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EOSC Landslides and Volcanoes coursehero.docx

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University of British Columbia
Earth and Ocean Sciences
EOSC 114

M,EOSC Landslides and Volcanoes 1. Explain how the impact of landslides depends on: a) Population density: Population density affects the impact of landslides because the more concentrated the population the higher the risk is of a larger death toll if a landslide occurs in that area. b) Economic infrastructure: The largest impact of landslides is their toll on the economy, the more economic infrastructure there is in a landslide prone area the higher the economic toll will be as more infrastructure will be affected by the landslide. c) Population Preparedness: The impact of landslides can be greatly reduced if the population is prepared because if evacuation is necessary it can be carried out and personal belonging can also be saved, accordingly population preparedness can also reduce the effect of landslides by the use of mitigation tools such as water drains or protection walls to reduce the effects of the landslide. 2. Explain why British Columbia has the highest frequency off landslides in Canada and what we should expect as our population expands into the mountains It has the highest frequency of landslides because of its mountainous terrain, the fact that it has lots of rain and due to its complex geology such as unconsolidated glacial sediments. British Columbia also has many triggers. The return period of Large Landslides is 25-70 years; they are over 20 million m3 of material. As our population expands into the mountains we can expect more landslides to occur because mountain sides will logged making the slopes unstable and also allowing the ground to absorb more rainwater 3. Distinguish between the 3 main failure modes (falls, flows, and slides) and how they are influenced by geology Falls: Occur on very steep slopes, usually rock. The material detaches because of weakness and it falls very fast due to gravity. Slides: Can vary from slow to fast they are usually soil rock or debris. In slides the material moves as a coherent mass along a surface of failure which can be either curved or strait. If the surface is curved the slide is rotational which means it will have intermediate speed, usually have weak material like sidement and this material rotates on a curved failure plane so this slide is often characterized by a curved scarp above the slide. Slides can also be translational if the surface is flat. Translational slides can vary from slow and fast and they are usually strong material moving on planes of weakness and is cohesive motion of material along a flat surface. Flows: Very flow to very fast, mudlows up to 80kmh, the materials involved are soil, mud and wet debris which is usually rock. In Flows water is usually very important because flows have a fluid or plastic flow of material that is chaotic. 4. Categorize, Identify, and Name a variety of different landslides Landslides are classified by the type of material and the type of movement. Eg. A rock fall has rock as its material and fall as the type of movement. 1) Define Angle of repose Steepest angle a slope can maintain without collapsing, the exact angle varies depending material. It is exactly balanced by the shear strength and shear stress. 2) Assess the balance between the strength of the slope and the destabilizing forces acting on it (Factor of Safety) If the ratio of the resisting force is greater than the driving force the slope is stable if it is less than 1 than the slope will fail. Driving force are gravity which manifests as shear stress which goes side to side but across a plane and the component of force which is parallel to the slope. Resisting forces are friction and cohesion of how the material holds together these manifests as shear strength which is the ability to resist shearing motion. 3) Compare and contrast landslide causes and landslide triggers Causes are factors (often long term) leading to instability of given slope, they reduce the shear strength of a slope but do not initiate movement. Triggers are factors (usually short events) that translate instability into motion; there can be many causes but only one trigger for a landslide occurrence. 4) List and describe several external causes of landslides External causes are factors outside of the slope that affect stability this includes: Highslope Angle: You must have a slope to have a mass movement and steeper slopes equal more movement. Undercutting: The lower part of the slope is removed, it removes material supporting the slope which is causes by roads, rivers, buildings, etc. Overloading: Adding weight which is causes by buildings, roads, landslides, trees, etc. Vegetation: Roots bind loose material, removal of vegetation can make slopes unstable, Heavy trees can increase instability, ie overloading. Climate: If average temperature and rainfall is high it means there will be more water and increased weather of rocks which means more fractures and more soul, if average temp is around 0 then it is internal causes. 1. List and describe 3 internal causes of landslides Water content: in sediment (loose rocks, sand, silt and clay), water can help or hinder cohesion and it depends on the amount of water. In sediment: No water means there is a low angle of repose, some water means there is a high angle and too much water means there is a very low angle. In solid rock water reduces shear strength along planes of weakness. Water content also causes frost wedging in colder climates, water gets into cracks and fractures in rock, if it freezes it expands forcing the cracks apart. Inherently Weak Materials: Some materials are very weak, they fail at relatively very low angles, such as volcanic rock and quick clay. Adverse Geologic Structures: unfortunate bedding or fracture orientation, structures angles in an instable direction or layered precariously. 2. List several landslide triggers - Earthquakes, Small Melt, Heavy Rainfall, Rain on Snow, Loud Noises, Vehicles, Volcanic Eruptions, Excavations, Skiing, Jumping up and down. 3. Compare and contrast key triggers and causes of landslides and how they affect the force balance equation. (i.e. Factor Safety) Triggers are a force or event that disrupts the equilibrium of a slope and initiates its mass movement. This means that a triggers makes the factor of safety less than one because it makes the shear stress stronger than the shear strength. 4. Explain how liquefaction landslides develop in sensitive marine clays When too much water is absorbed by water saturated silts and sands they lose their strength and flow as a liquid. These mineral grains are loosely packed and when disturbed sediment collapses. 5. List and describe the site conditions (Causes and Trigger) that lead to the development of the Rissa quick clay slide in Norway 1. Relate the type of landslide damage expected as a function of its velocity. The higher the velocity the more deaths will occur and the less chance there is that it can be prevented. 2. Identify tell‐tale signs of an unstable slope. - Cresent shaped cracks or terraces on a hillside, A scalloped or recessed crest of a valley wall, A tongue shaped area of bare soil or rock on a hillside, Large boulders or piles of talus at the base of a cliff, An area of tilted or jack strawed trees, Trees that are convex at their base but strait higher up, Exposed bedrock with a layering that is parallel to the slope, Tongue shaped masses of sediment at the base of a slope or at the mouth of a valley, A hummocky or irregular and undulating land surface at the base of a slope. 3. Compare and contrast avoidance, prevention, and protection strategies for dealing withlandslide hazards. - Avoidance is the best option but often is the least preferred, Prevention is to do something to make sure that events don’t occur and is usually the first option in mitigation and Protection is to armour or strengthen an area that might be affected. They all are mitigation issues and prevention and protection are relatively the same except prevention tries to prevent the event from occurring while protection focuses on embracing for the event. 4. List the mitigation techniques commonly used for avoidance, prevention and protection strategies. Prevention: Removal of material, Stabilizing Slopes, Anchors, Drainage Protection: Barriers and Netting, Rock Fall prevention 5. Identify the appropriate mitigation strategy for a variety of risk situations Debris Flow Protection: Separate water and debris by removing debris from the flow with barries, Prevent more debris from being entrained by concrete lined channel, Decrease flows velocity and erosive capabilities by boulder em
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