Mass Wasting

Question 0

Why do we need to know about terrain instability?

Answer 0

Well it is most common in mountainous regions that we work in. Working in unstable terrain can lead to failure and mass wasting events that could affect us legally. Building roads on unstable terrain is extremely risky.

Question 1

What is terrain instability?

Answer 1

A natural geological phenomenon in which slopes fail. Material is moved downwards from hills, mountains and coastlines, gradually flattening the topography.

Question 2

What things should one consider when working or planning work in unstable terriain?

Answer 2

• Slope
• Shape of slope
• Moisture status
• Soil texture
• Surficial indicators
• Bedrock type and strike

Question 3

What are some common triggers of mass wasting events?

Answer 3

• Plate tectonics
• Earthquakes
• Volcanism
• High precip. and flooding
• Isostatic rebound

Question 4

What is the main force responsible for mass wasting?

Answer 4

Gravity

Question 5

What are the two forces of gravity on a slope and how do they react to an increase in pitch?

Answer 5

Gp is perpendicular gravity that anchors an object to a slope
Gt is tangential gravity that acts to move the object downslope.

As the pitch of the slope increases, so will Gt, which will eventually result in the object being moved downslope.

Question 6

What force besides Gp resists downslope movement and what is it made up of?

Answer 6

Shear strength, which includes frictional resistance and cohesion among the particles that compose an object.

Question 7

In soil science, what is the strength of a material?

Answer 7

The greatest stress it can sustain. (consistence)

Question 8

What is Fs???

What happens if it is < 1?

Answer 8

Fs stands for safety factor and is the ratio of shear strength/shear stress.

If it is greater than 1 then slope failure is imminent.

Question 9

What role does water play in slope stability?

Answer 9

Water significantly decreases slope stability and is a factor in all mass wasting except for earthquakes.
By adding weight, volume and lubrication, any more than hygroscopic water increases the chances of slope failure.

Question 10

How does field capacity relate to mass wasting?

Answer 10

When field capacity is reached mass wasting is imminent

Question 11

How much does water weigh per meter cubed?

Answer 11

1000 kg/m cubed

Question 12

List 4 ways that water contributes to slope instability.

Answer 12

• lubricates soil and reduces frictional force
• likely to change the angle of repose
• increases outward pressure in soil (reducing adhesion and cohesion)
• Addition of mass and volume

Question 13

What is adsorption?

Answer 13

When water is attracted to and binds to the surface of soil particles through H bonding.

Question 14

What is absorption?

Answer 14

When water moves into the structure of soil particles. Only occurs with expansive types of clays like smectites and montmorillinites

Question 15

Explain at what level a soil would begins to lose cohesion?

Answer 15

Beyond the hydroscopic level of water content, soil particle cohesiveness is reduced which also reduces shear strength.

Question 16

Why are clayey soils weaker when they become wet

Answer 16

In most soils the clay fraction is random and patternless, the spaces are filled with binding agents like salts that allow the soil to maintain its structure. When the soil gets flooded, water acts as a solvent to dissolve the minerals and the clay fraction realigns to a parallel pattern which reduces the shear strength of the soil.

Question 17

Explain how fluid pressure happens?

Answer 17

In most soil types (besides basal tills) soil particles at depth tend to realign themselves.

• pore water is constrained to occupy the same space and is subject to higher pressures
• The water ends up supporting the weight of the overlying rock mass
• Reduction of friction and shear strength can lead to slope failure.

Question 18

What is strike?

Answer 18

rock layers oriented parallel to the slope they are on.

Question 19

Where is basalt formed?

Answer 19

On the earths surface

Question 20

Where is granite formed? how is it exposed?

Answer 20

Deep below the earths surface.

Granite is exposed by a combination of upward plate tectonics and downward weathering.

Question 21

Explain geological discontinuity

Answer 21

Igneous and metamorphic rocks are generally stable. Sedimentary rock however can be quite unstable due to bedding plane differences. This problem is worse if the bedding plane runs parallel to a slope.

Question 22

Explain how faults and fractures occur in mountains other than by earthquakes and plate tectonics…

Answer 22

Igneous and schistosic metamorphic rock that are subject to freezing and thawing events tend to fracture which leads to loss of shear strength and increases potential of failure.

Question 23

What groups are landslides categorized into with respect to type of material?

Answer 23

Rock - hard mass that was in natural place before movement
Debris - soil material where a range of sizes is represented
Earth - Soil material with mostly small particles

Question 24

What two groups can landslides be divided into?

Answer 24

Movement type or Type of material

Question 25

What are landslides?

Answer 25

A natural process that removes material from hills, mountains and coastlines , gradually lowering and flattening the topography

Question 26

What are some of the more prominent indicators of slope instability?

Answer 26

• Fine textured soil
• steep pitches
• Wet sites
• Thin soils to bedrock
• Un secured bedrock

Question 27

At what slope % is it mandatory to call in a geotech?

Answer 27

Above 60%

Question 28

What is a gully?

Answer 28

Area containing a stream with a gradient of = to or >25%

At least one reach of over 100m has a channel gradient of >20%, sidewall greater than 3 m and side wall slope >50%

Question 29

What are soil compaction and puddling?

Answer 29

Compaction is the increase in Db that results from the compression of soil particles by applied external forces.

Puddling is the destruction of soil structure and the associated loss of macro and microporosity that results from compacting the soil when wet.