❄️Earth Surface Processes Unit 4 – Mass Wasting and Slope Stability

Key Concepts and Definitions

• Mass wasting encompasses the downslope movement of rock, soil, and debris under the influence of gravity
• Slope stability refers to the resistance of an inclined surface to failure by sliding or collapsing
• Shear stress is the force per unit area acting parallel to the slope that drives mass movement
• Shear strength represents the resistance of the slope material to shear stress and failure
• Factor of safety (FOS) is the ratio of shear strength to shear stress, indicating the stability of a slope
  • FOS > 1 suggests a stable slope
  • FOS < 1 indicates an unstable slope prone to failure
• Angle of repose is the maximum angle at which a pile of unconsolidated material remains stable without sliding
• Cohesion describes the attractive forces between particles that hold them together, contributing to slope stability

Types of Mass Wasting

• Falls involve the rapid, free-fall movement of detached rock or soil from steep slopes or cliffs (rockfall)
• Topples occur when tall, narrow blocks of rock rotate forward and fall from a steep slope
• Slides are the downslope movement of a coherent mass along a distinct surface of rupture
  • Translational slides move along a planar surface parallel to the slope
  • Rotational slides involve a curved surface of rupture, causing the mass to rotate backward
• Flows are the continuous, fluid-like movement of unconsolidated material in which the individual particles travel separately (debris flow, mudflow)
• Creep is the slow, gradual downslope movement of soil or rock particles, often imperceptible
• Lateral spreads are the lateral extension of cohesive material due to liquefaction of underlying layers during earthquakes
• Complex mass movements combine two or more types of mass wasting processes (slump-earthflow)

Factors Influencing Slope Stability

• Slope angle and steepness directly affect the shear stress acting on the slope, with steeper slopes being less stable
• Geology and rock type determine the strength and cohesion of the slope material
  • Weak, fractured, or weathered rocks are more susceptible to failure
• Soil properties, such as composition, grain size, and moisture content, influence the shear strength and stability
• Vegetation provides root cohesion, intercepting rainfall and reducing erosion, thus enhancing slope stability
  • Deforestation or removal of vegetation can destabilize slopes
• Hydrological conditions, including groundwater levels and pore water pressure, affect the effective stress and stability
  • High groundwater levels or water saturation can reduce shear strength and trigger failures
• Seismic activity and earthquakes can induce strong ground motions, leading to slope instability and triggering mass movements
• Human activities, such as excavation, loading, or changes in land use, can alter the stress conditions and destabilize slopes

Mechanics of Mass Movement

• Mass movements are driven by the balance between driving forces (shear stress) and resisting forces (shear strength)
• Shear stress ($\tau$) is the force acting parallel to the slope, given by $\tau = \rho g h \sin \theta$
  • $\rho$ is the density of the material
  • $g$ is the acceleration due to gravity
  • $h$ is the thickness of the material
  • $\theta$ is the slope angle
• Shear strength ($s$) is the resistance of the material to shear stress, expressed as $s = c + (\sigma - u) \tan \phi$
  • $c$ is the cohesion
  • $\sigma$ is the normal stress
  • $u$ is the pore water pressure
  • $\phi$ is the angle of internal friction
• The factor of safety (FOS) is the ratio of shear strength to shear stress, given by $FOS = s / \tau$
  • FOS > 1 indicates a stable slope
  • FOS = 1 represents a critically stable slope
  • FOS < 1 suggests an unstable slope prone to failure
• Progressive failure occurs when the shear stress exceeds the shear strength in a localized area, leading to the propagation of failure along the slope

Triggers and Causes

• Rainfall and intense precipitation events can saturate the slope material, increase pore water pressure, and reduce shear strength
  • Antecedent moisture conditions also play a role in slope stability
• Rapid snowmelt can lead to increased groundwater levels and destabilize slopes
• Earthquakes generate strong ground motions that can exceed the shear strength of the slope material
  • Seismic shaking can also cause liquefaction of susceptible soils
• Volcanic activity, such as lava flows, pyroclastic flows, or lahars, can overload slopes and trigger mass movements
• Freeze-thaw cycles can cause mechanical weathering and weaken slope material over time
• Human activities, including deforestation, excavation, and construction, can alter the stress conditions and trigger mass movements
• Undercutting of slopes by rivers, waves, or human activities can remove support and destabilize the slope
• Loading of slopes by buildings, fill material, or waste dumps can increase the shear stress and trigger failures

Assessment and Prediction Methods

• Field observations and geomorphological mapping help identify past mass movements and assess slope conditions
• Geotechnical investigations, including soil sampling and laboratory tests, provide data on soil properties and shear strength
• Slope stability analysis involves calculating the factor of safety using limit equilibrium methods
  • Methods include the ordinary method of slices, Bishop's simplified method, and Janbu's method
• Numerical modeling techniques, such as finite element analysis, can simulate slope behavior under various conditions
• Remote sensing techniques, like LiDAR and InSAR, enable the detection and monitoring of slope deformation
• Rainfall thresholds and intensity-duration relationships can be used to predict rainfall-induced landslides
• Hazard mapping and zonation categorize areas based on their susceptibility to mass movements
• Early warning systems, based on monitoring of precipitation, slope deformation, or other precursors, can help mitigate risks

Mitigation and Prevention Strategies

• Slope stabilization measures, such as retaining walls, anchors, and soil nails, can increase the resisting forces and improve stability
• Drainage control, including surface and subsurface drainage, helps reduce pore water pressure and increase shear strength
• Vegetation management, such as planting deep-rooted species, can enhance slope stability through root reinforcement
• Grading and slope modification, like reducing the slope angle or creating benches, can reduce the driving forces
• Erosion control measures, such as geotextiles, mulching, and check dams, minimize surface erosion and prevent the initiation of mass movements
• Land use planning and zoning regulations can restrict development in high-risk areas and reduce exposure to hazards
• Public awareness and education programs can help communities understand the risks and adopt appropriate preparedness measures
• Monitoring and early warning systems can provide timely information for evacuation and emergency response

Real-World Case Studies

• The 2014 Oso landslide in Washington, USA, was a catastrophic event that claimed 43 lives and destroyed numerous homes
  • The failure occurred in glacial sediments and was triggered by heavy rainfall and groundwater saturation
• The 2017 Mocoa landslide in Colombia resulted from intense rainfall and caused the loss of over 300 lives
  • The event highlighted the vulnerability of communities living in steep, deforested areas
• The 1963 Vajont Dam disaster in Italy was caused by a massive landslide that displaced water from the reservoir, generating a wave that overtopped the dam
  • The tragedy emphasized the importance of thorough geological investigations and monitoring in dam projects
• The 2011 Christchurch earthquake in New Zealand triggered numerous rockfalls and landslides in the Port Hills area
  • The event demonstrated the susceptibility of steep, jointed volcanic rocks to seismic-induced mass movements
• The recurring landslides in the Darjeeling Himalayas, India, are influenced by a combination of factors, including heavy monsoon rainfall, deforestation, and infrastructure development
  • The region requires comprehensive landslide hazard assessment and sustainable land management practices