EVSC10001 Lecture Notes - Lecture 25: Mass Wasting, Regolith, Particle Size
10 Jun 2018
School
Department
Course
Professor
All slopes are unstable - constantly changing via MASS MOVEMENT processes
Mass movements are important to rock cycle
Initial step in SEDIMENT TRANSPORT
-
Significant agent of LANDSCAPE CHANGE
-
Natural movement often accelerated by humans
Mass wasting: movement of rock, regolith, or soil by GRAVITY(can be helped by
small amounts of water)
Downslope force: gravity (weight of material, added water, structures) or
shear stress
1.
Resisting force: material strength (cohesion, friction) - shear strength
2.
Steeper slopes -> larger downslope force -> more prone to mvoement
Particle size, shape, and surface roughness
○
-
Slope Stability and Failure
Fs = shear strength / shear stress
>1 failure shouldn’t occur
○
<1 failure should occur
○
-
Earthquakes ,blasting, etc. 1.
Shocks and vibrations abruptly decrease material friction
-
Fast triggering event not always necessary
Slope material weakens (weathering)1)
Gravity operates 2)
-
Changes in slope characteristics
-
Undercutting 1.
Reduction of vegetation or roots -> destabilization 2.
Steeping and loading of landscape 3.
Adds weight, pushes grains apart, lubricates (pore water between grains) 4.
Precipitation + steep slopes •
CLASSIFICATION
Water content (rockfall vs. mudflow)
-
Rate of movement (soil creep vs. avalanche)
-
Soil creep1.
Slow (1-3mm/yr)
-
Downslope movement of regolith (weathered rock fragments + soil)
-
Heaving processes due to wetting and drying, or freezing and thawing,
undercutting
Frost heave and needle ice
○
-
Solifluction2.
In places with permafrost (periglacial environments)
Summer melt water logs top layer, permafrost remains as
impermeable layer
○
Wet material slides on top
○
-
Low angled
-
Gradual movement of WET soil down slope
-
Landslides (slides + slumps) 3.
Failure of underlying stratum along a shear plane (failure surface)
-
Sliding of regolith as coherent blocks down a nonvertical slope
-
Usually excess water involved - large rainfall events, land clearance
-
Sliding rotational (slumps) or translational (landslide)
-
Rock slide = slide ONLY ROCKS
-
Debris slide = slide mostly REGOLITH
-
Debris flows and mudflows4.
Flows of fragments and mud with high water content
-
Grains supported by strength + buoyancy of matrix
-
Cohesive strength - behaviour difficult to predict using physical laws
-
Water reduces cohesion
-
Often after intense rainfall
-
Plug: shear stress < shear strength
-
Lahar: violent debris/mudflow composed of PYROCLASTIC VOLCANIC MATERIAL,
ROCKY DEBRIS, AND WATER
Rock falls / Topples 5.
Blocks of bedrock or regolith detach and fall vertically - impact of blocks ->
fragment -> keep moving
-
Well-jointed or well-bedded rock
-
Can trigger debris avalanches
-
Rock avalanche: larger volume of rock and mass appears to flow even
though no water is involved
-
IMPLICATIONS
dynamic + dangerous
-
Prevention by: revegetation, regrading and reshaping, dewatering, stabilizing
undercuts, engineering (retaining walls, rock staples, avalanche sheds,
controlled blasts)
-
Slope replacement
Rocks weather -> accumulated debris at foot slope (cone shaped) -> cones
grown and create more gradual slope as debris grows over time
Talus cones
○
-
Parallel retreat
Weathering, mass wasting (accumulation), erosion are balances
○
Slope moves back over time
○
-
Mass Wasting
Thursday, 7 June 2018
11:34 pm
All slopes are unstable - constantly changing via MASS MOVEMENT processes
Mass movements are important to rock cycle
Initial step in SEDIMENT TRANSPORT
-
Significant agent of LANDSCAPE CHANGE
-
Natural movement often accelerated by humans
Mass wasting: movement of rock, regolith, or soil by GRAVITY(can be helped by
small amounts of water)
Downslope force: gravity (weight of material, added water, structures) or
shear stress
1.
Resisting force: material strength (cohesion, friction) - shear strength2.
Steeper slopes -> larger downslope force -> more prone to mvoement
Particle size, shape, and surface roughness
○
-
Slope Stability and Failure
Fs = shear strength / shear stress
>1 failure shouldn’t occur
○
<1 failure should occur
○
-
Earthquakes ,blasting, etc.
1.
Shocks and vibrations abruptly decrease material friction
-
Fast triggering event not always necessary
Slope material weakens (weathering)
1)
Gravity operates
2)
-
Changes in slope characteristics
-
Undercutting
1.
Reduction of vegetation or roots -> destabilization
2.
Steeping and loading of landscape
3.
Adds weight, pushes grains apart, lubricates (pore water between grains)
4.
Precipitation + steep slopes
•
CLASSIFICATION
Water content (rockfall vs. mudflow)
-
Rate of movement (soil creep vs. avalanche)
-
Soil creep1.
Slow (1-3mm/yr)
-
Downslope movement of regolith (weathered rock fragments + soil)
-
Heaving processes due to wetting and drying, or freezing and thawing,
undercutting
Frost heave and needle ice
○
-
Solifluction2.
In places with permafrost (periglacial environments)
Summer melt water logs top layer, permafrost remains as
impermeable layer
○
Wet material slides on top
○
-
Low angled
-
Gradual movement of WET soil down slope
-
Landslides (slides + slumps) 3.
Failure of underlying stratum along a shear plane (failure surface)
-
Sliding of regolith as coherent blocks down a nonvertical slope
-
Usually excess water involved - large rainfall events, land clearance
-
Sliding rotational (slumps) or translational (landslide)
-
Rock slide = slide ONLY ROCKS
-
Debris slide = slide mostly REGOLITH
-
Debris flows and mudflows4.
Flows of fragments and mud with high water content
-
Grains supported by strength + buoyancy of matrix
-
Cohesive strength - behaviour difficult to predict using physical laws
-
Water reduces cohesion
-
Often after intense rainfall
-
Plug: shear stress < shear strength
-
Lahar: violent debris/mudflow composed of PYROCLASTIC VOLCANIC MATERIAL,
ROCKY DEBRIS, AND WATER
Rock falls / Topples 5.
Blocks of bedrock or regolith detach and fall vertically - impact of blocks ->
fragment -> keep moving
-
Well-jointed or well-bedded rock
-
Can trigger debris avalanches
-
Rock avalanche: larger volume of rock and mass appears to flow even
though no water is involved
-
IMPLICATIONS
dynamic + dangerous
-
Prevention by: revegetation, regrading and reshaping, dewatering, stabilizing
undercuts, engineering (retaining walls, rock staples, avalanche sheds,
controlled blasts)
-
Slope replacement
Rocks weather -> accumulated debris at foot slope (cone shaped) -> cones
grown and create more gradual slope as debris grows over time
Talus cones
○
-
Parallel retreat
Weathering, mass wasting (accumulation), erosion are balances
○
Slope moves back over time
○
-
Mass Wasting
Thursday, 7 June 2018 11:34 pm
Document Summary
All slopes are unstable - constantly changing via mass movement processes. Mass wasting: movement of rock, regolith, or soil by gravity(can be helped by small amounts of water) Downslope force: gravity (weight of material, added water, structures) or shear stress. Resisting force: material strength (cohesion, friction) - shear strength. Steeper slopes -> larger downslope force -> more prone to mvoement. Adds weight, pushes grains apart, lubricates (pore water between grains) Downslope movement of regolith (weathered rock fragments + soil) Heaving processes due to wetting and drying, or freezing and thawing, undercutting. Summer melt water logs top layer, permafrost remains as impermeable layer. Failure of underlying stratum along a shear plane (failure surface) Sliding of regolith as coherent blocks down a nonvertical slope. Usually excess water involved - large rainfall events, land clearance. Flows of fragments and mud with high water content. Grains supported by strength + buoyancy of matrix. Cohesive strength - behaviour difficult to predict using physical laws.
