9.2. Mass Movements Flashcards
Mass Movements
Any large-scale movement of the Earth’s surface that is not accompanied by a moving agent
Shear Stress
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
Shear Strength
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
What leads to mass movements
- increase in shear stress
- decrease in shear resistance
Factors contributing to increased shear stress
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
Factors contributing to reduced shear strength
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
Human activities causing mass movements
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
Slowest to fastest mass movements (increasing in speed going downwards)
- Soil creep
- Solifluction
- Debris flow
- Mud flow
- Rock slides
- Slumps
- Rock fall
- Rock avalanche
Flows
involve the internal movement of soil and rocks down slope.
Characteristics of flows
- 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.
Debris Flows
made up of loose rocks, mud and water. Debris flows also transport large objects such as trees.
Characteristics and Causes of Debris flow
- 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).
Mudflows
similar to debris flows but they are wetter and the soil / regolith particles are smaller.
Causes and Characteristics of Mudflows
- 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.
Slides
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.
Characteristics of Slides
- 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.
Rockslides
- 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.
Slumps
- 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.
Cause of Slumps
- 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.
Rock Falls
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.
Characteristics and causes of rock falls
- 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.
Impacts of Mass Movements on lives and property.
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
Reducing impacts of mass movements
DARNN!!!!
1) Drainage
2) Afforestation (Slope Greening)
3) Retaining Walls
4) Nailing
5) Netting
Drainage
Moving water away from vulnerable slopes reduces mass mass and lubrication
Afforestation (Slope Greening)
Increasing interception and binding soil together
Retaining Walls
Retaining walls are structures usually provided at the toe of a slope to stabilize it from sliding.
Nailing
metal rods inserted into rock forming an anchor in rock so that an unstable slope area is being reinforced.
Netting
Metal netting is fastened to road cuttings to prevent loose rocks from falling
Landslide Hazard Mapping
- 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
Predicting and Monitoring Mass Movements
- 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
Perception of risk from mass movements
- 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
