Unit 3 topic 3

• Mass Movement


  The downward, large-scale movement of material under the influence of gravity. It is caused when the stress exerted exceeds the internal strength of the slope, resulting in instability.
• Factors that Impact Slope Control


  • Climate
  • Geology
  • Regolith and soil
  • Aspect
  • Vegetation
• Mass Movements in Relation to the Speed of Movement and Water Involvement


  1. Soil creep

  2. Solifluction

  3. Mudflow/Earthflow

  4. Rockfall

  5. Rockslide

  6. Slump
• Soil Creep


  • The slowest form of mass movement, but it's almost continuous. Individual particles of soil move downhill. It occurs mainly in the winter. Individual soil particles are pushed to the surface by wetting, heating or freezing water. Rates of 1-3 mm per year in the UK, up to 10 mm in tropical rainforest.
• Solifluction


  • Occurs in areas of permafrost (tundra area). The top layer of soil thaws in the warmer summer but the layer below remains frozen (permafrost). The surface layer becomes saturated as the frost melts and flows over the subsoil and rock below. Movement is 5 cm to 1 m per year.
• Mudflow/Earthflow


  • Heavy rain causes a reduction in friction, and thus earth turns into mud and flows slowly over the bedrock. The material will become disorderly and flow down the cliff. Slumps occur on weaker rocks, especially clay. This is often rotational along a curved slip plane. Clay absorbs water and becomes saturated, and exceeds its liquid limit. It then flows along a slip plane.
• Rock Falls


  • Occurs when mechanical weathering like freeze-thaw breaks large chunks of the cliffs away. The cliff has to be at an angle of 40 degrees or more. Material that breaks off is called scree and bounces down the cliff to the bottom. On bare rock faces where joints are exposed, they are more common.
• Slides


  • Rocks that are jointed or have bedding planes parallel to the slope are susceptible to landslides. Again, an increase in water can reduce friction and make this easier. Slabs of rock will slide over the underlying rock, called a slip or plane. As the slide moves along the slip plane, it tends to retain its shape and structure until it hits the bottom of the slope. Slip planes occur at fault lines, along a bedding plane and at the junction of two layers. Rock slides are where a huge volume of rock moves together. Landslides include rocks, stones and soil.
• Slumps


  • These occur in saturated conditions. The difference between sliding and slumping is that there is a rotational movement (Walton-on-the-Naze is an example). These occur on moderate to steep slopes. Clays or sand overlying more resistant rock or impermeable rock like limestone or granite. Slumping causes rotational scars. It can repeat, forming a terraced cliff.
• How Does Water and Sediment Move on a Slope?


  1. Surface Wash

  2. Rills

  3. Sheet Wash

  4. Rain Splash
• Surface Wash


  This is sheet-like flow of water over the surface of the soil. There may be areas of high or low velocity. Sheet wash transports material that is dislodged by rain splash and erodes the top layer of the soil. It produces rills.
• Rills


  These are shallow channels that carry water and sediment for a short period of time. Rills are common in agriculture after harvesting, and bare land is left. It occurs after the compaction of the soil by heavy machinery.
• Sheet Wash


  This occurs when the soil is saturated, and water can no longer infiltrate the ground due to heavy rainfall.
• Rain Splash


  This is when raindrops cause erosion. It occurs mostly on slopes with an incline between 33 and 45 degrees. On a 5-degree slope, 60% of movement is downwards. On a 25-degree slope, 95% of movement is downwards.
• Slope failure occurs when there is a reduction in shear strength/resistance or an increase in shear stress.
• Factors that increase shear stress


  • Removal of lateral support through undercutting or steepening a slope
  • The extra weight of water, vegetation or accumulation of debris on a slope
  • Undercutting of a slope and removing the base of support
  • Lateral pressures such as water freezing in cracks
  • Movements caused by earthquakes or the movement of trees in the wind
• Factors that reduce the shear strength of a slope


  • Weathering
  • Changes in the water levels can cause softening of material pressure
  • Changes in the rock structure, such as cracks being created in clay
  • Removal of vegetation which helps binds the soil together
  • Burrowing of animals (biological weathering) and the decay of plant roots which bind the soil together
• What Causes Slope Failure?
• Human Factors that Impact Slope Shape and Stability


  • Building of roads
  • Farming practices such as creating terraces
  • Deforestation
  • Flood control methods such as building a dam
• Building of roads results in undercutting artificial valleys with steeper slopes on either side. These steeper slopes can be unstable. Traffic vibrations may also trigger mass movements.
• Farming practices such as creating terraces meant that steps are cut into the profile of a slope. This helps stabilise hillsides. However, the steep slopes between the steps can become less stable. Ploughing up and down slopes creates rills and channels of water. This destroys the soil structure. Overgrazing and over-cultivation also increase soil erosion.
• Deforestation causes soil erosions and means that the soil is less bonded by the roots of vegetation. This can lead to slides or flows. It also increases carbon dioxide levels in the atmosphere and can increase carbonation rates.
• Flood control methods such as building a dam can destabilise slopes surrounding the reservoir because they are cut into to create the reservoir. This changes the shape of the slope and loads the valley with water which can destabilise slopes and increase the shear stress.
• Urbanisation such as building houses means that material is removed to make a flat area for foundations for buildings. This changes the form of the slope.
• Mining and quarrying mean that large areas are excavated. Excavation at the top of the slope removes the support of the slope. This creates dips in the landscape, and vegetation is removed. This can cause landslides.
• Global warming increases the amount of carbon dioxide in the air, increasing chemical weathering rates. It can also result in acid rain, increasing the corrosion of buildings and structures.
• How Can The Stability Of Slopes Be Increased?

  1. Afforestation

  2. Pinning

  3. Soil Nailing

  4. Rock Bolting

  5. Netting

  6. Grading
• Afforestation


  • Planting of new forests in the upper parts of a catchment to increase interception and reduce overland flow. Takes many years to be effective as young, immature trees intercept relatively small amounts of water. More trees reduce soil saturation and therefore reduce the risk of mass movement triggered by saturated soil such as mudslides. The roots of trees bind the soil together, which means the slope can handle more stress before it fails.
• Pinning


  • Wire nets (or sometimes concrete blocks) are attached to a rock face or slope so that the risk of rock falls or erosion is reduced.

  
• Soil Nailing


  • Steel rods are drilled into the soil. These hold weight and increase the stability of the slope and reduce the risk of mass movement.
• Rock Bolting


  • Bolts are drilled into rock slopes in a particular pattern. This transfers the load from the weaker exterior to the stronger interior of the slope. It reduces the risk of rock falls and rock slides as it knits the rock together to strengthen the slope.

  
• Netting


  • Metal mesh nets are attached to the surface of a slope which helps collect fragments of scree, which can be safely removed at a later date. This is often used in areas where tourism is important, and where the risk of rock fall is high.

  
• Grading


  • The re-profiling of slopes so that they become more stable. This reduces the slope angle through excavation and reduces the risk of movement. Material has to be transported away, so this process is quite costly. However, it makes a more uniform topography.
• Hong Kong suffers from many landslides. These occur due to high rainfall, steep slopes and a high population density.
• The Hong Kong government has a responsibility to manage landslides. One of Hong Kong's government interventions is to ensure that private owners of slopes take responsibility for slope safety. The risk from landslides has been reduced by 50% since 1977. However, due to an increase in population growth, developers increasingly build further up the slopes. This has resulted in increased risk and damage from landslides.
• How does Hong Kong manage mass movements of slopes?


  Removing excess water from slopes is vital. Surface drains are very vulnerable to blockage. Without proper drain maintenance, landslides are more common than on slopes without drains. Man-made slopes are one of the main methods of slope stabilisation used in Hong Kong. These contain drains to intercept and direct water away from the slopes. The slope is usually protected from infiltration and the erosive effects of water by impermeable hard covers. A mulching system provides a protective cover that allows natural vegetation to grow on the slope. Long, rooting grass is a fast and cost-effective system to cover man-made slopes. A fibre-reinforced soil system is constructed by mixing polyester fibre into sandy soils.