mass wasting

Mass Wasting Study Guide

Slope Stability

  • Mass wasting:

    • Definition: The downslope movement of soil, rock, and debris under the influence of gravity.

    • Significance: A key process in shaping landscapes, transporting weathered material from mountainous and sloped areas toward lower elevations.

    • Ultimate destination: All weathered materials eventually move toward the ocean basins, completing the sediment cycle.

  • Normal Force:

    • Definition: The component of an object’s weight acting perpendicular to the slope surface.

    • Function: It counteracts gravity’s downward pull.

  • Weight Equation:

    • Formula: W = m \times g

    • Where:

      • W = weight

      • m = mass (changes with material)

      • g = constant gravitational acceleration, g = 9.8 \ m/s^2

  • Gravity Vectors on Slopes:

    • Shear Force (downslope):

    • Function: Pulls material downhill.

    • Normal Force:

    • Function: Pushes material into the slope, resisting downslope movement.

  • Slope Angle (θ):

    • Higher angle (θ): Increases downslope shear force, leading to a less stable slope.

    • Lower angle (θ): Increases normal force, contributing to a more stable slope.

  • Friction Force:

    • Definition: Acts opposite to downslope movement and stabilizes the slope.

  • Increasing Weight on Slope:

    • Factors: Adding water (rainfall, irrigation), constructing buildings, or depositing sediment.

    • Effect: Increases load and decreases stability.

  • Shear Strength:

    • Definition: The resistance of unconsolidated materials to movement.

    • Influencing Factors: Cohesion, friction, and particle interlocking.

  • Angle of Repose:

    • Definition: The maximum slope angle at which loose material remains stable.

    • Influences:

    • Angular grains interlock better than rounded grains.

    • Coarse grains provide more friction than fine grains.

    • Scale: Horizontal = 0°, vertical = 90°.

  • Water Effects:

    • Small amounts of water can increase cohesion (through surface tension), raising the angle of repose.

    • Excess water reduces friction, adds weight, and lubricates particles, leading to slope failure.

  • Vegetation:

    • Function: Roots bind soil, absorb water, and reduce erosion, thereby stabilizing slopes.

  • Water Instability:

    • Heavy rainfall or rapid snowmelt can saturate the soil, reducing shear strength and triggering landslides.

Types of Mass Wasting

  • Classification: By type of material (rock, soil, debris) and movement (fall, slide, flow).

  • Flows vs. Slides vs. Falls:

    • Flows:

    • Behavior: Material behaves like a fluid and moves chaotically.

    • Slides:

    • Behavior: A coherent block moves along a defined plane.

    • Falls:

    • Behavior: Free-fall of rock or debris from steep slopes.

  • Other Triggers of Mass Wasting:

    • Factors: Earthquakes, volcanic eruptions, heavy rainfall, human excavation.

  • Flow Velocity:

    • Depends on: Slope steepness, water content, and type of material.

    • Order of velocity (from slowest to fastest):

    • Creep → Solifluction → Earth flows → Debris flows → Mudflows.

  • Creep:

    • Definition: Very slow downslope soil movement.

    • Characteristics:

    • Common in temperate climates with freeze-thaw cycles; absent in arid tropics.

    • Soil particles move incrementally, leading to visual impacts like tilted fences and bent trees.

  • Solifluction:

    • Definition: Saturated soil flow in permafrost regions.

    • Characteristic: Thawed surface soil slides over frozen ground, moving faster than creep.

  • Creep Damage:

    • Examples: Cracked foundations, tilted poles, distorted roads.

    • Indicators: Bent trees or tilted structures.

  • Earth Flows, Debris Flows, Mudflows:

    • Differentiated by material size and water content.

    • Triggers: Rainfall, snowmelt, volcanic activity may initiate these flows.

  • Slides:

    • Triggers: Weak bedding planes, earthquakes, or water saturation.

    • Movement Types:

    • Translational Slide: Movement along a flat plane.

    • Rotational Slide (Slump): Curved surface movement.

  • Falls:

    • Characteristics: Occur on steep cliffs and are often triggered by freeze-thaw cycles, weathering, or seismic shaking.

Causes & Mitigation

  • Increasing Stability Methods:

    • Strategies: Reduce slope angle, plant vegetation, remove excess water, and build retaining structures.

  • Hazard Mapping:

    • Purpose: Identifies landslide-prone zones to guide safe development practices.

  • Monitored Variables:

    • Important parameters include: Rainfall, groundwater levels, slope movement, seismic activity.

  • Creep Damage Avoidance:

    • Recommended Actions: Use flexible foundations, monitor slopes, avoid construction on unstable ground.

  • Safe Slope Grade:

    • General guideline: Slopes less than 30° are usually stable.

  • Construction Precautions:

    • Avoid building in:

    • Steep slopes

    • Fault zones

    • Areas with unconsolidated sediments.

  • Preventing Falling Rock Damage:

    • Methods: Use of rock nets, building tunnels, barriers, and catchment fences.

  • Slope Reinforcement:

    • Techniques include: Retaining walls, rock bolts, terracing, and vegetation.

  • Retaining Walls:

    • Requirement: Must include drainage; failure occurs if water pressure builds behind the wall.

  • Water Removal Techniques:

    • Examples: Hydrauger pipes, contour drains, and slope landscaping to reduce saturation levels.

  • Deflection Walls:

    • Purpose: Divert debris flows or rockfalls away from structures.

  • Real Estate Warning Signs:

    • Indicators of potential issues: Tilted trees, scarps, cracked foundations, hummocky terrain, and fresh landslide scars.

Human-Induced Mass Wasting

  • Vajont Dam Disaster (Italy, 1963):

    • Cause: Weak limestone and clay layers combined with steep slopes resulted in a massive landslide into the reservoir.

    • Consequence: Displaced water created a wave that overtopped the dam, leading to approximately 2,000 fatalities.

  • Providence Canyon (Georgia):

    • Cause: Sandy sediments have eroded due to poor farming practices, such as overgrazing and insufficient vegetation.

    • Result: Gullies expanded into the canyon.

  • Control Attempts for Erosion:

    • Efforts include: Planting vegetation and constructing erosion control structures, though erosion persists due to the fragile nature of the sediments.