Study Guide For EESA05, Chapter 4

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Published on 19 Jul 2010
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
- 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
- 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
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- 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
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
and other debris below the soil
nature of the material underlying a slope can greatly influence the type of failure
o slope angle and topography
slope stability is strongly influenced by slope and topography, more specifically by slope steepness
and topographic relief
steeper the slope, greater the driving forces that promote failure
topographic relief refers to the height of a hill or mt above and land below
areas of high relief are hilly or mountainous, have dozens to 1000s of meters of relief, and are
generally prone to landslides
soil layers on steep slopes can become unsaturated with water and slide downhill. Such small slides
may transform into debris flows that travel long distances and cause much damage
debris flows are mixtures of mud, debris and water; range in consistency from thick mud soups to wet
concrete; they can move slowly or rapidly, depending on the conditions
o climate
climate is the characteristic weather typical of a place or region over years or decades; it is more than
just the average air temperature and amt of precipitation; it also includes the type of precipitation and
its seasonal patterns
climate influences the amt and timing of water that infiltrates or erodes a hillslope and type and
abundance of hillslope vegetation
in arid and semi-arid climates, vegetation tends to be sparse, soils are thin and base rock is exposed in
many areas. Free-face and talus slopes are common in these areas, and mass movements include rock
falls, debris flows and shallow soil slips
in subhumid and humid climates, abundant vegetation and thick soil cover most slopes. Mass
movement in these areas include deep complex landslides, soil creep, rockslides, slumps and debris
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