Mass Movement Hazards
Porosity and Permeability
Porosity – the % of cavities (spaces) between grains, fractures and joints
Porosity = Total volume of pore space x 100
Total volume of rock/sediment
• Primary Porosity – pore spaces between grains
• Secondary Porosity – fractures, joints and faults in a rock
Factors Affecting Porosity
Factor How does it affect porosity?
A well sorted rock has a high porosity. A poorly sored rock has a low porosity
Degree of Sorting
because the finer grains fill in the spaces between the courser grains
A loose unconsolidated rock has a much higher porosity than a rock that has
undergone compaction and cementation. There is a general decrease in
Amount of Diagenesis
porosity with depth – 40% porosity close to the surface to 10% at depths of
10km in some sedimentary rocks
Rocks containing rounded grains have a higher porosity than rocks containing
Grain Shape
angular grains that fit together
The way the grains fit together. Depth is important as compaction from
Packing of Grains
overburden pressure will tend to cause particles to pack closer together
Permeability – measures how fast fluid can flow through a rock, governed by the connectivity of the spaces
Permeability = Distance fluid has travelled
Time taken
Factors Affecting Permeability
• Grain Size – the smaller the grain size, the lower the permeability (usually) because there is more
resistance to the flow
Types of Mass Movement
Mass Movement – the movement of a mass of rock, debris, or earth down a slope under the direct influence
of gravity
Type of Mass Movement Description
• If slope is greater than 45° then loose rock will fall off
Rock Fall • Forms scree/talus
• Freeze-thaw and wetting and drying of clay weathers it
• Rocks slide along natural planar features such as bedding planes
• Beds dipping downslope
Sliding
• Hydrostatic pressure – when water goes down bedding planes it
increases pore pressure and reduces friction
• Failure along an arc shaped plane
• Blocks rotate as they slip with the foot material collapsing as a small
Slip
slump
• Curved landslip scars and backwards tilted block
• Crack develops in unconsolidated material and then get rotational
slide
Slump
• Forms lobes of highly disturbed material
• Fast or slow
Porosity and Permeability
Porosity – the % of cavities (spaces) between grains, fractures and joints
Porosity = Total volume of pore space x 100
Total volume of rock/sediment
• Primary Porosity – pore spaces between grains
• Secondary Porosity – fractures, joints and faults in a rock
Factors Affecting Porosity
Factor How does it affect porosity?
A well sorted rock has a high porosity. A poorly sored rock has a low porosity
Degree of Sorting
because the finer grains fill in the spaces between the courser grains
A loose unconsolidated rock has a much higher porosity than a rock that has
undergone compaction and cementation. There is a general decrease in
Amount of Diagenesis
porosity with depth – 40% porosity close to the surface to 10% at depths of
10km in some sedimentary rocks
Rocks containing rounded grains have a higher porosity than rocks containing
Grain Shape
angular grains that fit together
The way the grains fit together. Depth is important as compaction from
Packing of Grains
overburden pressure will tend to cause particles to pack closer together
Permeability – measures how fast fluid can flow through a rock, governed by the connectivity of the spaces
Permeability = Distance fluid has travelled
Time taken
Factors Affecting Permeability
• Grain Size – the smaller the grain size, the lower the permeability (usually) because there is more
resistance to the flow
Types of Mass Movement
Mass Movement – the movement of a mass of rock, debris, or earth down a slope under the direct influence
of gravity
Type of Mass Movement Description
• If slope is greater than 45° then loose rock will fall off
Rock Fall • Forms scree/talus
• Freeze-thaw and wetting and drying of clay weathers it
• Rocks slide along natural planar features such as bedding planes
• Beds dipping downslope
Sliding
• Hydrostatic pressure – when water goes down bedding planes it
increases pore pressure and reduces friction
• Failure along an arc shaped plane
• Blocks rotate as they slip with the foot material collapsing as a small
Slip
slump
• Curved landslip scars and backwards tilted block
• Crack develops in unconsolidated material and then get rotational
slide
Slump
• Forms lobes of highly disturbed material
• Fast or slow
