Study Guide For EESA05, Chapter 4

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University of Toronto Scarborough
Environmental Science
Ingrid L.Stefanovic

Chapter 4: Mass wasting 4.1 An introduction to Landslides - Mass wasting is a general term for ay type of downslope movement of rock or sediment because of gravity - Landslide is rapid movements of soil or rock, we consider the term in its broadest sense to also include rock fall, debris flows and slowly moving bodies of coherent rock Slope Processes - slopes comprise several segments that are either straight or curved - steep faces regularly shed rock fragments to the apron of talus below. Both the rock face and the talus slope are segments of the overall slope - most slopes outside mt ranges are gentle and lack free face - the shape of a particular slope depends largely on the rock type and climate - free-face development is more common in mts, on hard rocks and in arid environments - convex and concave slopes are more common on softer rocks or in humid environments - earth material on most slopes is constantly moving - through time, mass wasting erodes valley walls and is one reason that nearly all valleys are much wider than the steams they contain Types of Landslides - 4 variables that underpin most landslide classifications: (1) mechanism of movement (fall, topple, slide, flow or complex movement) - falling refers to free fall and bounding of rock or blocks of sediment from the face of a cliff - sliding is the downslope movement of a coherent block of rock or sediment along a discrete failure plane - slumping is a particular type of sliding in which the failure plane is curved upward - flow is the slow to rapid downslope movement of sediment in which particles move semi-independently of one another, commonly with the aid of water - very slow flow of rock or sediment, at rates ranging from mm to dozens of cm per year is called creep o sackung is a special type of creeping that involve movement of large masses of rock, up to many billions of cubic meters, along ill-defined deep failure planes - slow creep like movements in which rock masses pivot about a point are termed topples; these are common in rocks with joints or bedding planes that dip steeply into the slope (2) type of material (rock, consolidated sediment or organic soil) (3) amt of water present (4) rate of movementit is considered rapid if it can be discerned with a naked eye - many landslides are complex combo of sliding and flowage o most subaqueous landslides are of this type - a slump or slide on the submerged slope of a delta or the edge of a continental shelf can change into debris flow or a turbidity current that travels great distances from the point of failure - turbidity current is a turbid flow of mud, sand and water - some complex landslides may form when water-saturated sediments flow from the lower part of the slope and undermine the upper part, causing slump blocks to form Forces on Slopes - slope stability can be evaluated by determining the relation b/w driving forces that move rock or sediment down a slope and resisting forces that oppose such movement - largest driving force in the downslope component is the weight of the slope material and the water it contains - resisting force is the shear strength of the slope materialits resistance to failure by sliding or flow along potential slip planes - potential slip planes are surfaces of weakness in the slope material, such as bedding planes in sedimentary rocks, foliation in metamorphic rocks and fractures in all types of rocks slope stability is evaluated by computing a factor of safety (FS), defined as a ratio of the resisting forces to the driving forces o if factor of safety is greater than 1, the resisting forces exceed the driving forces and the slope is considered stable o if the factor is equal to 1, the driving forces equal the resisting forces and a slope failure can be expected - these forces are not stablethey change with local conditions - driving and resisting forces are determined by the interrelations of the following variables: o type of material material composing a slope can affect both the type and frequency of landslides important material characteristics include mineral composition, degree of cementation or consolidation and the presence of planes of weaknessplanes of weakness may be sedimentary bedding planes, metamorphic foliation, secondary joints, or zones along which Earth moved before these planes are especially hazardous if they are inclined more than about 15 degree and intersect or are parallel to the slope of a hill or mountain in the case of slides, the shape of the slip surface is strongly controlled by the type of material that fails. Slides have 2 basic patterns of movement: rotational and translational rotational slides or slumps have curved slip surfaces, whereas translational slides have planar slip surfaces rotational sliding tend to produce small topographic benches that tilt upslope; slumps are most common in unsolidated sediment and in mudstone shale or other weak rock types inclined slip plans of translational slides include fractures in all rock types, bedding planes in sedimentary rocks, weak clay layers and foliation planes in metamorphic rocks common type of translational slide is a debris avalanchea very shallow slide of sediment or soil over bedrock failure plane is either at the base of the organic soil or in the colluvium, a mixture of weathered rock
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