Mass Wasting: Concept, Factors and Types

2 years ago

Atiqur Rahman

17 minutes

Mass Wasting: Concept

The earth’s surface is everywhere, attacked by forces that lead to the decomposition and disintegration of rocks. The layer of waste this produces is called regolith. It is unconsolidated matter and a mixture of rock pieces and fine soil.

Lying on a sloping surface, regolith is pulled by the earth’s gravitational force and moves down. This down-slope movement of weathered material under the influence of gravitational influence is defined as ‘Mass wasting’, also termed ‘Mass movement’. Sometimes mass wasting may also involve intact rock beds.

According to A. S. Goudie – “A mass movement is the downward and outward movement of slope-forming material under the influence of gravity.” To this definition, Encyclopaedia Britannica adds “the rapid or gradual sinking of the Earth’s ground surface in a predominantly vertical direction.”

The process has a variety of rates and mechanics of movement; involves particles ranging from minute, fine clay to massive rock beds; leads to impacts ranging from insignificant events to large-scale disasters; and results in the creation of various landforms.

According to W. D. Thornbury, the importance of mass wasting as a process of creating surface features was realised much later. R. P. Sharpe (1938) was the first to pay serious attention to mass wasting processes, conditions and types.

Mechanics of Mass Wasting

The process of mass wasting and all the other related details can be understood if we look at the interplay of different forces acting on weathered mass sitting on a sloping surface. Basically, it is gravity, which is analyzed into two main components that together control the movement of regolith on all slopes (Fig.). These are:

Slide component, also called stress,
Stick component, also called friction or shear resistance.

The slide component works in the downslope direction and pulls the rock mass towards the foot of a slope; this leads to ‘stress’ or tension between the solid unweathered rock surface and the unconsolidated regolith lying on it.

It also exerts stress along bedding planes, joints, crevasses and fractures within a solid rock body. Even the mineral grains within a rock respond to this stress and internal stress is experienced by the matter.

The Stick component works perpendicular to the slope and creates friction between the regolith and slope and at all the other sites under stress, as mentioned above. The friction created by the stick component counteracts the downward pull. Whether a mass would move or not is decided by the critical balance between these forces, which in turn depends primarily on the steepness of the slope.

On a slope measuring 30°, if gravity exerts a force equal to 1 kg on a mass, the two components may be calculated as:

The slide component would show force equal to 0.5 kg (1kg sine 30) and
The Stick component is 0.85kg. (1kg cos 30).

Here frictional force opposing the motion is stronger. Hence it will neutralize the weaker slide force, and the mass will not move. In contrast, if the slope is steeper, such as 60°, then the mass movement would happen because, in this case, stress will have a stronger force, which is directed to the direction of movement (Fig.).

Thus chances of slope failure may be calculated using the following equation where Fs stands for ‘factor of safety’ (Chorley, Schumm and Sugden):

Fs = shear resistance / magnitude of stress

If the value of Fs is 1.0 or more, it indicates that gravity will not be able to move rock debris unless some other factor supports it. If Fs has a value less than 1.0 it indicates that the slope is vulnerable to mass wasting.

Besides many other factors, the balance between stress and shear resistance depends largely on the steepness of the slope. The rate and nature of the final movement may be influenced by several other factors that play an important role in the mass-wasting process.

Modes of Movement in Mass Wasting

Once a mass of regolith is displaced by gravity, the weathered material may follow different modes of movement downhill (Fig.). M. A. Carson and M. J. Kirkby consider three of the most important:

Slide,
Flow, and
Heave

Slide

In slide movement, motion is maximized along the base of the moving mass. There is a clear and easy-to-identify surface dividing the mobile upper layer and the intact, stable lower zone. This separating plane is called the shear plane.

Usually, the top of the mobile surface is able to keep pace with the rate of motion along the base, but sometimes it may lag behind. The slide can take place in absolutely dry matter. Hence it can happen anywhere. However, the presence of water facilitates it and induces greater speed.

Flow

In flow mode, the material above the shear plane accelerates to the maximum speed at the top, while the rate of movement diminishes with increasing depth till it reaches zero along the shear plane.

This differential rate of movement within a mass is caused by increasing friction towards the contact zone between the regolith and the stable rock surface. In this mode, water is an essential component of the process. As such, it is a feature of humid regions.

Heave

This type of mechanism can move any particle size from a fine clay grain to a large boulder. The rate is mostly very slow, and the actual movement is imperceptible unless it is measured using sophisticated methods. The ultimate result of the change in the position of matter can be seen and is taken as evidence of mass wasting activity.

In this mode of movement, regolith alternatively experiences swelling or expansion, and shrinking or contraction. Expansion may be a result of moisture absorption, heating or ice crystal formation.

As water changes into ice, there is about a 10% increase in the volume of moisture within the regolith, causing an upward push. Contraction happens when a moist particle dries up, a hot surface cools off, or ice crystals melt and convert into liquid.

During these expansion-contraction cycles, particles rise upwards perpendicular to the original surface as they expand (Points 2, 4, 6 in Fig.) and settle down vertically. At the same time, contraction compels downward movement (Points 3, 5, 7 in Fig.). Thus in Fig., a particle originally at point 1 would gradually shift to 2à3à4….till it reaches point 7 at the foot of the slope.

Besides these three modes, sub-varieties of mass wasting may be induced by factors like the shape of the shear plane and the presence of vegetation or boulder obstruction. Examples involving curved and rectilinear shear planes may be given here.

Rock debris moving on a curvilinear shear plane shows a tendency of difference in velocities at their base and top; mostly, speed increases with depth (Fig.). The resulting mass wasting has a characteristic backward rotation.

On the other hand, a rectilinear shear plane exhibits an equal rate of movement throughout the mass (Fig.).

Rotational slip on curved surface — Rotational slip on a curved surface

Rock gliding on rectilinear shear plane — Rock gliding on the rectilinear shear plane

Factors Controlling Mass Wasting

There are several natural and anthropogenic factors that regulate initiation, rate and type of mass wasting. Different views have been expressed in this regard. For example, Sharpe (Table-I) groups the controlling factors into:

Passive and
Active

This classification focuses mostly on the inherent characteristics of regolith and its environment. Others (D. J. Varnes 1978) have classified causes of mass wasting with a focus on the forces working on weathered material.

From this viewpoint, on the one hand, some forces enhance the pulling effect of gravity or the slide component. On the other, some conditions play a negative role with reference to the stick component and weaken the stability of matter. An ideal condition for mass wasting is created when a strong slide force combines with a weak stick force.

The two categories of forces are controlled by several supporting factors (Table II):

Factors that increase stress,
Factors that reduce the shear strength

Factors Affecting Stress and Shear Components:

The above two tables make it clear that interaction between several factors leads to conditions favouring slope failure.

Broadly speaking, on the one hand, regolith has some inherent favourable qualities – it becomes heavy, slippery, non-cohesive and ready to move. On the other hand, some external factors play a vital role, such as if the foothill area supporting the upper slope and stabilizing it is removed, or violent tremors destroy the shear strength and dislodge weathered material from the slope.

Both categories of such factors lead to the same result; that is, gravity succeeds in pulling down the weathered surface layer. In some cases, two or more conditions may combine and cause mass wasting of exceptional intensity.

For example, the April 2015 Nepal earthquake triggered several incidents of mass wasting in the area. Village Langtang suffered from a two to three-km-wide avalanche. The Trishuli River valley of Nepal witnessed several landslides.

Types of Mass Wasting

Classification of Mass Wasting

Different classifications of mass wasting have been suggested, founded on different bases. Arthur Bloom discusses a ‘Descriptive Classification of Mass Movement’; the mode of movement and nature of weathered material involved are the bases of his classification.

Carson and Kirkby suggest another classification of mass wasting. Their classification takes into account moisture conditions, along with the rate and mechanics of movement of weathered matter (Fig).

The following three mechanisms of movement are the primary basis for this classification of mass wasting:

Heave
Flow and
Slide

In this classification, the three basic categories replace one another as climatic condition change from extremely dry to extremely wet moisture regimes. Ultimately mass wasting is replaced by fluvial processes (warm regions) or glacial processes (cold regions). Particle size involved ranges here from soil to talus to rocks.

Depending on the availability of moisture and the nature of regolith, sub-types acquire specific characteristics and are identified as several separate classes of mass wasting. Thus the three basic mechanisms lead to several subtypes of the process (Fig.).

Types of Mass Wasting

Solifluction

It can be translated as ‘soil flow’. As the name indicates, it is a sub-type of flow movement. In this type, the surface is covered with water-saturated regolith. As water content increases, soil cover changes to a soggy matter and loses cohesive strength. This weakens friction or sticks components, enabling gravity to move the weathered layer on the slope.

The presence of an impervious sub-surface layer and ample soil moisture is vital for this process. It can happen both in warm and cold climatic regions. In warm regions, moisture is provided by precipitation or surface runoff. Here, the sub-surface impervious layer may be a bed of slate, schist, or any other such hard rock.

In cold regions, water is supplied as meltwater during spring, when all surface snow and ice starts melting. The impervious sub-surface layer here is the vast deep frozen ground (permafrost) that neither warms up nor thaws. To differentiate processes in these two climatic regimes.

A. L. Washburn (1967) suggested the usage of ‘gelifluction’ for the process in cold-climatic regions. A. L. Bloom also uses this term in his literature.

Solifluction/gelifluction is a slow process as compared to the other types. Mostly it covers extensive areas. The whole of Siberian tundra and Greenland hillsides are evidence of its regional scale. Ground influenced by this type of mass wasting is broken into gently sloping, terrace-like features, which are sometimes bound by vegetative growth (Fig.).

Soil Creep

This type of mass wasting is present almost everywhere on the earth’s surface, because first, it requires minimal support from the steepness of the surface and second, it can involve any type of debris. Third, it also does not need moisture. Hence can happen in all types of climates. It follows the mechanics of heave (fig.).

Areas experiencing soil creep show several symptoms that are easy to identify – trees with downward bending lower trunks (Fig.), tilt in exposed rock beds, broken fences and tilted electricity poles are some examples.

In mountainous areas, heaps of weathered matter collect at the base of slopes as talus and scree cones due to soil creep (Fig.). In these cones, the frontal margin of the cone migrates forward as soil creep continues and cones keep growing in size. Similar to soil creep is rock creep, which involves large boulders and rock chunks.

Mud Flow and Earth Flow

The mechanics of earth flow and mudflow are similar to any other flow type of mass weathering; however, both have certain distinguishing features. Mudflow is large-scale, channelized movement, while earth flow may happen on any surface in a localized manner.

Mudflow is confined to valleys and is a rapid movement. Arid regions have the most ideal conditions for it. Here seasonal streams cut valleys in mountainous areas. For most of the year, or sometimes for several years, the valleys stay dry and receive a large amount of weathered debris from slopes. When rains eventually come, they are usually torrential and last for a short while (a general characteristic of precipitation in hot arid areas). As water is channelized and moves down the valleys, it keeps on incorporating accumulated rock debris from the valley floor.

The absence of vegetative cover in these regions further supports this process. This mass has great velocity and erosive and transporting capacity. Movement stops under two conditions: one, the matter becomes thick in consistency and is no longer fluid; two, the flow reaches a flat foothill area. At the terminal point, all material brought down is left as a mound or in a fan-shaped feature called an alluvial fan.

Eliot Blackwelder (1928) considered mud flow an important geomorphic agent in arid and semi-arid regions. He explained the long-distance transportation of huge rocks in these regions as the work of mudflow.

Slide and Slump

Both these types follow slide mechanics, and their rate of movement is fast (Fig. 7 and 8). For initial movement presence of water or ice as a lubricant is not needed, but the rate of movement accelerates if the regolith is wet. Differences in their shear planes mark the distinction between these two. The slide has a rectilinear shear plane (Fig.), while the slump has a concave upward or spoon-shaped shear plane.

The slide is again classified according to the nature of the weathered material. The debris slide, rock slide, mudslide and landslide are some examples.

A landslide is the most devastating, sudden and rapid slide movement (Fig.). It can transport and bury settlements and destroy the ecosystem for a long time (Vedio1). It happens on slopes that have a combination of all positive stress and negative resistance factors, but the actual movement begins when some sudden change, like heavy rain or earthquake, triggers it.

Mountains experience most landslides during rainy seasons. Rainwater increases the weight of the regolith and also lubricates it. In cold regions, snow plays a similar role. Sometimes frozen rock mass may begin to slide, but as the heat of friction melts ice crystals and water content increases, the movement transforms into flow type.

Concavity of the slump’s shear plane results in the slumping blocks’ backward tilt (Fig.). The mass also breaks into blocks, creating a terraced surface (Fig.). The foot of the slope has a mound called ‘Toe’. The area from where the movement started is marked by a steep scar (Fig.).

Debris Fall and Rock Fall

Velocity in the fall type ranks highest among all mass-wasting types. Movement in fall type is vertical or almost vertical, and long cliff faces are an ideal site for it. The motion may start as a slide but may change to topple and terminate as a fall.

Slopes that have experienced glaciation are susceptible to rock falls (Arthur Bloom 1998) because unloading and frost action separate large rocks from the main slope; later, these blocks may be shaken off by triggers like earthquake tremors.

Falling rock pieces are dangerous to lower slopes, especially transport links and settlements. To manage this risk, rocks have been covered by steel net along the Sion-Panvel highway (Fig.).

Subsidence

This type of mass wasting does not involve a shear plane or horizontal displacement. Surface layers move almost vertically downwards when they lose support from beneath. Sub-surface weakness may be caused by several factors, for example:

Mining,
Excess withdrawal of underground water,
Removal of rocks by carbonation in Karst regions,
Removal of sub-surface fluid lava.

Other Types

Besides the above-mentioned types of mass wasting, there are other examples with minor variations in characteristics. Some of these are:

Avalanche: Any sudden and disastrous landslide may be called an avalanche (Bloom 2003). It is a phenomenon of humid regions (Thornbury, 1969). It involves weathered rock mass (that may have ice), both slide and flow mechanics, and rapid movement. If a slope experiences repeated debris avalanches, clear channels are cut, called ‘debris chute’.

Rockslide and Topple: Large rock blocks are moved if the surface beneath them is lubricated or is softened to move plastically. The surface blocks ride over the mobile underlying material. Destabilized while gliding, these blocks tend to fall, which is called toppling.

Spreading: This involves multiple blocks. The mechanism of this movement is similar to rock glide, but it is a lateral movement. Cambering is an example of this type. Cambering occurs in the glacial environment, where sediment moves from hillsides towards valleys and carries along sedimentary blocks.

Liquefaction: Solid surface (like sand and clay) is shaken during earthquake tremors, and grain compaction within the rock is loosened. This allows clay-rich rocks to behave like plastic matter, causing a typical spread movement known as ‘liquefaction’. It can uproot buildings from their foundations and move them.

Mass Wasting: Concept, Factors and Types

Mass Wasting: Concept

Mechanics of Mass Wasting