8 mass movement

Environmental & Geologic Hazards: Landslides and Mass Wasting

Introduction

  • Topic 8: Chapter 7

  • Focus: Landslides and Mass Wasting

  • Instructor: Dr. Pat Harris

  • Date: 22 March 2014

Washington (Snohomish) Landslide

  • Date: 22 March 2014

  • Repeated emphasis on the Washington (Snohomish) Landslide incident across various slides.

Geography of the Event

  • Lidar Map: Lidar (Light Detection and Ranging) is a laser mapping technique that strips away vegetation to reveal underlying landforms.

  • Slide-prone Terrain: Many past landslides have been mapped along the Stillaguamish River surrounding the area of Saturday's slide event.

  • Visuals: Maps provide insight into the locations of previous landslides, including areas affected in 2006 and locations where slide materials eventually ended up.

Earth’s External Processes

Definition

  1. Weathering: The physical breakdown and chemical alteration of rock at the Earth's surface.

  2. Erosion: The physical removal of materials by natural agents, including water, wind, ice, or gravity.

Sequence of Processes

  • Order: Weathering must occur before erosion can take place.

Types of Weathering
Mechanical Weathering

  • Definition: Physical breakdown of rocks into smaller pieces without changing their chemical composition.

  • Processes:

    1. Abrasion: Rocks collide and grind down into smaller pieces.

    2. Frost Wedging: Water enters cracks in rocks, freezes, expands, and causes fragmentation.

    3. Exfoliation: Removal of overlying rocks leads to pressure release, resulting in cracks parallel to surfaces.

    4. Biological Activity: Plant roots or organism burrowing into rock contribute to rock breakdown.

Chemical Weathering

  • Definition: Breakdown of rock components and internal structures of minerals primarily through chemical reactions.

  • Key Processes:

    1. Hydrolysis/Dissolution: Involves a weak acid/base dissolving minerals in water. Example: CaCO3 + H2O
      ightarrow H2CO3 + Ca^{2+}

    2. Oxidation: Oxygen interacts with minerals under previously reducing conditions, such as the oxidation of pyrite, leading to iron oxide formation, which weakens structures.

Impacts of Weathering on Slope Stability

  • Weathering significantly affects the structural integrity of slopes contributing to mass wasting.

What is Mass Wasting?

  • Definition: The downward movement of soil, rock, sediment, and regolith under the influence of gravity, without the need for a transport medium like water or ice.

Causes of Mass Wasting

  • Occurs when driving forces exceed resisting forces:

    • Driving Forces: Gravity, weight of materials, vegetation, fill materials, buildings.

    • Resisting Forces: Material strength, cohesion, internal friction, and external supports.

Stability Dynamics

  • On stable slopes:

    • Driving and resisting forces are balanced.

  • On unstable slopes:

    • Driving forces overpower resisting forces.

Landslide Terminology

  • Landslides: Can occur in various forms in all 50 states, typically costing between $1-2 billion annually and causing around 25-50 fatalities each year.

  • Notable Case Study: La Conchita Landslide in California (January 10, 2005) destroyed 36 homes and resulted in 10 deaths due to combination of steep slopes, weak rocks, and intense rainfall.

Factors Affecting Slope Stability

  1. Type of Earth Material: Harder materials can fracture; weaker materials can slide.

  2. Slope Angle and Topography: Steeper slopes have increased driving forces. The maximum angle before failure is the angle of repose.

    • Example: Dry sand can sustain an angle of repose of about 30°.

  3. Vegetation: Provides structural support; roots increase cohesion and resistance to movement.

  4. Water: Affects both cohesion and adds weight, impacting slope stability.

  5. Climate and Time: Influence the amount and timing of rainfall, thereby impacting vegetation and erosion rates.

Slope Failure Mechanisms

  • Oversteepening: Increased slopes from flooding or construction can lead to failures.

  • Liquefaction: This can occur during heavy rainfall, particularly in saturated conditions.

Classification of Mass Movement

  • Types of Material: Bedrock, unconsolidated material, soil, regolith, sediment.

  • Rate of Movement: Classified from slow (e.g., creep) to rapid (e.g., rock fall, mudflow).

  • Types of Motion: Falls, slides, and flows characterize how the material moves.

Types of Mass Movement

  • Falls: Free fall of debris from steep slopes.

  • Slides: Cohesive movement along defined surfaces. Includes rock slides and debris slides.

  • Flows: Viscous movements often saturated with water, including mudflows and debris flows.

  • Creep: Very slow movement caused by a combination of moisture and gravity effects over time.

Special Types

  • Solifluction: Movement typical in polar regions due to melting permafrost.

  • Quick Clays: Clay that behaves like a liquid upon disturbance, particularly in saturated conditions.

Mass Movement Prevention Strategies

  • Engineering Solutions: Netting and ledges can help prevent rock falls. Creating artificial cohesion can help stabilize slides.

  • Drainage of Groundwater: Essential in preventing liquefaction and enhancing slope stability.

  • Forecasting and Monitoring: Identify past hazards, weak rocks, large rain events, and steep slopes to anticipate risks.