MASS WASTING

Mass Wasting

Downslope movement of earths materials in response to the pull of gravity

• Regolith

• Bedrock

Regolith

Loose (unconsolidated) material (e.g., sediment, soil)

Bedrock

Solid rock underlying regolith/Exposed along Earth’s surface

Submarine Landslides

• Occurs when volcanic island give way, leaving massive debris fields on the deep seafloor

  • Volcanic rocks are inherently unstable

  • Hawaii ⇒ Oahu & Molokai

Pali

Steep cliffs, the edge where rocks fell off

Hope Slide

Largest recorded landslide (CAN) ⇒ Jan 9, 1965, 4 deaths

Rock avalanche (landslide)

• Fast fluid motion of broken rock by cushions of air

  • Rock may fragment to smaller pieces

Frank Slide

Weak fractured rock of Turtle Mountain ⇒ Canada’s most deadly rockslide (April 29, 1903)

• ~100 fatalities in the small mining town of Frank, AB

Rock Slide (Translational Slide)

Occurs as rock moves downslope along an inclined plane of weakness

Potential planes of weaknesses (Translational slides)

• Joints (sheeted joints)

• Clay/mudstone layers (slippery)

Rock bolts

Drilled into the rock, stitching weathered rock to unweathered rocks deep down

• Porteau Cove Rock Slide (2008)

Slump (Rotational Slide)

• Blocks slip along 1+ curved failure/rupture surface

  • Regolith commonly moves downslope as a slump

Slump features

• Head/main scarp

• Minor scarp

• Flank

• Slide surface

• Slump block

• Hummocky toe

Slump ⇒ Earthflow

Common to transition into an earthflow towards the base of the slope

• Fine-grained material liquefies & runs out in an hourglass shape 

• Rainstorms are the trigger

Undercutting

Cut-&-fill road construction has changed the slope angle

• Steeper slope = Increased chance of landslides

• Slumps are common along steep mountainsides

Water's Role

Involved in most mass wasting events

• In regolith, rainstorms are common triggers of slumps

  • Adds weight

  • Increase pore pressure ⇒ Lifts the grains

  • Decrease internal friction between grains ⇒ Lose cohesion

Debris Flow

Slurry of mud, water, rocks, & woody debris (Flowing downslope fast)

Quick Clay (Leda Clay)

Layers of glaciomarine clay are disturbed & suddenly lose strength 

• Large areas & many small pockets of ___ Clay underlie Canada’s capital

Creep

Slow, down-slope migration of regoliths due to the freezing & thawing of regoliths. Results in significant damage

Creep ⇒ Thawing

Grains settle back down with gravity 

Creep ⇒ Freezing

Grains lifted perpendicular to slope upon freezing

• occurs during thermal expansion too (slow shifts) 

Solifluction

Areas where thawing of permafrost occurs during summer months

Slow mass wasting types

Creep & solifluction

Mitigation

Build buildings elevated above the flowing surfac

Rock Fall

Free-falling of dislodged rock along steep cliffs

• Accumulates at the base as a steep cone-shaped pile of talus

Angle of repose

• The steepest-angle material of a certain size can attain

Rock fall Mitigation

• Wiremesh netting

• Scaling

• Tension wall

• Gabons

• Benching

• Shotcrete

Wiremesh netting

Wrap around cliffs to guide rocks down safely

Scaling

Removing loose & unstable rock

Tension wall

Reinforcements to absorb rock fall

Gabons

Ask

Benching

Retain falling rocks by angling the slope to fall into specific ledges

Shotcrete

Cement-like material sprayed onto a fractured slope