leakage from reservoirs, ponds, irrigation, canals and septic tanks!9

10 * Shale is made of clay, therefore, when wet it gets extremely slick* What Are the Different Types of Mass Wasting? - Three types: Flow, Slide, Fall - Classified on the basis of: - Rate of movement - fast or slow - Rapid movements involve visible movements of material - Slow movements are imperceptible except from their effects such as cracked walls and tilted trees or power poles - Type of movement - falling, sliding, or flowing - Type of material - rock, soil, and debris!10

11 - Falls - Occurs when unconsolidated material falls freely - Rockfalls are a common type of rapid mass wasting - May occur along steep canyons, cliffs, and road cuts! 11

12 Slides - slumps and block - Move along one or more surfaces of failure - May consist of soil, rock, or both - May move rapidly or slowly - Slumps involve movement along a curved surface - occurs when large chunks of rock or regolith moves due to a concave fracture! - Rock or block slides (avalanche) move along a planar surface, often where dip is the same as slope direction!12

13 ! - Flows move as a viscous fluid or show plastic movement -mudflows are fluid and move fastest, common in arid or mountainous regions - debris flows are more viscous - earthflows move as thick, viscous masses of wet regolith - Mudflows - Consist of at least 50% silt and clay sized particles, at least 30% water - Usually follow pre-existing channels until the slope decreases, then fan out!13

14 - Debris flows - Composed of larger sized particles than mudflows - clay, silt, sand and rock, in which more than ½ of the particles are larger than sand - Don t contain as much water as mudflows - Rarely confined to pre-existing channels!14

15 - Earthflows - thicker, less fluid, than a mudflow - Slumps form the upper part of a hillside - Occur most commonly in humid climates!15

16 - Quickclays - Spontaneous liquefaction and rapid flow of fine silt and clay - composed of fine silt and clay that was made by glaciers - Original pore space was filled with salt water and ionic bonds strengthened the clay particle attraction - Salt water flushed out, clays lose cohesion, and sudden movement liquefies - Solifluction - The slow downslope movement of water- saturated surface sediment -Most common in areas of permafrost!16

17 ! - Creep - Slowest type of flow, most common in humid climates such as the southeastern US - Particles move independently, not as a mass - rate = 1 centimeter/year!17

18 - Top moves faster than bottom, therefore, making things tilt - ex. telephone poles, trees, headstones - Extremely destructive over time; difficult to recognize or control - Complex Movements - Occurs when several of the recognized types are involved in a mass movement ex. slide-flow!18

19 ! ex. debris avalanche - Recognizing and Minimizing Mass Wasting - Conduct a thorough geologic investigation of the area in question - Assess risks and take steps to minimize the effects of!19

20 events - Slope stability maps indicate where to place roads, developments, and utility lines - Drainage of high areas or other water control measures helps prevent movement Drainage pipe - Reducing the angle of slope using cut-and-fill or benching!20

21 - Retaining walls and drainage pipe!21

22 !22

23 - Rock bolts hold unstable surface rock to solid bedrock!23