Mass Wasting Lecture-18

Mass Wasting Overview

• Definition: Movement of rock or soil downslope as a coherent mass.

Landslides

• Colloquial term for large ground mass movements.

Significance of Mass Wasting

• Fatalities (2004-2010): 80,058 deaths globally due to mass wasting events.

• Non-seismic landslide events: 2,620 with 32,322 deaths; seismic landslides: 47,736 deaths.

Factors Influencing Landslides

• Controls on spatial distribution: Climate (tropical cyclones, monsoonal moisture), population density, geology (active mountain belts).

• Local factors: Soil porosity, water content, rainfall (150-200% of average pre-Oso Mudslide).

Causes of Hillslope Failure

• Gravity: Provides potential energy for slope failure.

• Landscape gradient: Steeper slopes are less stable.

• Cohesion/strength of rock: Lower cohesion increases instability.

• Water content: Reduces friction; increased pore pressure can trigger failure.

• External triggers: Earthquakes, undercutting.

• Anthropogenic activities: Logging, construction.

Angle of Repose

• Defined as the maximum stable slope angle.

• For loose materials:

  • Sand: 30-35°

  • Gravel: 40-45°

  • Moist sand: up to 90°

  • Saturated sand: <10°

Stability Considerations

• Water impact:

  • Dry sand: Stability based on friction.

  • Wet sand: Stability increases due to surface tension.

  • Saturated sand: Decreases stability due to pore pressure.

Types of Mass Movements

• Slides/Slumps: Well-defined shear plane, coherent material movement.

  • Rotational slide/slump: Failure along a weakness plane, e.g., Oso slide.

• Flows: Material behaves like a fluid, no defined shear plane.

• Falls: Rapid downward movement from steep faces.

External Triggers for Failure

• Earthquakes, volcanic eruptions, extreme rainfall, natural undercutting, human activities.