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.