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

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