Landslides Ch. 13
There are warning signs to help predict mass movements in the case of heavy rainfall or volcanic eruptions. (T/F)
True
There is usually evidence in the form of distinctive deposits and geologic structures left by recent mass wasting events, making it possible, if resources are available, to construct maps of all areas prone to possible mass movement hazards (t/f)
True
Debris Avalanches
Very high velocity flows of large volume mixtures of rock & regolith that result from complete collapse of a mountainous slope.
Sediment Flows
a mixture of rock, and/or regolith with some water or air.
Mudflows move...
at velocities greater than 1km/hr and tend to travel along valley floors.
Evidence for creep is often seen in
bent tress, offsets in roads and fences, and inclined utility poles
Submarine Slumps
coherent blocks break and slip
Granular Flows
contain between 0-20% water. Fluid-like behavior is given low water flows by mixing with air.
Slurry Flows
contain between about 20-40% water (grade into streams)
Steep slopes can be...to prevent rock falls
covered or sprayed with concrete
and the perpendicular component of gravity
decreases
Slumps (also called Rotational Slides)
downward rotation of rock or regolith occurs along a concave-upward curved surgace
Debris Avalanches are often triggered by...
earthquakes and volcanic eruptions
Joints form as a result of
expansion due to cooling or relief of pressure
Debris Flows
flowage rates of 1 meter/year to 100 meter/hr
Earthflows usually remain active...
for long periods of time
Water can reduce the ____ along a sliding surface
friction
Mudflows
highly fluid, high velocity mixture of sediment and water that has a consistency ranging between soup-like to wet-concrete.
The perpendicular component of gravity helps to
hold the object in place on the slope
Vegetation
holds soil in place and slows the influx of water.
Slumps often form as a result of
human activities
In mountain valleys subject to mudflows, plans could be made to rapidly lower levels of water in...
human-made reservoirs to catch and trap the mudflows.
Grain flows usually form...
in a relatively dry material, such as a sand dune, on a steep slope
Sensitive Soils
in some soils the clay minerals are arranged in random fashion, with much more pore space between the individual grains
On a steeper slope, the shear stress or tangential component of gravity
increase
Changes in the groundwater system can...
increase or decrease fluid pressure in rock and also trigger mass movement events.
In general, for dry materials the angle of repose increases with, _____, but usually lies between about _____ and ______
increasing grain size, 30-45 degress
Slope Modification
modification of a slope either by humans or by natural causes can result in changing the slope angle so that it is no longer at the angle of repose
Submarine Debris Flows
moving material breaks apart
The media often refers to mudflows as...
mudslides (sometimes debris flows)... this is inaccurate because mud flows rather than slides down a slope.
A mass-movement event can occur any time a slope becomes
nstable
Rock Falls
occur when a piece of rock on a steep slope becomes dislodged and falls down the slope
What acts as glue in sensitive soils?
often the grains are held in this position by salts (such as gypsum, calcite, or halite) in the pore space that "glue" the particles together.
If a slope is very close to instability...
only a minor event may be necessary to cause a failure and disaster (something as simple as an ant removing a single grain of sand).
Peat
organic-rich material accumulated in the bottoms of swamps as decaying vegetable matter. It shows similar swelling & compaction as a result of addition or removal of water.
The two components of gravity are
perpendicular and tangential
Classification of mass movement processes is somewhat difficult because
processes generally grade into one another
Volcanic Eruptions
produce shocks like explosions and earthquakes, and can also cause snow to melt or empty crater lakes, rapidly releasing large amounts of water that can be mixed with regolith to reduce grain to grain contact resulting in debris flows, mudflows, and landslides.
Snow Avalanches
similar to debris avalanches, but involve snow.
Removal of trees and vegetation either by humans or by a forest fire, often results in
slope failures in the next rainy season.
Debris flows sometimes start with..
slumps & then flow down hill forming lobes with an irregular surface consisting of ridges and furrows.
Slump size
slumps can be isolated or may occur in large complexes covering thousands of square meters
Hydrocompacting Clays
soils that contain a high proportion of a type of clay mineral called smectites or montmorillinites
Weak Layers
some rocks are stronger than others. In particular, clay minerals and shales generally tend to have a lower shear strength.
Mitigating Mass Movement Hazards
stabilizing or avoing slopes and adding warning systems can minimize hazards in unstable areas
Water has the ability to change ______
the angle of repose
Angle of Repose
the angle that forms a stable slope, for unconsolidated material
Mass Movements
the down slope movement of rock and regolith near the Earth's surface mainly due to the force of gravity.
Mass movements are an important part of...
the erosional process
Foliation Planes
the foliation in metamorphic rocks is due to the alignment of sheet silicate minerals
Shear Strength
the forces resisting movement down the slop
If a weak rock or soil occurs between stronger rocks or soils, the weak layer will be the most likely place for failure to occur, especially if...
the layer dips in a down-slope direction.
The object will move down-slope when...
the sheer stress becomes greater than the combination of forces holding the object
The foliation or schistosity may become a slip surface because
the sheet silicates can break easily parallel to their sheet structure
Solifluction occurs in areas where...
the soil remains frozen and is then thawed for a short time to become saturated with water.
Slightly wet unconsolidated materials exhibit a very high angle of repose because...
the surface tension between the water & the grains tends to hold the grains in place.
Falls commonly occur where
there are steep cliffs
Mudflows are potentially very dangerous because...
they can travel for long distances over gently sloping stream beds at high velocities.
If the joints are parallel to the slope
they may becoming a sliding surface
Mass movements are an important part of the erosional process because...
they move material from higher to lower elevations where transporting agents like streams & glaciers pick up the material and move it to even lower elevations
Grain Flows
when a small disturbance sends the dry unconsolidated grains moving rapidly down slope.
When building a sand castle, if the sand is totally dry...
you cant build a pile with a steep face
___ could be inserted into the slope to more easily allow water to get out and avoid increases in fluid pressure, the possibility of liquefaction, or increased weight due to the addition of water.
Drainage Pipes
Changes in Hydrologic Characteristics
Heavy rains can saturate regolith reducing grain to grain contact and reducing the angle of repose, thus triggering a mass movement event.
What can water do to sensitive soils?
Infiltrating water can dissolve away the salts holding the "house of cards" together. The clay minerals will then line up with one another and result in slippage down slope.
Joints & Fractures
Joints are regularly spaced fractures or cracks in rocks that show no offset across the fracture
Submarine Mass Movements
Movements that occur on slopes in the ocean basins
3 Types of Slurry Flows (Fastest to Slowest)
Mud Flow, Debris Flow, Solifluction
Bedding Planes
Since they are planar and since they may have a dip down-slope, they can form surfaces upon which sliding occurs, particularly if water can enter along the bedding plane to reduce cohesion
Two type of sediment flows
Slurry Flows and Granular Flows
Quick Clays
Some water saturated clays are stable so long as they aren't disturbed, but when shaking occurs, just like sands, they can turn into a runny fluid.
Undercutting
Streams eroding their banks or surf action along a coast can undercut a slope making it unstable.
Sediment flows are categorized by...
1. Amount of water 2. Velocity
Two broad categories of mass movements
1. Slope Failures 2. Sediment Flows
Two components of shear strength
1. frictional resistance (acts in opposite direction to tangential g) 2. cohesion among the particles that make up the object
What event can restore the slope to its angle of repose?
A mass-wasting event can restore the slope to its angle of repose.
4 Types of Granular Flows (Fastest-Slowest)
Debris Avalanche, Grain Flow, Earth Flow, Creep
Solifluction
Flowage rates of centimeters per year of regolith containing water.
Mudslide is an...
ILLEGAL WORD!!!
_____ could be built to stabilize a slope
Retaining walls
Slides (Translational Slides)
Rock slides and debris slides result when rocks or debris slide down a pre-existing surface, such as a bedding plane, foliation surface, or joint surface.
3 Types of Submarine Mass Movements (Least to most disintegration)
Submarine Slumps, Submarine Debris Flow, Turbidity currents
How do subaqueous mass movements compare to land-based slope failures.
Submarine slope failures are much larger than land-based slope failures and are an important process sculpting adjacent land.
Talus
The accumulation of fallen material at the base of most cliffs
Creep
The gradual downslope movement of regolith (<1cm/yr)
____ or ____ could be ____ on bare slopes to help hold soil.
Trees or other vegetation could be planted on bare slopes to help hold soil.
Minor shocks like heavy trucks rambling down the road, trees blowing in the wind, or human made explosions can also trigger mass-wasting events (T/F)
True
Some slopes cannot be stabilized. (T/F)
True
T/F: Although water is not always directly involved as a transporting medium in mass movement processes, it plays an important role.
True
T/F: Mass movement processes are occurring continuously on all slopes.
True
Changes in Slope Strength
Weathering creates weaker material, and thus leads to slope failure.
Rock Glaciers
a lobe of ice-cemented rock debris (mostly rocks with ice between the blocks) that slowly moves downhill.
The tangential component of gravity causes
a shear stress parallel to the slope & pulls the object in the down-slope direction
Slope Failures
a sudden failure of the slope resulting in transport of debris down hill by sliding, rolling, falling, or slumping
Shocks and Vibrations
a sudden shock, such as an earthquake may trigger slope instability.
Creep occurs on...
almost all slopes, but rates vary
Slumps leave
arcuate scars or depressions on the hill slope
Gigantic subaqueous slope failures are widespread on the ocean floor, particularly...
around islands like Hawaii and off the east and gulf coasts of North America
When submarine slope failures occur they create
catastrophic tsunami
Submarine mass movements are differentiated based on
degree of disintegration
Solifluction produces...
distinctive lobes on hill slopes
Ice, although it is solid,
does have the ability to flow, and freezing and thawing cycles can also contribute to movement.
Debris Avalanches move
down slope and then can travel for considerable distances along relatively gentle slopes.
Earthflows
generally tend to be narrow tongue-like features that begin at a scarp or small cliff
Oversteepened slopes could be___or___ to reduce the slope to the natural angle of repose
graded or terraced
The main force responsible for mass movement events is
gravity
Earthflows are usualy associated with
heavy rains and move at velocities between several cm/yr-100s of m/day
Debris often result from...
heavy rains causing saturation of the soil & regolith with water.
Mudflows usually result from...
heavy rains in areas where there is an abundance of unconsolidated sediment that can be picked up by streams.
When the material becomes saturated with water, the angle of repose...
is reduced to small values and the material tends to flow like fluid. This is because the water gets between the grains and eliminates grain to grain frictional contact.
particularly if
it dips in the down-slope direction
The down-slope movement of material is commonly referred to as a
landslide
Combined with joints running perpendicular to the slope, the joint pattern results in fractures along which blocks can become
loosened to slide down-slope
Sediment Flows
material flows down hill mixed with water or air
Trees
put down roots that hold the ground together and strengthen the slope
The upper surface of each slump block remains
relatively undisturbed, as do the individual blocks
Slumps are also common along
river banks and sea coasts, where erosion has under-cut the slopes, heavy rains and earthquakes can also trigger
thus they are common along
roads where slopes have been over-steepened during construction
If the slope is made of highly fractured rock, ___ may be emplaced to hold the slope together and prevent failure
rock bolts
Heavy rains can also...
saturate rock and increase its weight.
Turbidity Currents
sediment moves as a turbulent cloud, called a turbidity current.
Down-slope movement is favored by
steeper slope angles and anything tat reduces the shear strength (aka increasing the shear stress or lowering the cohesion among the particles/the frictional resistance)
Hydrocompacting clays are troublesome because
such clays expand when they become wet as water enters the crystal structure increasing the volume of the mineral. When they dry out, the loss of water causes the volume to decrease and the clays to shrink or compact.
Sediment flows occur when...
sufficient force is applied to rocks and regolith that they begin to flow down a slope
short term predictions of mass movements are difficult (t/f)
true. most triggering events are impossible or require extreme attention to be able to predict.
On a slope, the force of gravity can be resolved into
two components
Sometimes, as in the case of creep or solifluction, the slope is...
unstable all of the time-the process is continuous
Mudflows can also result from
volcanic eruptions that cause melting of snow or ice on the slopes of volcanoes, or draining of crater lakes on volcanoes.
Mass movement in cold climates is governed by the fact that...
water is frozen as ice during long periods of the year.
Addition of water from rainfall or snow melt adds ______
weight, because it seeps into soil or rock and replaces the air, which is lighter.