EAS Midterm 2

Topic 7 -

what are some other names for landslides?

• mass movements

• mass wasting

define a landslide

the movement of a mass of rock, debris or earth down a slope

• moves as a mass under gravity

how does a landslide differ from erosion?

in landslides, materiel moves as a mass under gravity, not grain by grain in a transporting fluid like erosion (water, wind, ice)

what is the difference between weathering and erosion?

weathering is a set of physical and chemical processes that alter states of rocks and minerals at or near the earth’s surface, while erosion is the transport of materials at the earth’s surface

what are the two types of weathering?

• physical/mechanical - breaking up of rocks and minerals by physical processes (disintegration)

• chemical weathering - changing from one chemical state to another (decomposition)

freezing and thawing of materials would be considered what kind of weathering?

Physical/mechanical weathering, as it involves the physical breakdown of materials through temperature changes.

every landslide slope is a balance between ____ and _____ forces

driving and resisting forces

define what is driving shear stress?

the downslope part of the weight - W sin a

a lanslipe slope fails when shear _____exceeds shear _____

shear stress exceeds shear strength

define what is the available resisting force

friction plus cohesion along the potential slip surface

a steeper landslide slope means _____ driving force and _____ normal force

higher driving force, lower normal force

what effect does water pressure have on a slope?

• water pressure pushes against resisting force, which then reduces friction

• and increases the possibility of failure by reducing effective stress.

what are some ways you can increase the shear stress on a slope?

• steeper slope (River or wave undercutting)

• added load (fill, buildings, snow, saturated soil)

• removing support at the toe (excavation mining)

• shaking (earthquakes)

what are some ways to decrease shear strength of a slope?

• increase in pore water pressure (rain, snowmelt, reservoir filling)

• weathering - weakens rocks and soil

• loss of roots (logging, wildfire)

• thaw of ice-bonded ground (permafrost)

• weak layers - clay, bentonite, bedding parallel to the slope

how can water affect the shear stress of slopes?

can act on both sides - can add weight and raise pore-water pressure, reducing frictional resistance

what 2 specific things move a slope towards failure?

• raising shear stress

• lowering shear strength

what does the factor of safety measure?

how close a slope is top failure

how do you calculate the factor of safety, FS?

shear strength divided by shear stress

what do different values of FS indicate?

>1.5 - resistance exceeds driving force

1.0 - limiting equilibrium - threshold of sliding

<1.0 - insufficient resistance under assumed conditions

what parameters does the factor of safety rely on?

FS = [ c + (Wcosa-U) tanΨ] / Wsina

• c - cohesion of materials

• W - weight of the sliding stress

• a - slope angle

• U - pore water pressure (force) on the base

• Ψ - internal angle of friction of the material

what is the target FS for long term static design?

1.5

true or false - water alone is not enough to tip a slope into failure

false - depending on conditions, slope’s shape can remain the same and only the addition of water can cause the slope to faile (saturated slope) by lowing the friction

landslides are classified by the type of _____ and type of _____

type of movement and type of material

what are the types of movement in a landslide?

• fall

• topple

• slide

• spread

• flow

• complex

what are the two main types of material to consider in landslides?

• bedrock (cohesive rock)

• soil (unconsolidated debris or sediment)

how many speeds can landslides span?

span more than ten orders of magnitude

why does the speed of landslides matter?

speed controls whether people can escape

• fast slopes - more lives at risk

• slow slopes - mainly damage to infrastructure such as roads, pipes and buildings

what are some examples of very rapid, rapid, moderate and slow moving landslides?

extremely rapid - rock falls and avalanches

very rapid/rapid - debris flows, some slides

moderate - earthflows

slow - active slides

very slow - creep of slopes

define the movement of fall landslides

• rock detachment from a steep face

• free fall, bouncing, rolling

• rock materials

• extremely rapid

describe the topple movement of landslides

• forward rotation about a hinge

• rock material with steep joints

• slow to extremely rapid

describe the slide movement of a landslide

• along a rupture surface - curved (rotational) or planar (translational)

• rock or soil materials

• extremely slow to rapid

describe the spread movement of landslides

• lateral extension over a weak or liquefied layer

• clay, silt and sand materials

• slow to extremely rapid

describe the flow movement of a landslide

• like a viscous fluid

• debris, mud and earth material

• very rapid to extremely rapid movement

describe the complex type of landslide movement

• two or more types of movement combined

• can be any material

• varies in speed

• ei - frank slide

what can trigger rock falls?

• freeze - thaw

• rain

• roots

• earthquakes

when rock falls accumulate at the base of a slope, they are called ______

talus

how are rock falls managed?

• fence wires

• rock netting

• signals - if fence wires break

• warning signs and signals

describe a rock topple, and the slope required to cause it

• blocks rotate outward about a point near their base

• needs steep joints oe bedding dipping into the slope

• topples rotate forward about a hinge before they fall

describe slides and their two types

slide - movement of large slabs/blocks or consolidated materials along one or more surfaces

• rotational (curved)

• translational (parallel)

describe the parts of a slide

• main scarp - steep cliff in the rock at the top of the slide

• head - top of the moved mass

• main body - rest of the moving mass

• toe - the leading edge of moving mass

• surface of rupture - where movement occurs on slope

describe a rotational slide

• curved, spoon-shaped surface of rupture

• block tilts back toward the scarp

• typical of thick, uniform clay or weak rock

• often leave back-tilted blocks of main scarp

describe a translational slide

• planar surface - bedding, a joint or a frozen layer

• mass moves parallel to the slope

• often shallow and can travel far

rotational slides leave a ______ scarp

curved

describe the effects of river movement on slope erosion

river movement causes erosion on the outside of each meander

• creates cutbanks from erosion

• erosion increases sloped and sheer action

• toe erosion on slope

describe why the Grierson Hill slid in edmonton

• weak bentonite seams in cretaceous bedrock

• coal mine workings under the slope

• excavation steepened the slope, fill loaded it

describe what are landslide flows and lateral spreads

• mass movement in which material deforms in an irreversible way

• movement is similar to viscous fluid - decreased velocity at interface with lower/lateral surface

• usually involve water but also known to occur in dry materials

describe why flows and lateral spreads can be so damaging?

• boulders and large masses can be caught and carried in flows

• houses and infrastructure can be carried and destroyed

describe the ground movement and triggers of a lateral spread

• ground pulls apart sideways, often on very gentle slopes

• usually triggered by earthquake shaking that liquefies saturated sand or sensitive clay

• coherent blocks ride on a weak or liquefied layer

describe the movement of landslide flows and their movement speed

• flows move like a viscous fluid

• internal deformation - no single rupture surface

• usually involve water but dry flows can occur

• range from very slow (earthflows) to extremely rapid (debris flows)

what phenomenon can trigger landslide flows

• heavy rainfall - creates saturated slopes from unconsolidated material

describe what is a debris flow

• a fast moving slurry of mud, sand, rock, wood and water

• course particles in suspension of thick, viscous fluid

debris flows deposit where the gradients drops, which creates a ______

fan

describe the movement of channelized debris flows on steep slopes

• debris flows scour loos debris and wood from the channel

• creates levees on either side of channels from elevated viscous material buildup

• can pick up more material from basin - ie in mountains, collecting material from snow runoffs

• can be triggered by heavy rainfall

the ____ of a landslide controls who is at risk

speed

• can be from mm per year to ten of m per second

water ____ strength by ____ pore pressure, but it does not push the slope

lowers strength by raising pore pressure

What are two events linked to mass-extinctions

regional volcanism and asteroid impacts

what are the major extinction events?

• permian-triassic extinction

• triassic-jurassic extinction

• end-cretaceous extinction

when did the permian-triassic extinction event occur in absolute age and in geological time

• about 252 MYA

• after the permian period (end of the paleozoic era)

when did the Triassic-Jurassic extinction occur?

• about 200 MYA

• at the end of triassic period (near the start of the mesozoic era)

which eon did the major mass-extinctions occur during?

the phanerozoic eon

the Permian-Triassic extinction brought about the start of which era?

the Mesozoic Era

when did the End-Cretaceous extinction occur?

• about 65.5 MYA

• end of the cretaceous period, as well as the end of the Mesozoic Era.

what is the order of periods during the mesozoic era?

• triassic - 251 MYA - 200 MYA

• jurassic - 200 MYA - 145 MYA

• cretaceous - 145 MYA - 65.5MYA

which period was at the end of the paleozoic era?

the permian period

what catastrophic event increased throughout the Phanerozoic eon?

volcanism and large igneous provinces

which extinctions had volcanism as a contributing factor