6.2 Classification of Mass Wasting

GEOL1340

Mass Wasting (Mass Movement)

The downslope movement of rock, regolith, and soil driven directly by the force of gravity.

Geological Role of Mass Wasting

Represents the first step in sediment transport within the rock cycle and is the most rapid mechanism for modifying slope shapes and developing landscapes.

Fracturing & Weathering (Precondition)

Fresh, intact rock is too strong to slide; weathering and fracturing must first weaken the geological material.

Unstable Slope (Precondition)

A slope positioned near its stability threshold (e.g., high local relief, thick soil/debris, structural weaknesses, or sparse vegetation).

Initiating Event / Trigger (Requirement)

An event that upsets the balance and starts mass movement.

Downslope Force ($F_d$)

The gravitational component pulling material down a hill.

Resistance Force ($F_r$ / Friction)

The shear strength resisting mass movement.

Stable Slope Condition

$F_d < F_r$ (friction exceeds gravity; material remains in place).

Slope Failure Condition

$F_d > F_r$ (gravity overcomes friction; slope fails).

Angle of Repose

The steepest angle at which loose, unconsolidated sediment remains stable without sliding.

Fine Sand Angle of Repose

$$\sim 35^\circ$$

Coarse Sand Angle of Repose

$$\sim 40^\circ$$

Angular Pebbles Angle of Repose

$\sim 40^\circ$ (interlocking grains increase friction).

Role of Water in Small Amounts

Adds cohesion through surface tension, holding grains together and helping inhibit movement.

Role of Water in Oversaturation

Adds substantial weight to the slope overburden, destroys grain cohesion, increases pore water pressure, pushes grains apart, and drastically lowers friction/strength.

Structural Planes of Weakness

Slopes are especially prone to failure when internal planes of weakness (sedimentary bedding planes, metamorphic foliation, or exfoliation joints) dip parallel to the slope face.

Vegetation Cover Stabilization

Plant root networks anchor soil and stabilize potential failure surfaces.

Vegetation Cover Destabilization

Stripping vegetation (due to wildfires, deforestation, or land clearing) removes root support and increases mass wasting risk.

Oversteepening Slopes (Trigger)

Caused naturally by river/wave erosion undercutting cliffs, or artificially by road construction.

Reducing Slope Strength (Trigger)

Weathering, vegetation removal, adding heavy structures to slope tops, or saturating soil with water.

Shocks & Vibrations (Trigger)

Ground shaking triggered by earthquakes, construction activity, or heavy traffic.

Fundamental Rule of Landslide Prediction

"Landslides occur where they have occurred before."

Best Predictor of Future Mass Wasting

Evidence of past landslides (e.g., historical scars, hummocky terrain, uneven vegetation age).

British Columbia & California Mass Wasting Hotspots

Exceptional hotspots due to active weathering/tectonics, steep relief, and seismic/climate triggers.

Four Primary Criteria for Classifying Mass Movement

Rate of movement (velocity), type of material (bedrock vs. regolith), nature of movement (coherent vs. fluid slurry, wet vs. dry), and environment (subaerial vs. submarine).

Creep (Soil Creep)

Extremely slow downslope movement (millimeters to centimeters per year) affecting only the upper several centimeters of soil.

Visible Field Clues of Creep

Tilted telephone poles, fence posts, and gravestones; curved tree trunks ("pistol butts"); cracked building foundations; and damaged road surfaces.

Solifluction

Gradual downslope movement driven by freeze-thaw activity in cold or permafrost environments where water-saturated surface soil flows downhill over frozen subsoil.

Slump (Slumping)

Downslope movement of a coherent mass of loosely consolidated sediment or rock along a concave-upward (spoon-shaped) curved surface, leaving a crescent-shaped head scarp.

Rockslide / Slide

Sudden, rapid sliding of intact bedrock along pre-existing flat planes of weakness (bedding planes, joints, or foliation).

Mudflow (Mudslide)

A fast-moving flow of dirt and fine debris liquefied by abundant water.

Debris Flow

Water-laden mass of soil, mud, and larger rock fragments that funnels into stream channels and entrains trees, rocks, and structures.

Lahar

A specific volcanic mudflow composed of volcanic ash mixed with meltwater or heavy rain (e.g., Mount St. Helens eruption).

Rockfall

Free-fall of detached bedrock blocks dropping vertically from steep cliffs or slopes at or near terminal velocity.

Rock Avalanche

A massive, terrifyingly fast rush of snow, ice, and shattered rock travelling down a steep mountain supported by a cushion of air.

Frank Slide (Alberta, 1903)

Classic Canadian rock avalanche where 44 million cubic meters of limestone collapsed off Turtle Mountain at 120 km/h.

Submarine Mass Movements

Mass wasting occurring on the ocean floor along continental slopes and deep trenches (includes submarine slumps, debris flows, and turbidity currents).

Turbidity Currents

High-density suspensions of sediment and water that flow rapidly down submarine canyons, depositing graded beds on deep abyssal plains.

Controlled Blasting / Explosives (Mitigation)

Intentionally triggering small avalanches or rockfalls before dangerous volumes build up.

Slope Grading / Terracing (Mitigation)

Reducing overall slope angle to bring it below the material's angle of repose.

Rip-rap / Undercutting Protection (Mitigation)

Placing heavy boulders along riverbanks or coastlines to resist wave/current erosion at the slope base.

Retaining Walls & Benched Cuts (Mitigation)

Barrier walls equipped with drainage ditches and culverts to hold back debris and route water safely away.

Rock Bolts, Wire Mesh, & Shotcrete (Mitigation)

Steel bolts, mesh curtains, and sprayed concrete used to seal fractures, stabilize rockfaces, and prevent water infiltration.