Landslides Notes.docx

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Earth and Ocean Sciences
EOSC 114
Leah May Ver

Lesson 1 A. Why Study Landslides B. Socioeconomic Impacts C. The Effects of Gravity and Factor of Safety Lesson 2 F. Slope Stability and Volcanic Activity  When volcanic ash is released on hillsides, there is an accelerated erosion and mud or debris flow triggered by intense rainfalls.  Volcanic eruptions have triggered the largest landslides. G. Slope Stability and Earthquakes  Strong shaking is a common triggering factor for landslides.  Earthquakes M 4.0 can trigger landslides in susceptible slopes, and larger earthquakes can generate many landslides.  Small magnitude earthquakes less than 4.0 can generate falls over less 2 than 10km and earthquakes over magnitude 6.0 can generate avalanches affecting areas over 3 magnitudes larger.  Liquefaction can easily be generated by smaller earthquakes.  Earthquake shaking destabilizes loose rock on steep slopes. Seismic slopes destabilize slopes by increasing shear stress as the internal structure of the slope material weakens. H. Slope Stability and Anthropogenic Activity  Humans can increase the likelihood of a landslide by: o Excavation of a slope at its toe o Loading of a slope at its crest  Creating flat surfaces to build on  In residential construction, loading the slope at its crest is one way to increase the size of the lot by extending its backyard  Daylight bedding are weak fracture planes that dip with the slope. Bedding planes create a less stable condition on the side of the valley, where they are sub parallel to the slope vs when they are sub perpendicular to the slope o Deforestation for construction and development, which is discussed in slope stability and vegetation o Irrigation Through changes in ground water o Mining flow and surface water drainage o Water Leakage o Artificial Vibration J. Case Study: UBC’s Towers Beach  The Point Grey headlands and the recession of the cliffs that bound UBC are a concern because material the university is built upon is composed of unconsolidated sands and silts.  Wave actions erode and undermines the base of the cliffs which support the surrounding area of UBC.  The unconsolidated material was deposited during the last glacialization.  Berms have been put in place, which widen the natural beach and prevent the waves from reaching the base of the cliffs. o In 1974, the Berm was composed of sand, grave, and cobbles overlain by pit run gravel. o Groins are structures made of rocks, concrete, or wood to preserve sediments on the beach. o A short-term solution to the problem is to use the sands from Fraser River to cover and widen the beaches so that waves are further from the cliff. K. Case Study: The Vaiont Dam  First geological investigation reported that there would be some local detachments of materials, but would not be of serious magnitude.  This was later proven wrong by uncovering a prehistoric slide of unknown dimensions on the left reservoir slope.  There were three fillings of the damn to test it.  October 9, 1963 was when the landslide occurred. Lesson 3 A. The Principles of Landslide Classification  Landslide includes a variety of ground movements at different speeds. o It can be broken down into specific landslide types based on movement mechanism and type of geologic material and involves local geology of an area as well as the surficial/climatic conditions.  There are rapid-moving landslides and slow-moving landslides.  Landslides can also be classified by correlating mass movement speed and water content. o Landslides move at different speeds ranging from millimeters per year to meters per second. o The velocity is important as it determines whether mitigation or evacuation is the most appropriate response, which is the primary concern when evaluating hazards and saving lives. B. Falls  Fall is a sudden vertical movement of material and occurs on steep slopes with loose rock, which can sometimes detach from the surface.  Falls are fast moving events involving rocks.  Falls can be further described by the type of material that is falling such as: o Rock fall where broken bedrock is falling o Debris fall where unsorted soil and other material is falling  Areas of active rock falls can be identified by the formation of talus cones and aprons at the base of the cliffs.  Gravity has the greatest influence on falls that require near-vertical cliff faces. o The further an object falls, the greater its velocity, which increases the likelihood of greater damage. o Velocity = (2gz) 1/2  Mechanical weathering is the physical break-up by freeze-thaw cycles, salt crystal growth, root wedging and penetration, and absorption of water without chemical changes. o It plays a significant role in creating situations where falls are likely to occur. C. Topples  Topples require the forward rotation of material such as rocks, about a pivot point below the center of gravity of a unit or in the slope.  Topples require fractured material oriented perpendicular or parallel to a slope face, which allows the material to be broken up into pieces that roll forward. o There is no such thing as debris topple, as debris is unsorted and consists of material of a variety of sizes which does not remain in blocks when it fails. D. Slides  Slides involve a large volume of rock or soil material that is usually a coherent mass on a sliding surface.  There are two ways slides can occur: o Rotational slides
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