Physical Causes Lesson 2


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Physical Causes Lesson 2

  1. 1. Physical causes of slope failure and mass Need Bishop books movement Waugh page 50 A3 paper
  2. 2. Recap on types of movement
  3. 3. Slope development Use Waugh page 50 • Produce on A3 an illustrated mind map to summarise the factors which influence slope development. • Include relevant details and refer to case studies where appropriate.
  4. 4. Forces acting on material on a slope (Bishop pg 124) Decrease shear strength Increasing Shear strength • Increase water content of slope • An increase in slope angle, especially at materials. This reduces internal the base of the slope due to river cohesion and friction. High pore water undercutting, building a road pressure can be enough to lift sediment • An additional weight on the slope due to and start movement increased water content – building on a • Stress relaxation. As overlying material slope, increased material from mass is removed by mass movements the movement higher up the slope material below has less weight on it. This • Shocks and vibrations from an release of pressure opens up cracks earthquake or heavy machinery which increases porosity and allows water to enter. • Weathering breaks down rock minerals into clay minerals which expand when water is present. • Burrowing animals or development of soil pipes by throughflow will weaken the slope materials. • Removal of vegetation – wildfires, overgrazing, building and deforestation surface materials are looser with the loss
  5. 5. Your turn … Use Bishop pg 124 Fig 7.1 to draw up a matrix Classifying landslides by causes: Decrease in Increase in shear strength shear stress Physical processes Natural processes
  6. 6. How does building increase the risk of slope failure?
  7. 7. Waugh page 56 task 3 • Make a copy of the graph • Label each of the following types of slope movement to identify their speed of flow: – Earth / mud flow – Solifluction – Rockfall – Slide – Soil creep For any two of the flow movements explain how the process occurs and describe the landform shape which results. (10 marks)
  8. 8. Earth / mud flow Solifluction Rockfall Slide Soil creep
  9. 9. Case Studies • Earthquakes, avalanches and rockfalls Peru 1970 • Volcanic eruptions – mudflows (Colombia 1985) • Heavy rainfall – Nicaragua 1998, Italy 1998 • Waugh pg 52 & 53 • Bishop pg 128
  10. 10. Case study • Deadly landslide hits Indonesia 12.01.07 • pacific/6254469.stm • Indonesia landslides bury dozens 26.12.07 • pacific/7160138.stm • Indonesian landslide toll mounts 27.12.07 • pacific/7161480.stm • Two dead in Indonesia landslide 6.2.08 •
  11. 11. Consequences and Responses of mass movement. Responses can be made at a collective and individual level. • Responses depends on how people perceive the hazard which depends upon factors such as: – Past experience – Values, – Personality – expectations
  12. 12. Response is influenced by: • perception • economic ability to take courses of action • technological ability to carry out courses of action
  13. 13. Classification about ways in which hazards can be perceived… • Fatalism – hazards are natural events which are part of living. Losses are accepted as inevitable and people remain in the area • Adaptation – people see that they can prepare for and therefore survive by prediction, prevention, protection depending on the type of hazard and the economic and technological circumstance of the area. • Fear – people feel so vulnerable to the event that they no longer want to remain living in the area.
  14. 14. Management of hazards • Prediction – it is possible to give warnings of some hazards. Action can then be taken to reduce their impact. Improved monitoring, information and communications technology have improved prediction and warnings in recent years. • Prevention – this cannot be achieved for most hazards, at best is some form of control through modification of the environment • Protection – aim is to protect people and property from the impact of the hazard, insuring losses, supply of aid
  15. 15. Approaches to management of landslides…. Modify the event: 1. Engineering techniques can be used to make a slope more stable: • Improving groundwater drainage and diverting surface water away from gulley areas • Terracing – excavation and filling of slopes to level them • Building restraining structures such as gabions and stone walls • Stabilising the base of the slope • Stabilising by driving anchors into rock strata • Erosion control such as rock armour, revetments and use of ‘gunite’ a sprayed mixture of sand,
  16. 16. Slope stability can be increased by a variety of engineering techniques which although expensive, enable control of landslides to be successful However … cost and responsibility Governments may fund emergency stabilisation techniques but rarely fund permanent slope stabilisation. Building codes can enforce some use of these techniques by private developers Bishop page 129
  17. 17. Modify vulnerability Most mass movements are not rapid and so forecasting, warning and evacuation are possible techniques. Community preparedness can be used if people are aware of the early signs of movement such as bulging walls, tension cracks, tilted poles and fences, new areas of waterlogged ground and minor slumps. Educating the public is necessary Hazard mapping – used to restrict development on potentially vulnerable slopes – however, this may reduce land values • Landslide vulnerability can be reduced by diversion of roads and infrastructure from known active areas • Planning control to prevent development in landslide prone areas • Evacuation warnings when a landslide is imminent e.g. when heavy rain is forecast • Banning logging on hillsides
  18. 18. Sharing the loss • Private insurance for landslide hazards is not easily available due to the high risks involved for insurance companies • In New Zealand – complete landslide insurance protection is available