Environmental Geology: Study Notes on Mass Movements

SCI250: Environmental Geology - Mass Movements

What are Mass Movements?

  • Mass movements (or mass wasting) refer to the downhill movement of masses of bedrock, sediments, or soil (i.e., geologic material) under the pull of gravity.
  • Mass movements can occur at varying speeds:
    • Slow movements can take place over years, while rapid movements can occur in seconds.
  • The term "landslides" serves as a general term for rapid mass movements of earth materials.
  • Mass movements cause significant socioeconomic impacts, resulting in billions of dollars of damage and numerous fatalities each year:
    • In Canada alone, they result in estimated damages of $200-$400 million annually.
  • With proper planning, mass wasting is considered one of the most easily preventable of major geological hazards.
  • A notable example of mass movement is the largest recorded landslide in California in May 2017, which isolated Big Sur for five months due to cut access routes.

Factors Influencing or Triggering Mass Movements

Static Conditions (Influences)
  • Slope Angle: Steeper slopes can increase the risk of failure.
  • Water: Both the amount and saturation level can alter slope stability.
  • Slope Materials and Vegetation: The type of materials and vegetation on a slope affects its integrity and strength.
  • Geological Structures: Faults, joints, and bedding planes influence how materials move.
Dynamic Processes (Triggers)
  • Earthquakes: Seismic activity can trigger landslides.
  • Heavy Rains: Intense rainfall can increase soil saturation and instability.
  • Rapid Snow Melt: Sudden melting can increase water content behind slopes.
  • Human Activity: Actions such as construction and logging contribute to instability.

Effects of Slope and Materials

  • Gravity is crucial in shaping landscape features by tearing mountains down.
  • Mass movement occurs when the downward pull of gravity overcomes frictional forces.
  • Key concepts related to slope movement:
    • Shearing Stress: This is the stress in which parts of an object slide past each other across a plane.
    • Shear Strength: The ability of an object to resist shearing stress, which influences slope stability.
  • Factors contributing to slope failure include:
    • Increase in shearing stress.
    • Decrease in shear strength.
    • Increase in weight or mass on the slope.
    • Decrease in friction:
    • Erosion, fracturing, or weathering of material can reduce friction, leading to failure.
    • Weakness at bedding planes and slow tectonic deformation can also increase risk.
  • The Angle of Repose is defined as the maximum slope angle at which dry, unconsolidated material can remain stable without movement.

Effects of Fluids

  • The role of fluids varies with different materials:
    • In dry soils, adhesion is increased, contributing to stability.
    • In saturated soils, excess water reduces friction.
    • Certain soils can absorb water easily, which can negatively affect their strength.
    • Others may expand or contract with moisture changes, destabilizing slopes.
  • Water also adds weight, and can increase slide potential between bedding planes.
  • A triggering event such as rainfall can lead to weight increase, friction decrease, and higher pore pressure:
    • Example: The rain-induced landslide in Washington, USA, that resulted in 45 houses being buried and 43 fatalities.
  • Frost Wedging: Occurs due to the freeze-thaw cycle, causing cracks in rocks or driving chunks of material apart in soils. Ice volume increases approximately 10% compared to liquid water volume, which can lead to destabilization.
  • Frost Heaving: The process whereby wet soil loosens during freeze-thaw cycles.

Effects of Vegetation

  • Vegetation plays a dual role in slope stability:
    • Positive Impact: It helps hold unconsolidated sediment together and absorbs moisture, increasing slope stability.
    • Negative Impact: Adds weight and can lead to soil desiccation, which may decrease slope stability.
  • Evidence suggesting vegetation's protective role includes increased mass movements in burn areas. For instance, during the summer of 2021, wildfires in British Columbia resulted in areas experiencing landslides during subsequent rainfall events.

Earthquakes

  • Earthquakes can serve as triggers for landslides by:
    • Generating seismic waves that increase shear stress on slopes.
    • Causing shaking that diminishes friction among sediments.
  • Regions situated near subduction zones are particularly vulnerable:
    • Example: The 1970 earthquake in the Peruvian Andes (7.7 magnitude) caused massive landslides, leading to 18,000 deaths.

Quick Clays

  • Quick clays are primarily found in northern polar regions, originating from rock flour (very fine clay) produced by glaciers.
    • Following uplift from marine environments, sodium chloride (NaCl) in pore water connects particles, but rainfall can leach this out.
    • Seismic waves (even from distant activity) can cause quick clays to behave like quicksand when the glue between particles is dissolved.
  • Sensitive Clays: Similar to quick clays but consist of different materials like volcanic ash, can exhibit similar destabilizing effects. Example: The liquefaction event of Leda Clays in Quebec, May 2010.

Leda Clay

  • The Ottawa Valley's landscape was once covered by the Champlain Sea approximately 15,000 years ago during the last glacial maximum, leading to the formation of stable clays.
  • Upon wetting, the salt that binds the clay evaporates, leading to instability in the clays, with more than 250 recorded landslides in the region.

Types of Mass Movements

  • Creep: A slow downslope movement of soil or rock, causing damage to structures.
  • Landslide: A general term for rapid mass movement.
Specific Types of Mass Movements
  1. Falls: Fast, free-fall motion of materials (e.g., rockfalls) with talus accumulating at the bottom.
  2. Slides: Cohesive movement of materials along a defined surface.
  3. Slumps: Movement incorporating rotation, often resulting in a scarp at the top and a flow at the base.
  4. Flows: Chaotic movement of materials, including:
    • Earthflows (dry), mudflows (wet), snow avalanches, debris flows (mix of various materials).

Typical Rates of Movement and Characteristics

  • Falls: Rapid (rockfall)
  • Slides: Highly variable (rockslide/slump)
  • Flows: Rapid (ranging from meters/day to meters/second)

Flooding as a Consequence of Mass Movements

  • Landslides can obstruct rivers, leading to the formation of natural reservoirs.
    • Example: Attabad Lake in Pakistan formed by a landslide on the Hunza River in 2010, displacing 6,000 people.

Human Influence on Mass Movements

  • Clearing vegetation (e.g., logging): Reduces slope stability, altering landscape resilience to extreme weather.
  • Construction activities: Create steep slopes or add instability through added weight and water.
  • Removal of material: This can undermine slopes, increasing vulnerability to slides.
  • Artificial Reservoirs: The construction and filling of reservoirs can induce failures due to reduced shear strength in surrounding rock.
  • Vajont Dam Incident:
    • Located in northeastern Italy; at 262m high, it was one of the tallest dams at the time.
    • On October 9, 1963, a massive landslide sent water over the dam, causing devastation and resulting in approximately 2,500 deaths.

Preventive Measures Against Mass Movements

  1. Structural Measures: Building features that redirect avalanches and debris flows on highways and railways.
  2. Slope Stabilization: Includes:
    • Reducing slope angles.
    • Adding material for support at the base.
    • Removing weight from slopes.
  3. Soil Stabilization: Plant vegetation with strong roots to secure topsoil and improve drainage.
  4. Water Drainage Solutions:
    • Implementing impermeable surfaces to divert runoff.
    • Using boreholes and pipelines for subsurface drainage.
  5. Reinforcement:
    • Driving piles into slopes (less effective).
    • Utilizing rock bolts as a more effective solution.

Recognizing Hazards and Warnings

  • Mass movements are natural events that frequently recur in certain regions; evidence such as rockslides and scars can indicate past occurrences.
  • Signs of Potential Slope Failure: When assessing property:
    • Cracking/titling of house elements.
    • Visible cracks in slopes.
    • Buldges or cracks in retaining walls.
      - Water pooling may indicate drainage issues.
  • The importance of monitoring slide-prone areas and implementing warning systems.
    • Example: California and Washington states have systems to warn of landslide risks.

Case Study: Bingham Canyon Mine

  • The Bingham Canyon Mine in Utah, known as the largest man-made excavation, witnessed the biggest nonvolcanic landslide in North American history in 2013, with about 70 million cubic meters of debris moving at high speed.
  • The application of multiple protective measures, including regular inspections, geotechnical analysis, and monitoring systems, imposed effective risk management and led to no loss of life during the incident due to timely evacuation.

Additional Information

  • Another significant landslide event occurred on May 31, 2021, at the Bingham Canyon site, drawing attention to ongoing geotechnical challenges in mining operations.