EOSC 114 Unit 3 Landslides.docx

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Earth and Ocean Sciences
EOSC 114
Leah May Ver

Unit 3: Landslides  Explain how the impact of landslides depends on 1) population density, 2) economic infrastructure, and 3) population preparedness. o As population density increases, increasing damage. o Most fatalities from a slides are under-reported as they accompany major disasters  Japan has the most deaths from landslides per year, but highest pop density as well o Largest landslides in Canada happen on West coast (here), between BC and Alberta and in Quebec + Ontario  Explain why British Columbia has the highest frequency of landslides in Canada and what we should expect as our population expands into the mountains. o Mountainous terrain, lots of rain, complex geology (unconsolidated glacial sediments) and many possible triggers  South BC: return period is 25 – 70 years for large slides  Distinguish between the 3 main failure modes (falls, flows, and slides) and how they are influenced by geology. o Falls: very fast.  Occur on very steep slopes (usually rock)  Material detaches because of weakness – falls very fast due to gravity o Slides – cohesive block moves on a surface  Vary from slow to fast. Usually soil, rock or debris  Curved/bowl shaped surface: Rotational slide (slump)  Usually weak material (sediment). Characterized by scarp above slide.  Flat/planar surface: Translational slide  Usually strong material. Cohesive block slides down. o Flows – fluid motion (Chaotic)  Very fast – mudflows can be up to 80 km/h  Soil, mud, wet debris, rock. Water usually involved o Complex movements (combination of above)  Categorize, identify, and name a variety of different landslides o Landslides classified mainly by type of material and type of movement. Rate of movement also important.  Materials: Rock, Soil/Earth, Mud, Debris  Define Angle of repose o Steepest angle a slope can maintain without collapsing. Depends on material.  Assess the balance between the strength of the slope and the destabilizing forces acting on it (Factor of Safety) o Stability of a slope depends on  Driving forces: Gravity  Gravity is manifested as Shear stress “T”  Gravity force is parallel to the slope, moving downwards  Resisting forces (Shear strength) – “Tf  Friction – resistance to sliding  Cohesion – material holding together  Normal stress – perpendicular to slope o Factor of Safety (Fs) = (Tf/T) = (Shear strength / Shear Stress) = (Strength / Stress)  FS >> 1, stable slope  FS < 1, unstable – failure may occur  At angle of repose, FoS = 1 or just above.  Compare and contrast landslide causes and landslide triggers. o Causes – usually long term factors that lead to instability of slope  Reduce shear strength of a slope o Triggers – translate instability into motion. Usually short term events. o There can be many causes, but only one trigger.  List and describe several external causes of landslides o Factors outside of slope that affect stability o High slope angle o Undercutting of a slope (removal of bottom) by roads, rivers, buildings o Overloading a slope with excess weight o Vegetation is weird…  R
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