Mass Wasting2.doc

Mass Wasting and Classification

Introduction Mass wasting is the down slope movement of rock, regolith, and soil, under the influence of gravity, also called mass movement. Mass wasting is referred has non-technical term "landslide", is the down-slope movement of a mass of sediment or rock due mainly to the force of gravity. The "mass" part of the name implies that a somewhat coherent grouping of sediment/rock begins moving downward due to the force of gravity, and usually in combination with some triggering mechanism such as an earthquake or rapid erosion of the base of a slope. The "wasting" part of mass wasting means that a cliff or mountain slope is diminishing in size, or wasting away. This can occur suddenly with tremendous destructive force, or very slowly with only a gradual alteration of Earth’s surface over a period of many years. Mass wasting, also known as slope movement is the geomorphic process by which soil, regolith, and rock move down slope under the force of gravity. Mass wasting include creep, slides, flows, topples, and falls, each with their own characteristic features, and take place over timescales from seconds to years. Mass wasting occurs on both terrestrial and submarine slopes. When the gravitational force acting on a slope exceeds its resisting force, slope failure (mass wasting) occurs. The slope material's strength and cohesion and the amount of internal friction between materials help maintain the slope's stability and are known collectively as the slope's shear strength. The steepest angle that cohesion less slope can maintain without losing its stability is known as its angle of repose. When a slope possesses this angle, its shear strength perfectly counterbalances the force of gravity acting upon it. Department of Applied Geology

Mass Wasting and Classification

Mass wasting may occur at a very slow rate, particularly in areas that are very dry or those areas that receive sufficient rainfall such that vegetation has stabilized the surface. It may also occur at very high speed, such as in rock slides or landslide, with disastrous consequences, both immediate and delayed, e.g., resulting from the formation of landslide dam. Factors that change the potential of mass wasting include: change in slope angle; weakening of material by weathering; increased water content; changes in vegetation cover and overloading. Gravity factor The force of gravity is downward, towards Earth’s center. As gravity pulls downward on material comprising a tilted or sloping portion of Earth’s surface, a translational force is formed within the slope sediment/rock. This force creates shear stress within the slope's material, reducing the slope's strength and making it more prone to mass wasting. So, since gravity is always in effect, there is always the possibility of mass wasting of a sloped surface. Note that the steeper the slope, the more in line its material components (sediment and/or rock) are with gravity, so the more likely is mass wasting of that slope. See the diagrams below to better visualize the effects of gravity on slope material.

Mass wasting is much more likely on the slope shown in this diagram because the slope is more in line with the force of gravity than in the top diagram. Force of Gravity Department of Applied Geology

Mass Wasting and Classification

Rock and or sediment comprising a slope is held together in a variety of ways, giving that slope its shear strength. This shear strength resists the shear stress placed on the slope material by gravity. Listed below are some of the factors related to a slope's shear strength, and therefore its ability to resist mass wasting. If slope material is composed of sediment then mass wasting can be resisted by: 1. Friction between sediment grains in contact with each holds the loose grains together. The greater the friction between sediment grains, then the greater the shear strength of a slope. 2. The presence of a small amount of water which sticks sediment grains to each other. Note that too much water has an opposite effect, reducing a slope's shear strength. 3. Plant roots which physically bind sediment grains together, and anchor the sediment to bedrock. The best situation is to have a combination of small plants which protect slope sediment from the impacts of rain drops and water runoff, and trees which send roots deeply into the sediment as well as underlying rock. If slope material is composed of rock then mass wasting is resisted by: A. the formation of natural cement which locks sediment grains together (present in sedimentary rocks) B. Interlocking mineral crystals within the rock (common to igneous and metamorphic rocks). A slope composed of solid rock will be much more resistant to the many causes of mass wasting than a slope composed of sediment, no

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Mass Wasting and Classification

matter how many plants are binding the sediment together. So, given the option, always choose a building location underlain by solid rock. CLASSIFICATION The movements of rock waste may be either slow or fast. A classification of movements may be done on the basis of 1. Kind of material moved. 2. Size of the materials. 3. Rate of movement. 4. Water content. 5. Texture of the material. 6. Relation of moving material to the solid surface. The types of the materials moved are rock, earth, solid debris and mud. The classification is essentially based on the amount of water present in the debris, because water reduces the cohesive strength of finegrained materials and act as a lubricant in the down slope movement of debris. As the proportion of water present in the debris rises, the slope angle required for the transportation of materials becomes less and less. In other words, as the amount of debris in proportion to water present increases, steeper and steeper slopes are needed for the transport of the materials. Mass movements are divided into three groups based on rate of movement: 1. Flowage. Department of Applied Geology

Mass Wasting and Classification

2. Sliding. 3. Subsidence.

A. FLOWAGE

B. SLIDING

C.

Translational Rotational Falls

SUBSIDENCE A. Natural

Slow

Rapid

flowage (creep)

flowage slides Earth Rock slides

slides Single

(toppling) subsidence Rock

1. Shallow flows

Rotational falls

2.

slides

Deep

creep

Mud

Debris slides

flows

Multiple

Debris falls

1. Soil, 2.

Sub-

rotational

Mass

aqueous

slides

creep

B.

Artificial

subsidence

Rock glaciers

1.

Talus

Creep,

Stone

2. Rock

streams

Creep Solifluction Flowage: When water is present in the debris then the debris tends to flow down with the water. When the amount of water present is relatively less, the movement of debris is slow and when the amount of water is more, the movement of debris is faster. Consequently, it is possible to identify two sub-divisions. a. Slow flowage: The ground may be moving down slope at as such low rate as few centimeters a year or even less, include

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Mass Wasting and Classification

Slow flowage is gradual, almost imperceptible down slope transit of soil and may sometimes take places even under the cover of vegetations is observed in both tropical and temperate climate. The rate of downgrade movement may vary from one millimeter to several centimeters in a year. In most cases, soil creep is essentially a surface phenomenon in which only the top one meter or so of the soil is involved in the failure. 1. Soil creep, 2. Talus creep, 3. Rock creep and 4. Solifluction. b. Rapid flowage: In the rapid flowage the movement of failing mass may be easily visible and the mass may travel a few meters or more a day. The conditions causing flowage in the two classes may be closely related or entirely different.

1. Soil creep: Soil creep in which movement is relatively fast (10 centimeters a year or more) and the material involved is mostly made of rock fragments. It is a account of soil creep that telegraph, poles and trees are found bend downwards on slopes Department of Applied Geology

Mass Wasting and Classification

Talus creep: Talus or Scree is a slope built up by an accumulation of rock fragments at the foot of a cliff or ridge.

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Mass Wasting and Classification

2. Solifluction:

Slow flowing of water saturated earth or soil gravity is known as solifluction is found in permanently frozen grounds. Solifluction differs from soil creep in water content. In solifluction the soil movement occurs in almost saturated condition where as soil creep may start without much water, due to presence of enough water. To solifluction the conditions are: 1. The water supply from melting of ground ices and snow. 2. Slope 3. Permanently frozen ground. 4. Supply of the rock waste. 5. Solifluction is a slower, continuous and surfaceal movement. B. Rapid flow movement 1. In rapid flow movement the amount of water present in the debris is more, so that these is through lubrication of the debris and mass movement takes place principally or wholly in the form of flow. Rapid movement includes 1. Mud flow, 2. Earth flow or slump, 3. Avalanches, 4. Rock slide or debris slide and Falls

Department of Applied Geology

Mass Wasting and Classification 1. Mud flows: Mud flow is a highly fluid, high velocity mixture of

sediment and water that has a consistency of wet concrete. These usually result from heavy rains in areas where there is an abundance of unconsolidated sediment that can be picked up by streams. Thus, after a heavy rain streams can turn into mud flows as they pick up more and more loose sediment. Mud flows can travel from long distance over gently sloping streams beds, because of their high velocity and long distance of travel they are potentially very dangerous. Debris flows or mudflows Commonly occur in volcanic areas, where they are called lahars. Mudflows generally follow established drainage patterns (valleys).

Road damaged by the mudflow in central Italy. 2. Earth flows (slump): Department of Applied Geology

Mass Wasting and Classification

Earth flows are usually associated with heavy rain and move at velocities between several Cm/yr and 100s of m/day. They usually remain active for long period of time; they generally tend to be narrow tongue – like features that begin at a scarp or small cliff. Slumps

Slumping involves the rotational movement of a material downward and outward along a curved shearing plane. Or slides wherein downward rotation of rock or regolith occurs along a concave-upward curved surface (rotational slides). The upper surface of each slump block remains relatively undisturbed, as do the individual blocks. Slumps leave arcuate scars or depressions on the hill slope. Slumps can be isolated or may occur in large complexes covering thousands of square meters. They often form as a result of human activities, and thus are common along roads where slopes have been over steepened during construction. They are also common along river banks and sea coasts, where erosion has under-cut the slopes. Heavy rains and earthquakes can also trigger slumps. Slump Or Slope failure Movement of a mass of rock or unconsolidated material as a unit along a curved surface occurs along over steepened slopes

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Mass Wasting and Classification

Large-scale landslides of the Slump variety have occurred in the arid and semi-arid. The blocks formed by these slumps are called Toreva blocks

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Mass Wasting and Classification

2. Rock Avalanches: Any large catastrophic landslide may be called an Avalanche. They can include broken rock, ice and snow, usually so mixed that the material term debris is most appropriate. Debris avalanches are very high velocity flow of large volume mixture of rock and regolith that results from complete collapse of a mountainous slope. They move down slope and then can travel from considerable distances along relatively gentle slopes and they are often triggered by earth quakes and volcanic eruptions.

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Mass Wasting and Classification

B. Rock Sliding: In sliding there is a rapid down slope movement of principally dried materials. There are different kinds of slide such as a. rock slide, b. debris slide and c. land slide.

Rocks falls Separated rock blocks move down on step surface of a cliff. These are known as rocks falls. These are common in mountains areas during spring season when freezing and thawing is repeated. They may involve large rock masses and result in severe loss of life and damage to property.

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Mass Wasting and Classification

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Mass Wasting and Classification

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Mass Wasting and Classification

Rock falls from steep hillsides frequently disrupt road traffic in the mountainous regions or when a piece of rock on a steep slope becomes dislodged and falls down the slope.

In land slide a mass of rock and debris separates from the underlying rock along a shear plane and most rapidly down slope under the influence of gravity as a result of failure along the shear plane. In rock slide and debris slide the failure surface is shallow and frequently parallel to the surface and when there is mass movement of consolidate rockes is called rock slide and when there is mass movement of unconsolidated debris or regolite it is called debris slide. Debris falls:-

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Mass Wasting and Classification

Debris falls are similar, except they involve a mixture of soil, regolith, vegetation, and rocks. A rock fall may be a single rock or a mass of rocks and the falling rocks can dislodge other rocks as they collide with the cliff. Because this process involves the free fall of material, falls commonly occur where there are steep cliffs. At the base of most cliffs is an accumulation of fallen material termed talus. Debris falls are similar, except they involve a mixture of soil, regolith and rocks. C. SUBSIDENCE:- It is defined as sinking or settling of the ground in almost vertically downward direction which may occurs because of removal of natural support from the underground or due to compactions of the weaker rocks under the load from overlying mass. Types of subsidence :There are two types of subsidence:1. Natural subsidence:These are the type of landslides which occur usually in low lying area due to natural activities. Department of Applied Geology

Mass Wasting and Classification

Example:- Plate tectonics (regional changes in land and water) Karst (chemical weathering of soluble rocks)

1. Artificial subsidence:These are type of the landslides which occur usually in low lying area due to manmade activities. Example: Mining, loading, oil extraction, ground water etc. Sinking may vary from a few centimeters to many meters and may be due either to natural or artificial causes. Natural causes responsible for subsidence:a.

Solution of sub- surface rocks:Ground water is a active solvent many rocks especially

lime stones, dolomite and gypsum. When such chemically susceptible rocks support the ground in any given region, there is possibility of dissolution (partial or complete) of these rocks with the passage of time. This may eventually results in settling or subsidence of the ground from above. b.

Geological constitution:Sometimes the geological region may be responsible for

settling of the ground. Thus, when the layers of weak plastic character such as those of peat and shale or deposits of coarse sand and silt are over lain by other deposits, they may result from settlement due to the load of the over lying material.

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Mass Wasting and Classification

Among the artificial causes of the subsidence, the following deserve special mention:Removal of material- It is a matter of common knowledge that subsidence are settling of the ground is more in the mining and oil extraction regions. It is because with the extraction of economically important minerals and also oil and ground water, the ground loses critical support below and hence sinking take places at the surface. Settling of ground associated with removal of ground water is a commonly observed phenomenon. In other case, ground may contract because of drainage. These are especially true for swampy regions. POSSIBLE PREVENTIVE MEASURES A. Slope Reduction 1. Reduce slope angle 2. Place additional material at toe of slope 3. Reduce the load on slope B. Retention Structures 1. Plant ground cover 2. Retaining walls and structures 3. Terrace slope (farms) C. Fluid Removal 1. Diverting surface runoff 2. Subsurface drainage

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Mass Wasting and Classification

3. Hot air blown through boreholes (good use for politicians) D. Others 1. Drive vert. piles into foot of shallow slide (? effective) 2. Rock bolts (used in tunneling and mining, too) 3. Harden soil (dry and bake clay-rich soil) 4.

Modify

slope

geometry,

load,

and

dewatering

combinations 5. Expensive 3.

Slides

Rockslide: Blocks of bedrock slide down a slope, Generally very fast and destructive

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Mass Wasting and Classification

Rock slides and debris slides result when rocks or debris slide down a preexisting surface, such as a bedding plane, foliation surface, or joint surface (joints are regularly spaced fractures in rock that result from expansion during cooling or uplift of the rock mass). Piles of talus are common at the base of a rock slide or debris slide. Slides differ from slumps in that there is no rotation of the sliding rock mass along a curved surface. Earth flows Form in humid areas on hillsides following heavy rain or melting snow, in fine-grained materials (clay and silt). Also occurs at the toe of slumps. Rate of movement varies (less than 1 mm per day to several meters per day), but may be long-lived (days to years). Includes the liquefaction associated with earthquakes.

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Mass Wasting and Classification

Avalanche is a flow of snow down a mountainside, though rock slides and debris flows are also sometimes called avalanches. Avalanches are one of the biggest dangers in the mountains for both life and property. Many factors contribute to avalanches. Pointrelease avalanches occur when the weight of the snowpack exceeds the shear strength within it, and are most common on steeper terrain. In fresh, loose snow the release is usually at a point and the avalanche then gradually widens down the slope as more snow is entrained, usually forming a tear-drop appearance. This is in contrast to a slab avalanche. Slab avalanches account for around 90% of avalanche-related fatalities, and occur when there is a strong, stiff layer of snow known as a slab. These are usually formed when snow is deposited by the wind on a lee slope. When the slab fails, the fracture, in a weak layer, very rapidly propagates so that a large area, that can be hundreds of metres in extent and several metres thick, starts moving almost instantaneously. The third starting type is a slush avalanche which occurs when the snowpack becomes Department of Applied Geology

Mass Wasting and Classification

saturated by water. These tend to also start and spread out from a point. As avalanches move down the slope they may entrain snow from the snowpack and grow in size. The snow may also mix with the air and form a powder cloud. An avalanche with a powder cloud is known as a powder snow avalanche. The powder cloud is a turbulent suspension of snow particles that flows as a gravity current. Powder snow avalanches are the largest avalanches and can exceed 300 km/h and 10,000,000 tonnes of snow, they can flow for long distance along flat valley bottoms and even up hill for short distances. Creep Creep is a long term process. The combination of small movements of soil or rock in different directions over time are directed by gravity gradually downslope. The steeper the slope, the faster the creep. The creep makes trees and other shrubs curve to reach the sun light. These often trigger land slides because the dirt underneath is not very strong. The trees most of the time die out because of lack of water and sun, and these rarely happen in wet climates. The rate of soil creep down a slope depends on the steepness (gradient) of the slope, water absorption and content, type of sediment and material, and lastly vegetation. The rate of creep will take into account all of these factors to decide whether or not the hillside will progress downward. Creep is what is responsible for the rounded shape of hillsides.

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Mass Wasting and Classification

Water is a very important factor when discussing soil deformation and movement. When building a sand castle at the beach you would notice that the presence of water making damp sand aids in keeping your castle standing up. The water will offer cohesion to the sand which will bind the sand particles together. However, when you pour a bucket of water over your sand castle or it gets hit by a wave, it destroys it. This is because the presence of too much water fills all the pores between the grains with water creating a slip plain between the particles and offering no cohesion causing them to slip and slide away. This holds true not only for sand castles but for hillsides and creep as well. The presence of water may help the hillside stay put and give it that cohesion but in a very moist/wet environment or during/after a large amount of precipitation the pores between the grains could become saturated with water and cause the ground to slide along the slip plain it creates. Creep can be caused by the expansion of materials such as clay when they are exposed to water. Clay expands when wet, then contracts after drying. The expansion portion pushes downhill, then the contraction results in consolidation at the new offset. Vegetation also can play a role with slope stability and creep. When a hillside contains many trees, ferns, and shrubs their roots can create an interlocking network that can strengthen unconsolidated material. They also aid in absorbing the access water in the soil to help keep the slope stable. They also however, add to the weight of the slope giving gravity that much more of a driving force to act on in pushing the slope downward. Slopes with the absence of vegetation have a greater chance of movement. Department of Applied Geology

Mass Wasting and Classification

Design engineers sometimes need to guard against downhill creep during their planning to prevent building foundations from being undermined. Pilings are planted sufficiently deep into the surface material to guard against this behavior. Solifluction

In geology, solifluction, also known as soil fluction or soil creep, is a type of mass wasting where waterlogged sediment slowly moves downslope over impermeable material. It can occur in any climate where the ground is saturated by water, though it is most often found in periglacial environments where the ground is permanently frozen (permafrost). A term often used for deposits formed under periglacial conditions is Gelifluction. During warm seasonal periods the surface layer (active layer) melts and literally slides across the frozen underlayer, slowly moving downslope due to frost heave that occurs normal to the slope. This type of mass wasting can occur on slopes as shallow as 0.5 degrees at a rate of between 0.5 and 15 cm per year. In Germany the solifluction deposits from the Younger Dryas are found to have a consistent thickness of 0.4–0.7 metres. Subsidence Department of Applied Geology

Mass Wasting and Classification

subsidence is the motion of a surface (usually, the Earth's surface) as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation.

Land subsidence occurs when large amounts of ground water have been withdrawn from certain types of rocks, such as fine-grained sediments. The rock compacts because the water is partly responsible for holding the ground up. When the water is withdrawn, the rocks falls in on itself. You may not notice land subsidence too much because it can occur over large areas rather than in a small spot, like a sinkhole. That doesn't mean that subsidence is not a big event -- states like California, Texas, and Florida have suffered damage to the tune of hundreds of millions of dollars over the years.

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Mass Wasting and Classification

Dissolution of limestone Subsidence frequently occurs in karst terrains, where dissolution of limestone by fluid flow in the subsurface causes the creation of voids (i.e. caves). If the roof of these voids becomes too weak, it can collapse and the overlying rock and earth will fall into the space, causing subsidence at the surface. This type of subsidence can result in sinkholes which can be many hundreds of meters deep and can provide areas of ecological isolation which see the evolution of new branches of animal and plant life. Mining-induced Several types of sub-surface mining, and specifically methods which intentionally cause the extracted void to collapse (such as pillar extraction, longwall mining and any metalliferous mining method which utilises "caving" such as "block caving" or "sub-level caving") will result in surface subsidence. Mining induced subsidence is relatively predictable in its magnitude, manifestation and extent, except where a sudden pillar or near-surface underground tunnel collapse occurs (usually very old workings). Mining induced subsidence is nearly always very localised to the surface above the mined area, plus a margin around the outside [1]. The vertical magnitude of the subsidence itself typically does not cause problems, except in the case of drainage (including natural drainage) - rather it is the associated surface compressive and tensile strains, curvature, tilts and horizontal displacement that are the cause of the worst damage to the natural environment, buildings and infrastructure. Where mining activity is planned, mining-induced

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Mass Wasting and Classification

subsidence can be successfully managed if there is co-operation from all of the stakeholders [2]. This is accomplished through a combination of careful mine planning, the taking of preventative measures, and the carrying out of repairs post-mining.

4.

Causes of land subsidence Land subsidence is most often caused by human activities, mainly from the removal of subsurface water. This pictures shows a fissure near Lucerne Department of Applied Geology

Mass Wasting and Classification

Lake in San Bernardino County, Mojave Desert, California. The probable cause was declining ground-water levels. Here are some other things that can cause land subsidence: Ground-water pumping and land subsidence Compaction of soils in some aquifer systems can accompany excessive ground-water pumping and it is by far the single largest cause of subsidence. Excessive pumping of such aquifer systems has resulted in permanent subsidence and related ground failures. In some systems, when large amounts of water are pumped, the subsoil compacts, thus reducing in size and number the open pore spaces in the soil the previously held water. This can result in a permanent reduction in the total storage capacity of the aquifer system. Topples

Topples are instances when blocks of rock pivot and fall away from a slope. Triggers of mass wasting Soil and regolith remain on a hill slope only while the gravitational forces are unable to overcome the frictional forces keeping the material in place (see Slope stability). Factors that reduce the frictional resistance relative to the down slope forces, and thus initiate slope movement, can include: 

Seismic shaking



Increased overburden from structures



Increased soil moisture



Reduction of roots holding the soil to bedrock

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Mass Wasting and Classification



Undercutting of the slope by excavation or erosion



Weathering by frost eave



Bioturbation

Causes and consequences of Mass Movements Basically, mass movements occur whenever the downward pull of gravity overcomes the forces resisting sliding or flow. The down slope pull tending to cause mass movements, called the shearing stress, is related to the mass of material and to slope angle. Counteracting the shearing stress is friction or, in the case of a cohesive solid, shear strength, sliding occurs. Sudden movements may be set off by a triggering mechanism, such as an earthquake. Mass Movements, in turn may cause secondary problems, such as folding. 1)Earthquake The vigorous shaking of an already-unstable slope by seismic waves may cause it to fail. Typically, the higher the magnitude of an earthquake, the more mass wasting will occur. 2) Over-steepening of a slope A slope whose material is stable at a fairly gentle slope angle may become unstable if its slope angle becomes steeper. This can occur where a stream cuts into a valley slope, or where ocean waves remove the base (toe) of a slope. Also, sometimes humans over-steepen slopes when constructing building sites, or roads in mountainous areas as shown in the image to the left.

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Mass Wasting and Classification

3) Removal of slope vegetation A slope denuded of vegetation loses surface protection from the impacts of raindrops, which can mobilize sediment grains with water flowing down slope. The roots of plants on a slope can play a significant role in binding sediment together, reducing the likelihood of rapid or sudden mass wasting of a slope.

Removal of the vegetation, due to human cutting or

harvesting, or due to fire, reduces strength of the slope 4) Introduction of water into slope material An excessive amount of water within a slope increases its mass, increasing shear stress within parts of the slope, especially along rock fractures tilted in the same direction as the slope surface. If the slope is composed of sediment where grains are not cemented together, excess water can float the grains apart, reducing friction (and shear strength).

Both of these

situations, often associated with heavy rainfall or rapid snowmelt, can lead to mass wasting 5) Ice wedging Water can flow into even the narrowest of rock fractures.

If the

temperature then drops below freezing, ice crystals will form, expanding in volume by 9 %. This is a very powerful force that can wedge apart rocks, often causing them to fall from steep slopes in mountains and canyons 6) Biological activity Animals moving along steep slopes may loosen rocks, sending them crashing down slope. Some animals are more destructive than others, rolling rocks down slope on purpose. Department of Applied Geology

Mass Wasting and Classification

Causes of Mass Movements Mass movements are caused by various conditions: 

Volcanic activity many times causes huge mudflows when the icy cover of a volcano melts and mixes with the soil to form mud as the magma in the volcano stirs preceding an eruption.



Mudslides can also develop when water rapidly accumulates in the ground, such as during heavy rainfall or rapid snow melt, changing the earth into a flowing river of mud or "slurry.".



Earthquake shocks cause sections of mountains and hills to break off and slide down.



Human modification of the land or weathering and erosion help loosen large chunks of earth and start them sliding downhill.



Vibrations from machinery, traffic, weight loading from accumulation of snow; stockpiling of rock or ore; from waste piles and from buildings and other structures.



However, the trigger mechanism for mass movement is the gravitational pull of the earth on soil, rocks, and mud.

Preventing Measures of landslides The key to preventing damage from landslides is to identify and avoid developing landslide prone areas such as steep, unstable hillsides. However, if some of these areas must be developed then building codes should require extensive efforts to insure slope stabilization: 

Vegetation of unstable slopes



Installation of drainage and runoff channeling structures

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Mass Wasting and Classification



Benching and regrading of slopes to lessen their steepness



Stabilization structures such as retaining walls, deeply sunk pylons, and backfilled supports

Good slope engineering is expensive and the temptation to cut corners is great. However, landslide damage is far more expensive and estimates have shown that for every dollar spent on slope stabilization, between 10 and 2000 dollars are saved over the long term. For example, a landslide in Utah in 1983 dammed a river and caused flooding of the town of Thistle, a railroad, and a major US highway. Total damage was about $200 million. The slide was triggered by a high water table due to high precipitation and was a reactivation of an older slide that had a well known history of movement. Estimates suggest that the slide could have been predicted to be imminent and could have been prevented for about $300,000 worth of drainage engineering. Benefit to cost ratio: about 100:1.

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Mass Wasting and Classification

Creep

Creep is a long term process. The combination of small movements of soil or rock in different directions over time are directed by gravity gradually downslope. The steeper the slope, the faster the creep. The creep makes trees and other shrubs curve to reach the sun light. These often trigger land slides because the dirt underneath is not very strong. The trees most of the time die out because of lack of water and sun, and these rarely happen in wet climates. The rate of soil creep down a slope depends on the steepness (gradient) of the slope, water absorption and content, type of sediment and material, and lastly vegetation. The rate of creep will take into account all of these factors to decide whether or not the hillside will progress downward. Creep is what is responsible for the rounded shape of hillsides.

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Mass Wasting and Classification

Water is a very important factor when discussing soil deformation and movement. When building a sand castle at the beach you would notice that the presence of water making damp sand aids in keeping your castle standing up. The water will offer cohesion to the sand which will bind the sand particles together. However, when you pour a bucket of water over your sand castle or it gets hit by a wave, it destroys it. This is because the presence of too much water fills all the pores between the grains with water creating a slip plain between the particles and offering no cohesion causing them to slip and slide away. This holds true not only for sand castles but for hillsides and creep as well. The presence of water may help the hillside stay put and give it that cohesion but in a very moist/wet environment or during/after a large amount of precipitation the pores between the grains could become saturated with water and cause the ground to slide along the slip plain it creates. Creep can be caused by the expansion of materials such as clay when they are exposed to water. Clay expands when wet, then contracts after drying. The expansion portion pushes downhill, then the contraction results in consolidation at the new offset. Vegetation also can play a role with slope stability and creep. When a hillside contains many trees, ferns, and shrubs their roots can create an interlocking network that can strengthen unconsolidated material. They also aid in absorbing the access water in the soil to help keep the slope stable. They also however, add to the weight of the slope giving gravity that much more of a driving force to act on in pushing the slope downward. Slopes with the absence of vegetation have a greater chance of movement.

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Mass Wasting and Classification

Design engineers sometimes need to guard against downhill creep during their planning to prevent building foundations from being undermined. Pilings are planted sufficiently deep into the surface material to guard against this behavior.

Solifluction

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Mass Wasting and Classification

In geology, solifluction, also known as soil fluction or soil creep, is a type of mass wasting where waterlogged sediment slowly moves downslope over impermeable material. It can occur in any climate where the ground is saturated by water, though it is most often found in periglacial environments where the ground is permanently frozen (permafrost). A term often used for deposits formed under periglacial conditions is Gelifluction. During warm seasonal periods the surface layer (active layer) melts and literally slides across the frozen underlayer, slowly moving downslope due to frost heave that occurs normal to the slope. This type of mass wasting can occur on slopes as shallow as 0.5 degrees at a rate of between 0.5 and 15 cm per year. In Germany the solifluction deposits from the Younger Dryas are found to have a consistent thickness of 0.4–0.7 metres. Subsidence subsidence is the motion of a surface (usually, the Earth's surface) as it shifts downward relative to a datum such as sea-level. The opposite of Department of Applied Geology

Mass Wasting and Classification

subsidence is uplift, which results in an increase in elevation.

Land subsidence occurs when large amounts of ground water have been withdrawn from certain types of rocks, such as fine-grained sediments. The rock compacts because the water is partly responsible for holding the ground up. When the water is withdrawn, the rocks falls in on itself. You may not notice land subsidence too much because it can occur over large areas rather than in a small spot, like a sinkhole. That doesn't mean that subsidence is not a big event -- states like California, Texas, and Florida have suffered damage to the tune of hundreds of millions of dollars over the years.

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Mass Wasting and Classification

Dissolution of limestone Subsidence frequently occurs in karst terrains, where dissolution of limestone by fluid flow in the subsurface causes the creation of voids (i.e. caves). If the roof of these voids becomes too weak, it can collapse and the overlying rock and earth will fall into the space, causing subsidence at the surface. This type of subsidence can result in sinkholes which can be many hundreds of meters deep and can provide areas of ecological isolation which see the evolution of new branches of animal and plant life. Mining-induced Several types of sub-surface mining, and specifically methods which intentionally cause the extracted void to collapse (such as pillar extraction, longwall mining and any metalliferous mining method which utilises "caving" such as "block caving" or "sub-level caving") will result in surface subsidence. Mining induced subsidence is relatively predictable in its magnitude, manifestation and extent, except where a sudden pillar or near-surface underground tunnel collapse occurs (usually very old workings). Mining induced subsidence is nearly always very localised to the surface above the mined area, plus a margin around the outside [1]. The vertical magnitude of the subsidence itself typically does not cause problems, except in the case of drainage (including natural drainage) rather it is the associated surface compressive and tensile strains, curvature, tilts and horizontal displacement that are the cause of the worst damage to the natural environment, buildings and infrastructure. Where mining activity is planned, mining-induced subsidence can be successfully managed if there is co-operation from all of the stakeholders [2]. This is

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Mass Wasting and Classification

accomplished through a combination of careful mine planning, the taking of preventative measures, and the carrying out of repairs post-mining.

Causes of land subsidence Land subsidence is most often caused by human activities, mainly from the removal of subsurface water. This pictures shows a fissure near Lucerne Department of Applied Geology

Mass Wasting and Classification

Lake in San Bernardino County, Mojave Desert, California. The probable cause was declining ground-water levels. Here are some other things that can cause land subsidence: Ground-water pumping and land subsidence Compaction of soils in some aquifer systems can accompany excessive ground-water pumping and it is by far the single largest cause of subsidence. Excessive pumping of such aquifer systems has resulted in permanent subsidence and related ground failures. In some systems, when large amounts of water are pumped, the subsoil compacts, thus reducing in size and number the open pore spaces in the soil the previously held water. This can result in a permanent reduction in the total storage capacity of the aquifer system.

Department of Applied Geology

Mass Wasting and Classification

A Talus cone at the base of Mount Smith, Canadian Rockies. Causes of Mass wasting When there is a little friction between particles they will not lie on a high angle slope. Another factor in the down slope movement of material is water or fluid. Material that is stable below it critical angle of repose

may become unstable when saturated water. The water has the

effect of adding significant weight and reducing friction between a load of materials and the surface upon which it rests. Heavy rain an a slope known to be near the angle of repose , therefore can spell trouble in a populated area.

Department of Applied Geology

Mass Wasting and Classification

The angle of repose is different for different kinds of material. Coarser material may have an angle of repose of up to about 37 0, while the angle decreases with grain size down to a range of 20 to 35 0 for fine dry sand. Angular material generally can have a higher angle of repose than rounded material. There are other factors, aside from lubrication and Steepness of slope, that can contribute significantly to down slope movement where vegetation is lacking, for e.g. there is no root system to stabilize the soil an a slope and erosion is promoted

. Down slope

movement is also promoted as material is loosened during freeze thaw cycles. Certain rock types, such as shale’s, may become very slippery when wet so that overlying rock layers may slide along the shale. Finally bedrock structures such an bedding planes and alignment of crystals with in the rock, may form planes along which the material may be weaker and movement takes place along these planes. However an additional force is often responsible for actually initiating the mass movement, often materials on the slopes are ready to move downward if only a little extra push is given push. This additional push sometimes

comes in the form of an earthquake, large amounts of

precipitation or changes in slope by natural or human activity Conclusion Mass wasting is the down slope movement of rock, regolith, and soil, under the influence of gravity, also called mass movement. The "wasting" part of mass wasting means that a cliff or mountain slope is diminishing in size, or wasting away. This can occur suddenly with tremendous destructive force, or very slowly with only a gradual alteration of Earth’s surface over a period of many years. Mass wasting, also known as slope movement is the geomorphic process by which soil, regolith, and Department of Applied Geology

Mass Wasting and Classification

rock move down slope under the force of gravity. Factors that change the potential of mass wasting include: change in slope angle; weakening of material by weathering; increased water content; changes in vegetation cover and overloading.

Reference: Text book of Geomorphology By Dayal , Engineering Geology book by Parbin Singh. Text book of geomorphology Blooms.

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Mass Wasting and Classification

Contents

Introduction Force of Gravity Classification Type of Mass Wasting 1. Flowage. 2. Sliding. 3. Subsidence. Causes of Mass Wasting Conclusion Reference:

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