Slope Failures - Falls, Slides, and Flows

what is mass wasting?

• gravitational downslope transport of rock, regolith (unconsolidated material), snow or ice

• also called mass movement

factors that affect slope shape?

• material strength

  • ability to avoid failure

  • material strength depends on if it is coherent or unconsolidated or has planes of weakness

    • roundedness plays a part too

• climate

  • affects which agents or erosion are at work (ice, water, wind) and how fast; also affects soil and vegetation formation on slopes

    • more veg → more stability

• process of formation

  • ex. glacially carved valley have a ‘U’ shape

  • river erosion causes a ‘V’ shaped valleys

factors controlling downslope movement of material?

• slope angle

• critical angle of repose

• cohesion & water Saturation

• Material Strength

slope angle stress?

• Shear Stress (σd): stress resulting from application of force parallel to a surface (force pulling the boulder/grain downslope)

• Normal Stress σn): component of stress perpendicular to the Earth’s planar surface (force keeping the boulder/grain from moving)

when is a slope stable?

• when σd<σr

  • when resistance stress (friction and cohesion) is bigger than downslope shear stress

when is slope unstable?

• when σd>σr

  • shear stress bigger than resistance

what is the critical angle of repose?

• max angle at which sediment particles can stand w/o falling depends on:

• grain size

• grain angularity

  • rounded grains (more friction) < angular grains

• moisture content

  • dry sediments typic is less stable than damp sediment

  • oversat sediment becomes less stable

what is cohesion?

• attraction between small soil particles that is provided by the surface tension of water between the particles

• friction and surface tension of water can provide some resistance to sliding

material strength categories?

• loose, poorly cemented, dry material → weak

• solid, well-cemented material → stronger

• weak between sedimentary layers & along well dev factures

  • diff types of rocks layer up → change between one type of rock to other can be failure

what are the diff types of slope failure?

• creep and solifluction

• slumps

• debris flows & mud flows

• rockslides and rockfalls

• sturzstroms (rock avalanches)

what is a creep?

• extremely slow downslope flow of sediment on the surface

• caused by cycles of freeze-thaw, wetting-drying and/or warming-cooling

  • when water freeze, expands ground (rises) n continues

  • so winter slope is higher than summer slope

how do creep occur?

• slow, so loss is mostly from damage to buildings and infrastruct, rather than casualties

what is solifluction?

• downslope creep driven by sequential freezing and thawing in tundra regions

• results in soggy layer of ground above frozen permafrost that then moves downhill

  • this water can’t go back down

what are translational slump?

• landslide that moves along a regular sloping planar surface

• typic occurs when a block of material is underlain by a weak surface that is more or less parallel to the slope

  • planes between sedimentary beds

  • old faults and fractures

  • debris or cohesive mud over underlying bedrocks

• block can travel down slope as a cohesive unit or lose internal cohesion and break up as it travels downslope

  • if it breaks, turns into diff landslide

what are rotational slumps?

• landslide in which the mass rotates on a concave failure surface

examples of slumps?

• Slumps dev along the South Saskatchewan River near Medicine Hat, Alberta

• Slump destroyed Holbeck Hall in Scarborough, England, 1993

what are flows?

• slope failures involving material that has lost internal cohesion

what is mud flow?

• flow of mud, rock, and water dominated by clay-sized particles

  • includes lahars

what is debris flow?

• slurry of rock, sand, and water flowing downslope; water usually makes up less than half of the flow vol

what does speed of flow depend on?

• steepness of the slope

• water content

example of debris flow?

• Andres of Northwest Argentina that was rich in cobble and boulders

example mudflow?

• Afghanistan, 2014

• heavy rain turned a slump into a mudflow

• buried part of a village in Afghanistan

Rio De Janeiro and landslides

• highly weathered Sugarloaf mountains surround the city

• poor communities (favelas) built on material where landslides are common

• 8 million mostly poor Brazilians face mudflow risk

• wealthy communities build homes on stable ground

Vargas Tragedy - Venezuela 1999

• several debris and mudflows over a 2-day period caused by heave rainfall

• towns were built on debris from past flows

• 30,000 deaths

• 75,000 people displaces

• dutch helped out (Netherlands)

• Some buried to death

• mudline on clothes suggest that the landslide was much higher than what videographer saw next day

what is quick clay?

• water-sat mud deposited in salty water composed of clay flakes with large pore spaces between the flakes

• highly unstable

example of quick clay landslides?

• Led Clay in the ST. Lawrence, Saguenay, and Ottawa Valleys

• Lemieux Landslide 1993 →

  • The headscarp retrogressed 680 m into level

    ground above the riverbank. About 2.8 million tons of clay and silt liquefied and flowed \n into the South Nation River valley, damming the river

• Alta, Norway 2020

what is a rockslide?

• sudden movement of rock and debris down a non-vertical slope

Vaiont Slide

• October 9, 1963; Southern alps in northwest Italy

• dam changed water tables (raised it) → caused surface above to get wet, making it unstable → cause slide + river was blocked

• slide displaced all the water in downstream half of the reservoir

• 125m high wave crashed over the dam

• tsunami rushed upstream

• 2500 lives lost due to water spilling over dam

what is a rockfall?

• slope failure that occurs when a mass free-falls down a cliff or a steep slope; slope is often vertical

rock fall examples?

• Swiss Alps

• India

what is frost wedging?

• splitting of rock thru pressure exerted when water freezes

• freezing water expands by 9.2%

• coastal areas prone to frost wedging where temperature oscillates around freezing pnt

• example: cliff in Torbay, Newfoundland and Labrador

what is Sturzstrom?

• extremely rapid downslope movement of large vols or rocks and debris

• largest and most destructive

• typic being as a rock slide, but breaks up, entraining air n water

• travel velocities as high as 100-300 km/hr

• can travel distances up to 20x their vertical fall

• mecha that causes sturzstroms to have such hig velocity and long flow paths is still under debate

• sturzstrom may flow as a fluid composed of rock fragment suspended in air (fluidization)

examples of struzstroms?

• 1903 Frank, Alberta

• 1965 Hope, BC

Frank, Alberta

• started as a translational slide

  • grained speed and dev into sturzstrom

• event lasted less than 100 secs

• slope failure occurred along fractured limestone planes

• based on the speed and distance of the sturzstrom, it is thought to have traveled on a cushion of compressed air

• The rock moved from Turtle Mountain (lower \n left), buried the town of Frank, crossed the river and surged 120m up the other slope.

• The river eventually cut through and opened the channel again

hazard mapping & assessment

• GIS software can help identify areas of past and potential failures by mapping attributes that contribute to slope failures

  • slope

  • bedrock type (lithology)

  • sediment texture and depth

  • vegetation

  • drainage

  • historic slope failures

reducing or mitigating risk?

• roots of plants

• reservoir lowering

• river undercutting flow

• retaining wall

• pipes

• metal screening

• debris basin

• rock bolt

how to reduce risk from slope failures?

• installation of detection devices, such as acoustic flow monitors

  • only gives a couple of mins of warning

  • not enough time to evacuate peep downslope

• prohibit building at the base of mountains and is dangerous areas along steep slopes

• zone hazardous area as open space for parks, golf courses and agriculture

• construct walls and dams to deflect or slow debris flows

• construct debris basins large enough to channel and contain debris