Mass Movement Hazards

Question 1

Define Porosity

Answer 1

The % of cavities between grains, fractures and joints

Question 2

Porosity =

Answer 2

Total volume of pore spaces/Total volume of rock x 100

Question 3

Define Primary Porosity

Answer 3

Pore spaces between grains

Question 4

Define Secondary Porosity

Answer 4

Fractures, joints and faults in a rock

Question 5

What are four factor affecting porosity?

Answer 5

Degree of sorting, Amount of diagenesis, Grain shape, Packing of grains

Question 6

How does degree of sorting affect porosity?

Answer 6

A poorly sorted rock has low porosity because the finer grains fill the spaces between the coarser grains

Question 7

How does amount of diagenesis affect porosity?

Answer 7

A loose unconsolidated sediment has a much higher porosity than a rock that has undergone compaction and cementation

Question 8

How does grain shape affect porosity?

Answer 8

Rocks containing rounded grains have a higher porosity than rocks containing angular grains that fit together

Question 9

How does packing of grains affect porosity?

Answer 9

With depth compaction from the overburden pressure will cause the grains to pack closer together, reducing the pore spaces

Question 10

Define Permeability

Answer 10

Measures how fast fluid can flow through a rock, governed by the connectivity of the spaces

Question 11

Permeability =

Answer 11

Distance fluid has travelled/Time taken

Question 12

What factor affects permeability?

Answer 12

Grain size

Question 13

How does grain size impact permeability?

Answer 13

The smaller the grain size, the lower the permeability because there is more resistance to flow

Question 14

What are the seven types of mass movement?

Answer 14

Rockfall, Sliding, Slip, Slump, Flow, Creep, Solifluction

Question 15

Define Mass Movement

Answer 15

The movement of a mass of rock, debris or earth down a slope under the direct influence of gravity

Question 16

What are the four forces acting on a slope?

Answer 16

Shear strength, Normal force, Gravity, Shear force

Question 17

If the angle of a slope increased, what would happen to shear force, shear strength and gravity?

Answer 17

Shear force = increase
Shear strength = decrease
Gravity = never changes

Question 18

Define Angle of Repose

Answer 18

The maximum steepness (angle) a sediment can be at before it will move downslope

Question 19

What are three factors influencing the angle of repose?

Answer 19

Size, Shape, Sorting

Question 20

Define Pore Pressure

Answer 20

Pressure exerted by water in pore spaces

Question 21

Define Cohesion

Answer 21

The tendency for water molecules to stick together

Question 22

What are six factors influencing slope stability?

Answer 22

Groundwater, Dip amount, Dip direction, Weathering, Ground vibration, Vegetation

Question 23

How does groundwater affect slope stability?

Answer 23

(1) Pore pressure increases (2) Decreased cohesion and friction (shear strength) (3) Increase shear force (4) Slope becomes unstable as the forces are unbalanced

Question 24

How does dip amount of rock strata affect slope stability?

Answer 24

Horizontal or very shallow dipping beds are more stable than beds with a high angle of dip

Question 25

How does dip direction of rock strata affect slope stability?

Answer 25

Beds dipping towards the cliff face are more unstable than beds dipping away

Question 26

How does weathering affect slope stability?

Answer 26

Weathering loosens the sediment, making it less stable due to there being less friction

Question 27

How does ground vibration affect slope stability?

Answer 27

More vibration means more grains are shaken, so slopes become less stable

Question 28

How does vegetation affect slope stability?

Answer 28

Vegetation holds the sediment together, preventing it from becoming unstable

Question 29

What are seven monitoring techniques?

Answer 29

Ground levelling and surveying, Microseismic survey, Surface strain measurement, Borehole distortion, Tiltmeter, Electronic distance measurement, Water table and pore pressure

Question 30

Describe Ground Levelling and Surveying

Answer 30

Conventional surveying equipment or GPS to monitor the change in height of the land, Height of land should drop over time if mass movement is about to happen

Question 31

Describe Microseismic Survey

Answer 31

Sound is detected through geophones around the slope as rocks break or slide past each other, Increases in size and frequency before a mass movement

Question 32

Describe Surface Strain Measurement

Answer 32

Inclinometers and extensometers are placed across cracks that have developed on a cliff face, If cracks get bigger the inclination of the bar will increase

Question 33

Describe Borehole Distortion

Answer 33

Inclinometers are placed in permanent boreholes which are drilled into the hillside, Any change in the tilt of the borehole could suggest movement along the plane

Question 34

Describe Tiltmeters

Answer 34

A network of tiltmeters are set up around the head of a mass movement, Any increase in tilt suggests a mass movement could occur

Question 35

Describe Electronic Distance Measurement

Answer 35

Lasers are used to measure the distance between two fixed points, If there is a decrease in distance a mass movement might be about to occur

Question 36

Describe Water Table and Pore Pressure Surveys

Answer 36

Piezometers are used to measure the pore pressure in boreholes around a mass movement, If pore pressure increases it could indicate there is a trigger for mass movement

Question 37

What are three methods of modifying slope geometry to stabilise slopes?

Answer 37

Removal of material, Adding material to the toe, Reducing the angle of the slope

Question 38

What are three methods of drainage to stabilise slopes?

Answer 38

Surface drains, Deep drains, Vegetation

Question 39

What are seven methods of retaining structures and slope reinforcements to stabilise slopes?

Answer 39

Retaining walls, Rock armour, Gabions, Retention nets, Rock traps, Rock bolts and anchors, Vegetation