Landslides and Mass Wasting: Lecture 7

Mass Wasting

• Definition: Mass wasting, commonly referred to as landslides, involves the downslope movement of materials which may include:
    - Rocks
    - Sediment/soil (regolith)
    - Snow/ice

• Driving Force: Gravity is the primary force driving mass wasting movements.

Observation 1: El Salvador

Introduction to Mass Wasting

Formation of Relief

• Relief: The difference in elevation between two locations due to up and down movements on the land surface. Relief is influenced by:
    - Types of slopes (gentle, moderate, and steep).
    - The presence of resistant rock layers.
    - Variation in slope angle.

Slope Analysis

• Types of Slopes by Angle:   - Gentle slopes: angles <5°   - Moderate slopes: angles 5° – 35°   - Steep slopes: angles >35°
    - Vertical slopes: angle = 90°

• Susceptibility to Mass Wasting:
    - Gentle slopes are less susceptible to mass wasting compared to steeper slopes.

Critical Thinking Questions

1. How does relief form?

2. Which of these slopes is more susceptible to mass wasting?

Slope Shapes

Slope shapes are determined based on several factors:

• Material Strength: Strong materials can create steeper slopes.

• Climate Impacts: The effect of wind, water, ice, and vegetation on slope formation.

• Formation Process: Natural processes lead to variations in slope shapes.

Examples of Slope Shapes:

• U-Shaped Glacial Valley

• V-Shaped River Valley

Slope Stability

Engineering Concepts

• Stable Slope: A slope remains stable when the resistance force (σr) is greater than or equal to the downslope shear stress (σd).

• Unstable Slope: A slope is classified as unstable when σd > σr.

Stress in Mass Wasting

• Stress (σ): Represents the force exerted on a material. It can be summarized as follows:
    - Normal Stress (σn): Force perpendicular to a surface.
    - Downslope Shear Stress (σd): Force acting parallel to a surface.
    - Resistance Stress (σr): Represents friction coefficient and the resistance to motion.

• Safety Factor Calculation: The stability of a slope can be evaluated using the formula:
    $ext{Safety Factor} = rac{ ext{Resistance Stress (σr)}}{ ext{Downgrade Shear Stress (σd)}}$
    - If the safety factor is >1, the slope is stable; if <1, it is unstable.

The Weakness of Earth's Surface

• Rock Formation: Many rocks possess joints and fractures, which are areas where stress is distributed unevenly, making them weaker and prone to breaking.

• Joint Formation: In sedimentary rocks, joints often form perpendicular to bedding layers. Their spacing varies depending on bed thickness.

• Exfoliation Joints: Particularly common in granite surfaces due to their layered structure.

Understanding Joints

Weakness Example:

• A pre-cut paper illustrates how small cracks can lead to easier breakage, comparing it to a rock that has been naturally fractured or jointed.
  

Friction and Cohesion in Mass Wasting

Friction

• Small material protrusions serve as anchors preventing movement until sufficient stress breaks them.

Cohesion

• Represented by:
    - Friction properties that hold sediment grains together.
    - Surface tension effects of water can also affect grain movement, especially when saturated.

Angles of Repose

• Definition: The maximum angle of steepness at which loose material can sit without sliding. The angle of repose varies depending on material:
    - Rounded gravel/sand: 30° - 37°
    - Irregular rocks: up to 45°

Observation of Slope Angles

• Critical Thinking 3: How to determine if the material is rounded or angular based on its slope angle.

Types of Weakness in Rock

• Planes of Weakness:
    - Joints: Natural fractures in rocks.
    - Bedding Planes: Boundaries between different rock layers.
    - Foliation Planes: Found in metamorphic rocks where minerals align in planes due to pressure.

• Slope Stability: Slope stability is impacted significantly when planes of weakness are aligned parallel to slope orientation, making them unstable.

Types of Mass Wasting

• Classifications based on:
    - Material Type: Such as rock, mud, snow, etc.
    - Movement Characteristics: Fast or slow, coherent or non-coherent movement.
    - Environmental Context: The climate of the area such as tropical or polar territories.

Common Types of Mass Wasting:

1. Creep: Slow movement of regolith, often caused by repeated expansion/contraction cycles (freeze/thaw, wet/dry).
      - Identification: Observe for tilted objects and land deformations such as sagging foundations or leaning gravestones.

2. Solifluction: A type of creep occurring in areas with permafrost.

3. Slump: Movement characterized by a coherent mass of rock or regolith, typically sliding down a slope.
      - Example: Holbeck Hall's destruction in 1993 due to slump.

4. Mudflows and Debris Flows: Non-coherent flows of water-saturated materials creating a viscous slurry.

Mudflow and Debris Flow Analysis

• Notable case studies include mudflows in La Conchita, California occurring in 1995 and 2005, highlighting the recurrence of mass wasting events in the same region.

Causes of Landslides

• Various factors can trigger landslides, including:
    - Shocks and Vibrations: Caused by earthquakes, wind, and anthropogenic activities.
    - Liquefaction: Occurs during earthquakes leading to the movement of sediment.
    - Undercutting: Results from natural and artificial erosion processes.
    - Changing Slope Angle/Strength: Various external forces can exacerbate slope instability.

Detailed Case Studies

• Peru (1970 Earthquake): A magnitude 7.9 earthquake triggered significant ground movements consequence.

• Palu, Indonesia (2018 Earthquake): An example of liquefaction leading to devastating outcomes.

Mitigation Strategies

Identify Historical Landslide Areas:

• Use aerial imagery, surveys, and mapping to identify where past slides have occurred.

• Implement hazard maps highlighting varying risk levels.

Design Management Models:

• NASA's landslide potential model uses real-time rainfall data for risk assessments.

Landscape Indicators:

• Look for clues in the terrain such as cracked roads or buildings to assess slope integrity.

Prevention Methods:

1. Revegetation: Plant roots stabilize potential failure surfaces.

2. Water Management: Lowering water levels in reservoirs and redirecting water courses to reduce pressure on slopes.

3. Engineering Solutions: Create benches, drainage systems, retaining walls, and other structures to manage and divert movement effectively.

Engineering Solutions: Detailed Understanding

• Benches: Cut steps into slopes to create stability.

• Drainage Systems: Install perforated pipes to drain water away from slopes.

• Retaining Walls: Use walls to hold debris and prevent spills into essential areas.

Conclusion

Understanding the complexities of mass wasting is essential for environmental management and engineering practices to reduce risks and protect structures from landslides.