9.2 Hazardous Environments resulting from mass movements

• Rockfall:

  A rapid mass movement that is associated with dry weather conditions.

  
  

  -Occurs on steep slopes with a slope angle of more than 40 Degrees, where detached regolith from the free face falls under the influence of gravity usually downwards.
• Mudflow (Mudslide)


  A type of fast movement on slopes that are associated with saturation:

  
  

  -Occurs at slopes with angles of 5-15 Degrees.

  
  

  -Occurs within slopes with internal deformation.

  
  

  -Water absorbs into weaker rocks, reducing the internal cohesion --) Mudflow
• Landslide


  A mass movement that is triggered by earthquakes (under-cutting accelerates the process)

  
  

  -Occur on steep slopes with angles of 15-40 Degrees

  
  

  -Occurs when the bedding plane is parallel to the surface

  
  

  -Has a uniform movement so causes no internal disruption of the slope.
• Slump (Rotational Landslide)


  A mass movement on slopes that are associated with saturation.

  
  

  Occurs on steep slopes with angles of 15-40 Degrees

  
  

  Occurs when clay absorbs water, which changes the internal cohesion and the stability of clay decreases- slumping
• Mass Movement


  The downslope movement of large amounts of material under the force of gravity of a slope.
• Landslide


  The sudden and unexpected downhill movement of part of a hillside.
• Slope System:

  →Slope is an open system influenced by external and internal factors.

  
  

  →They are influenced by climate, geology, biotics and human activity.

  
  

  →Driving forces promote downslope movement of internal , whereas resisting forces deter movement. So when driving forces over come resisting forces, the slope is unstable and results in mass movements.
• Angle of Repose:


  →The Angle of Repose is the maximum angle at which loose material or granular substance can remain stable on a slope without sliding and collapsing.

  
  

  →When the Angle of Slope exceeds the Angle of Repose, it can lead to slope failure.

  
  

  There are two primary mechanisms through which this can occur:

  
  

  →Sliding Failure: When the angle of the slope surpasses the angle of the repose, the shear stress acting on the material exceeds its shear strength.

  
  

  →This causes the material to start sliding downhill, the shear stress is influenced by factors such as the weight of the material, the slope angle, and the cohesion and internal friction of the material.

  
  

  →Flow Failure: In some cases, rather than sliding, the material may start to flow downhill when the slope angle surpasses the angle of repose.
• Factors that affect the Angle of Repose:

  Particle Size: The size of particles in a material plays a significant role in determining the angle of repose. Generally, smaller particles tend to have a lower angle of repose compared to larger particles. This is because smaller particles have a higher surface area relative to their volume, leading to increased inter-particle friction and cohesion, which helps maintain stability at steeper angles. (More Energy required to erode)

  
  

  Particle Shape: The shape of particles also affects the angle of repose. Rough, irregularly shaped particles tend to interlock with each other more effectively, resulting in a higher angle of repose. In contrast, smooth, spherical particles have a lower angle of repose since they tend to roll and slide more easily.

  
  

  Surface Roughness: The roughness of particle surfaces can influence the angle of repose. Rougher surfaces increase interlocking between particles, enhancing stability and leading to a higher angle of repose. On the other hand, smoother surfaces reduce interlocking, resulting in a lower angle of repose.

  
  

  Moisture Content: The moisture content of a material significantly affects its angle of repose. For dry materials, the particles are typically more frictional, resulting in a higher angle of repose.

  
  

  As the moisture content increases, the particles can become lubricated, reducing inter-particle friction and cohesion. This leads to a decrease in the angle of repose, making the material more prone to sliding or flowing.

  
  

  
  

  Compaction: The degree of compaction or density of the material can impact the angle of repose. Well-compacted materials tend to have a higher angle of repose due to increased inter-particle friction and stability. Loose or poorly compacted materials have a lower angle of repose as there is less interlocking between particles.

  
  

  Temperature:

  
  

  More Vegetation- Slope stability increases
• Van't Hoff's Law


  The law that states that a 10 degree increase in temperature leads to a 2.5x increase in the rate of chemical weathering.
• Human Factors + Physical Factors:
• Shear Strength


  Forces resisting movement downslope. It includes frictional resistance and the cohesion of the particles that make up the object.

  
  

  Both Human and Physical Factors can influence shear stress and shear strength.

  
  

  Mass Movements Occur when Shear Stress exceeds Shear Strength= Slope Failure
• Factors affecting the movement of materials on the slope:

  Gravity - gravity moves the material downslope (rockfall and landslide)

  - gravity sticks the particle to the slope (mudflow and landslide)

  
  

  Slope angle - the downslope movement on slopes is proportional to the weight of the particle and the slope angle

  
  

  - the steeper the slope, the faster the movement

  
  

  Pore pressure - water lubricates particles and fills the spaces between the particles, which

  forces them to break apart under pressure (pore pressure)

  - this pore pressure increases the ability of the material to move

  - important in influencing the movements of wet material on low-angle slopes, where gravity and slope angle doesn't play much of a role
• Leading Reasons of Landslides:

  It is expected to increase due to:

  
  

  →Urbanisation and increasing development in land-slide prone areas.

  
  

  →Deforestation and Climate Change leading to increased precipitation in places.
• Ways in which Shear Stress can be exceeded:

  ) Removal of lateral support through undercutting or slope steepening

  Undercutting due to erosion by rivers and glaciers or wave action would cause erosion of material at the cliff's base. This means that the lateral support (surrounding soil or rock) will weaken -> shear stress

  Slope steepening could be caused by previous rockfalls or slides. Steeper slopes will increase the weight of the material -> shear stress

  
  

  2) Removal of underlying support

  Undercutting by rivers and waves would cause erosion of material at the cliff's base. This means that the remaining material above the eroded material has to carry the weight -> shear stress

  
  

  3) Loading of slope

  The weight of water, vegetation and the accumulation of debris -> shear stress

  
  

  4) Lateral pressure

  Freeze-thaw weathering occurs when water enters through the cracks and expands by 9% as it turns into ice, exerting pressure on the rock -> shear stress

  Pressure release occurs when over-lying rocks are removed (by erosion), causing a great release of pressure, which is previously caused by the weight of the rocks removed -> shear stress

  
  

  5) Transient stresses

  Earthquakes or movements of trees in wind could create sudden, short-lived stress on the slope
• Decrease Shear Strength:

  Weathering effects:

  
  

  Weathering such as the disintegration of granular rocks and hydration of clay material is influenced by climate. This decomposes the rocks and so weakens the bonds between the rocks -> decrease shear strength

  
  

  2) Changes in pore-water

  The saturation of rocks and the softening of material pressure occurs when the water enters the clay, reducing the cohesion -> decrease in shear strength

  
  

  3) Changes in the structure

  The creation of fissures in clays and the remoulding of sands and clays change the structure of rocks within the slope -> decrease shear strength

  
  

  4) Organic effects

  Burrowing of animals and the decay of tree roots can cause the removal of materials within the slope -> decrease shear strength
• Why is mass movement a secondary hazard of: Earthquakes


  Repeated earthquakes can cause deformation of the materials comprising a slope. This deformation leads to a decrease in the intrinsic characteristics of the materials that resist movement, making the slope more prone to failure.

  
  

  Additionally, earthquakes can increase pore pressures within the slope. Compression waves from earthquakes compress loosely compacted soils, and if these soils are saturated with water, the water pressure within the soil pores rises. Under normal circumstances, the water would drain from the soil, but if the earthquakes occur rapidly or frequently, the water cannot escape in time. As a result, the increased water pressure weakens the soil mechanically, further increasing the likelihood of slope failure.
• Why are people against Hazard Mapping?

  Hazard Mapping, is the consideration of areas that are vulnerable and in risk.

  
  

  All hazard maps are not unconditionally acceptable to residents. Landowners and land developers may fear about the fall of land prices and oppose to the public release.
• Rotational slides are caused by heavy rainfall or snowmelt that saturates soil and weakens it, making it more susceptible to sliding.
• Italian Mudslides (1998) - Mudslides swept through towns and villages in Campania killing 300 people.

  • 2 years of rainfall fell in two weeks.
  • 1170 serious landslides since 1892.
• Causes of the Italian Mudslides:

  • Bed of River Sarno had been cemented over.
  • Deforestation and forest fires loosed clay soils to dangerous levels.
  • 20% of houses in Sarno had been built without permission.
  • Many houses were built on Lava layers from Vesuvius, which turns liquid and gets washed away every year.
• Avalanches:

  • Mass Movements of snow and ice.
  • Avg. speeds of 40-60km/h but recorded as high as 200km/h.