Mass movements
Definitions:
- Mass Movement
- Mass movement refers to the gravitational or downslope movement of weathered rock debris.
- Gravity is the primary force responsible for transporting materials downhill.
- Other transporting mediums such as water, wind, waves, and glaciers are not directly involved but can contribute to mass movement.
- Weakening internal cohesion and structural instability are the most common cause of mass movements.
- Landslides and Avalanches are the two most common types of mass movements.
- Landslides
- Landslides are the perceptible downslope movement of rock, debris, or earth.
- They are caused by a large volume and combination of rock, debris, soil, and water moving under the influence of gravity and water.
- Landslides can be triggered or aggravated by factors such as additional weight, seepage pressure, or lubrication.
- Methods of stabilizing hill slopes can help reduce the impact and vulnerability of landslide-related hazards.
- Avalanches
- Avalanches refer to the fall or rapid downslope movement of snow.
- They are caused by structural weaknesses or gravity.
- Structural weaknesses can include the existence of solid bedrock acting as a slide plane below weak material.
- Avalanches tend to occur in the same places and can be recurrent.
- Ski resorts are particularly prone to avalanches.
| Movement category | Types of materials | Speed of onset | ||
|---|---|---|---|---|
| Bed rock | Coarse | Fine | High (slope angle or gradient plays the major role) Low (water plays an important role) |
|
| Fall | Rock fall | Debris fall | Earth fall | |
| Topple | Rock topple | Debris topple | Earth topple | |
| Slide or Slump | Rock slide | Debris slide | Earth slide | |
| Spread | Rock spread | Debris spread | Earth spread | |
| Flow | Rock flow | Debris flow | Earth flow | |
| Creep | Rock creep | Talus creep | Soil creep/Terracettes | |
Debris flow – Flow of coarse materials typically mixed with abundant water, creating long runouts at the slope base. Flows are commonly separated into debris flow (coarse material) and earthflow (fine material) depending on the type of material involved and the amount of water. Some of the largest and fastest flows on land are called sturzstroms, or long-runout landslides. Source: OpenGeology.Â
Solifluction (sometimes termed gelifluction in periglacial environments) is the slow flow of saturated soil downslope. Movement of these saturated soils can be initiated by thawing, creating excess pore-pressure in the soil, resulting in slope movements (Mackay, 1981). The relief of the slope, depth of thaw, and water content are significant factors in solifluction rates (Walker, 1986; Hjort et al., 2014). Source: ScienceDirect.
Rotational slides: slow movement along a curved rupture surface. A rotational landslide, also known as a slump, is a type of landslide that involves the downward and outward movement of a mass of soil or rock along a curved surface. Rotational landslides often occur in areas with slopes or hillsides that have a concave shape. Translational slides often are rapid movements along a plane of distinct weakness. Slides can be further subdivided into rockslides, debris slides, or earth slides depending on the type of the material involved.
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Falls: Rocks separate along existing natural breaks such as fractures or bedding planes. Free-falling, bouncing, and rolling movements are common in case of fall, strongly influenced by gravity.Â
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Factors that promote Landslides
Prolonged rainfall: Saturate soil and increase the possibility of soil movement
Removal of Vegetation: Roots bind the soil and intercept the water flow.
Rock type and Permeability: Permeable rocks get saturated quickly and is more likely to move than the solid bedrock
Construction of buildings and excavation: Increase weight on the slop and add to downward pull of gravity. Excavation is responsible for the undercutting of slopes and increases instability.
Methods of stabilizing slopes and reducing the risk landslides
Draining of extra water: This increases the shear strength of the rocks by reducing the pore-water pressure.
Terracing: Generates more stable angle
Toe stabilization and hazard resistant design by using retaining wall, anchors (rock armour), gabions: Retaining wall are used to stabilize the upper slope. Steel-mesh curtain are often used as retaining wall. The materials deposited at the slope foot (toe) are also stabilized by a retaining wall. Anchors are normally used at the upper slopes.
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