9.2. Mass Movements

Question 1

Mass Movements

Answer 1

Any large-scale movement of the Earth’s surface that is not accompanied by a moving agent

Question 2

Shear Stress

Answer 2

stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock

Question 3

Shear Strength

Answer 3

a measure if the resistance of earth materials to be moved. The interlocking of soil particles increases the ability of material to stay in place

Question 4

What leads to mass movements

Answer 4

• increase in shear stress

- decrease in shear resistance

Question 5

Factors contributing to increased shear stress

Answer 5

1) Removal of lateral support through undercutting or slope steepening
- erosion by rivers and glaciers, wave action, faulting, previous rock falls or slides
2) Removal of underlying support
- Undercutting by rivers and waves, subsurface solution, loss of strength by exposure of sediments
3) Loading of slope
- Weight of water, vegetation, accumulation of debris
4) Lateral pressure
- Water in cracks, freezing in cracks, swelling, pressure release
5) Transient stress
- Earthquakes, movement of trees in wind

Question 6

Factors contributing to reduced shear strength

Answer 6

1) Weathering effects
- Disintegration of granular rocks, hydration of clay minerals, solution of cementing minerals in rock or soil
2) Changes in pore-water pressure
- Saturation, softening of material
3) Changes of structure
- Creation of fissures in clays, remoulding of sands and clays
4) Organic effects
- Burrowing of animals, decay of roots

Question 7

Human activities causing mass movements

Answer 7

WELVD
1) Waste heaps
Waste heaps from mining are often steep and porous which is unstable
2) Excavation
- Ground is removed eg in a road and railway cuttings
- In areas where there is soft/unconsolidated rock this creates a steep slope liable to movement
3) Loading
- Building on slopes adds mass that may trigger mass movement
4) Vibrations
- Movement of heavy vehicles can trigger movement
5) Deforestation
- Decreases interception and takes away binding agents

Question 8

Slowest to fastest mass movements (increasing in speed going downwards)

Answer 8

• Soil creep
• Solifluction
• Debris flow
• Mud flow
• Rock slides
• Slumps
• Rock fall
• Rock avalanche

Question 9

Flows

Answer 9

involve the internal movement of soil and rocks down slope.

Question 10

Characteristics of flows

Answer 10

• Flows occur under wet conditions than slides.
• Flows range in speed from very slow to fast.
• They occur on relatively gentle slopes. Slope gradients are commonly from 10% to as steep as 30%.
• Flows involve a lot of mixing of soil and rock particles within the moving mass.
• Flows usually consist of unconsolidated material.

Question 11

Debris Flows

Answer 11

made up of loose rocks, mud and water. Debris flows also transport large objects such as trees.

Question 12

Characteristics and Causes of Debris flow

Answer 12

• Debris flows commonly occur on moderately steep slopes. Slope gradients are commonly from 10% to as steep as 30%.
• This type of flow happens when the regolith is saturated (very wet).
• It happens at 1-15km per year (faster than creep or solifluction)
• Happens in humid (wet) areas on hillsides following heavy rain or melting snow, and where the regolith consists of fine-grained materials (clay).

Question 13

Mudflows

Answer 13

similar to debris flows but they are wetter and the soil / regolith particles are smaller.

Question 14

Causes and Characteristics of Mudflows

Answer 14

• They have a higher water content and are made up mostly of clay.
• They are very rapid (the fastest flow!), flowing at 1km/hour.
• They are likely to occur in mountain areas after heavy rain or in permafrost areas at the time of maximum thaw.
• Mudflows are a serious volcanic hazard – melted snow and ice from a volcano top can combine with ash to form lahars.

Question 15

Slides

Answer 15

Slides do not experience internal movement.

- The material moves as one mass along a planar surface - with the material moving at the same rate as at depth.

Question 16

Characteristics of Slides

Answer 16

• Slides occur under drier rock conditions than flows.
• They occur on steep slopes. Slope gradients are commonly from 35% to as steep as 70%.
• The sliding surface may be curved or flat in shape.
  Slides consist of consolidated materials.

Question 17

Rockslides

Answer 17

• A Rockslide is a rapid movement of newly detached large pieces of rock along a flat (planar)surface.
• Rockslides take place under slower and drier conditions than slumps.

Question 18

Slumps

Answer 18

• A mass of weathered rock is moved along a curved surface.
• It is likely to happen when weathered rock becomes saturated with water and gains weight.
• Most of the groundwater lubricates the curved surface underground.
• Slumps involve a mass of soil or rock sliding along a curved, rotational surface (shaped like a spoon.)
• At the bottom (or toe) of the slump, flow of soil occurs.
• Typically a steep scarp is developed where the moving material has pulled away from the upper slope. A pile of material collects at the front, or toe, of the slump.

Question 19

Cause of Slumps

Answer 19

• Erosion along the base of a slope, which removes support for the overlying material.
• This may be caused naturally by stream erosion along its banks or by wave action at the base of a coastal cliff.
• It can also be caused by human activity, such as the construction of roads and in quarry walls.

Question 20

Rock Falls

Answer 20

Rockfalls occur when a piece of rock or rocks on a steep slope (more than 40°) breaks off and falls down the slope but not along any particular type of surface.

Question 21

Characteristics and causes of rock falls

Answer 21

• The conditions are dry.
• The individual rocks move at very rapid speeds, speeds ranging from 1m/sec to more than 100 m/sec.
• At the base of most cliffs is an accumulation of fallen material termed talus / scree
• Can be caused by Pressure release weathering and Earthquakes.

Question 22

Impacts of Mass Movements on lives and property.

Answer 22

1) Loss of life
2) Disruption of transport and communication
3) Damage property + infrastructure

Dangerous areas include areas of steep slopes, supersaturated soils and plate tectonic movement

Question 23

Reducing impacts of mass movements

Answer 23

DARNN!!!!

1) Drainage
2) Afforestation (Slope Greening)
3) Retaining Walls
4) Nailing
5) Netting

Question 24

Drainage

Answer 24

Moving water away from vulnerable slopes reduces mass mass and lubrication

Question 25

Afforestation (Slope Greening)

Answer 25

Increasing interception and binding soil together

Question 26

Retaining Walls

Answer 26

Retaining walls are structures usually provided at the toe of a slope to stabilize it from sliding.

Question 27

Nailing

Answer 27

metal rods inserted into rock forming an anchor in rock so that an unstable slope area is being reinforced.

Question 28

Netting

Answer 28

Metal netting is fastened to road cuttings to prevent loose rocks from falling

Question 29

Landslide Hazard Mapping

Answer 29

• Landslide Hazard Mapping maps are drawn up using factors that affect slope stability and knowledge of previous movements
• Rocks are classified as having high, medium or low susceptibility to landslides and the degree to which they have been affected in the past
• Maps of the routes taken by rock falls in mountain areas reduce the chances of building in their paths

Question 30

Predicting and Monitoring Mass Movements

Answer 30

• warning signs that a mass movement is likely to occur include cracks resulting from tension in buildings and in ground, tilted structures, bulging walls and steep slopes
• arcuate-shaped cracks on ground above cliffs indicate the site of future slopes
• once noticed the slopes can be closely monitored for change
• Can also be monitored using GPS and laser surveys to determine amount of movement each year of a number of fixed markets on the surface of the landslip
• Tiltmeters can detect change in slope of soil
• Rainfall and changes in soil moisture are also monitored.
• Measurements in boreholes indicate changes in groundwater content and the pressure it exerts; the weight of additional water can trigger mass movements

Question 31

Perception of risk from mass movements

Answer 31

• depends on distance from the site of hazard, the amount of knowledge a person or group of people have and the length of time since a hazard last occurred
• insurance companies have a clearly defined assessment of the risk, whereas a visitor to the area may have no knowledge of the risk at all