Mass Movement Hazards Flashcards
Define Porosity
The % of cavities between grains, fractures and joints
Porosity =
Total volume of pore spaces/Total volume of rock x 100
Define Primary Porosity
Pore spaces between grains
Define Secondary Porosity
Fractures, joints and faults in a rock
What are four factor affecting porosity?
Degree of sorting, Amount of diagenesis, Grain shape, Packing of grains
How does degree of sorting affect porosity?
A poorly sorted rock has low porosity because the finer grains fill the spaces between the coarser grains
How does amount of diagenesis affect porosity?
A loose unconsolidated sediment has a much higher porosity than a rock that has undergone compaction and cementation
How does grain shape affect porosity?
Rocks containing rounded grains have a higher porosity than rocks containing angular grains that fit together
How does packing of grains affect porosity?
With depth compaction from the overburden pressure will cause the grains to pack closer together, reducing the pore spaces
Define Permeability
Measures how fast fluid can flow through a rock, governed by the connectivity of the spaces
Permeability =
Distance fluid has travelled/Time taken
What factor affects permeability?
Grain size
How does grain size impact permeability?
The smaller the grain size, the lower the permeability because there is more resistance to flow
What are the seven types of mass movement?
Rockfall, Sliding, Slip, Slump, Flow, Creep, Solifluction
Define Mass Movement
The movement of a mass of rock, debris or earth down a slope under the direct influence of gravity
What are the four forces acting on a slope?
Shear strength, Normal force, Gravity, Shear force
If the angle of a slope increased, what would happen to shear force, shear strength and gravity?
Shear force = increase
Shear strength = decrease
Gravity = never changes
Define Angle of Repose
The maximum steepness (angle) a sediment can be at before it will move downslope
What are three factors influencing the angle of repose?
Size, Shape, Sorting
Define Pore Pressure
Pressure exerted by water in pore spaces
Define Cohesion
The tendency for water molecules to stick together
What are six factors influencing slope stability?
Groundwater, Dip amount, Dip direction, Weathering, Ground vibration, Vegetation
How does groundwater affect slope stability?
(1) Pore pressure increases (2) Decreased cohesion and friction (shear strength) (3) Increase shear force (4) Slope becomes unstable as the forces are unbalanced
How does dip amount of rock strata affect slope stability?
Horizontal or very shallow dipping beds are more stable than beds with a high angle of dip
How does dip direction of rock strata affect slope stability?
Beds dipping towards the cliff face are more unstable than beds dipping away
How does weathering affect slope stability?
Weathering loosens the sediment, making it less stable due to there being less friction
How does ground vibration affect slope stability?
More vibration means more grains are shaken, so slopes become less stable
How does vegetation affect slope stability?
Vegetation holds the sediment together, preventing it from becoming unstable
What are seven monitoring techniques?
Ground levelling and surveying, Microseismic survey, Surface strain measurement, Borehole distortion, Tiltmeter, Electronic distance measurement, Water table and pore pressure
Describe Ground Levelling and Surveying
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
Describe Microseismic Survey
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
Describe Surface Strain Measurement
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
Describe Borehole Distortion
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
Describe Tiltmeters
A network of tiltmeters are set up around the head of a mass movement, Any increase in tilt suggests a mass movement could occur
Describe Electronic Distance Measurement
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
Describe Water Table and Pore Pressure Surveys
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
What are three methods of modifying slope geometry to stabilise slopes?
Removal of material, Adding material to the toe, Reducing the angle of the slope
What are three methods of drainage to stabilise slopes?
Surface drains, Deep drains, Vegetation
What are seven methods of retaining structures and slope reinforcements to stabilise slopes?
Retaining walls, Rock armour, Gabions, Retention nets, Rock traps, Rock bolts and anchors, Vegetation
