Soil compaction
Encyclopedia
In Geotechnical engineering
, soil compaction is the process in which a stress applied to a soil causes densification as air is displaced from the pores between the soil grains. When stress is applied that causes densification due to water (or other liquid) being displaced from between the soil grains then Consolidation (soil), not compaction, has occurred. Normally, compaction is the result of heavy machinery compressing the soil
, but it can also occur due to the passage of (e.g.) animal feet.
In Soil Science
and Agronomy
Soil compaction is usually a combination of both engineering compaction and consolidation, so may occur due to a lack of water in the soil, the applied stress being internal suction due to water evaporation as well as due to passage of animal feet. Affected soils become less able to absorb rainfall, thus increasing runoff and erosion
. Plants have difficulty in compacted soil because the mineral grains are pressed together, leaving little space for air and water, which are essential for root
growth. Burrowing animals also find it a hostile environment, because the denser soil is more difficult to penetrate. The ability of a soil to recover from this type of compaction depends on climate, mineralogy and fauna. Soils with high shrink-swell capacity
, such as Vertisols, recover quickly from compaction where moisture conditions are variable (dry spells shrink the soil, causing it to crack). But clays which do not crack as they dry cannot recover from compaction on their own unless they host ground-dwelling animals such as earthworm
s — the Cecil soil series
is an example.
. Generally, those that can apply significant amounts of shear as well as compressive stress, are most effective. The available techniques can be classified as:
The construction plant available to achieve compaction is extremely varied and is described elsewhere
.
When an area is to be filled or backfilled the soil is placed in layers called lifts. The ability of the first fill layers to be properly compacted will depend on the condition of the natural material being covered. If poor material is left in place and covered over, it may compress over a long period under the weight of the earth fill, causing settelment cracks in the fill or in any structure supported by the fill. In order to determine if the natural soil will support the first fill layers, an area can be proofrolled. Proofrolling consists of utilizing a piece heavy construction equipment (typically, heavy compaction equipment or hauling equipment) to roll across the fill site and watching for poor areas to be revealed. Poor areas will be indicated by the development of rutting or ground weaving.
To ensure adequate soil compaction is achieved, project specifications will indicate the required soil density or degree of compaction that must be achieved. These specifications are generally recommended by a geotechnical engineer in a geotechnical engineering report.
The soil type - that is, grain-size distributions, shape of the soil grains, specific gravity of soil solids, and amount and type of clay minerals present - has a great influence on the maximum dry unit weight and optimum moisture content. It also has a great influence on how the materials should be compacted in given situations. Compaction is accomplished by use of heavy equipment. In sands and gravels, the equipment usually vibrates, to cause re-orientation of the soil particles into a denser configuration. In silt
s and clay
s, a sheepsfoot roller
is frequently used, to create small zones of intense shear
ing, which drives air out of the soil.
Determination of adequate compaction is done by determining the in-situ density of the soil and comparing it to the maximum density determined by a laboratory test. The most commonly used laboratory test is called the Proctor compaction test
and there are two different methods in obtaining the maximum density. They are the standard Proctor and modified Proctor tests; the modified Proctor is more commonly used. For small dams, the standard Proctor may still be the reference.
Solutions to overcome compaction include tillage
and the zaï
-system.
Geotechnical engineering
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering is important in civil engineering, but is also used by military, mining, petroleum, or any other engineering concerned with construction on or in the ground...
, soil compaction is the process in which a stress applied to a soil causes densification as air is displaced from the pores between the soil grains. When stress is applied that causes densification due to water (or other liquid) being displaced from between the soil grains then Consolidation (soil), not compaction, has occurred. Normally, compaction is the result of heavy machinery compressing the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
, but it can also occur due to the passage of (e.g.) animal feet.
In Soil Science
Soil science
Soil science is the study of soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils.Sometimes terms which...
and Agronomy
Agronomy
Agronomy is the science and technology of producing and using plants for food, fuel, feed, fiber, and reclamation. Agronomy encompasses work in the areas of plant genetics, plant physiology, meteorology, and soil science. Agronomy is the application of a combination of sciences like biology,...
Soil compaction is usually a combination of both engineering compaction and consolidation, so may occur due to a lack of water in the soil, the applied stress being internal suction due to water evaporation as well as due to passage of animal feet. Affected soils become less able to absorb rainfall, thus increasing runoff and erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
. Plants have difficulty in compacted soil because the mineral grains are pressed together, leaving little space for air and water, which are essential for root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
growth. Burrowing animals also find it a hostile environment, because the denser soil is more difficult to penetrate. The ability of a soil to recover from this type of compaction depends on climate, mineralogy and fauna. Soils with high shrink-swell capacity
Shrink-swell capacity
The Shrink-swell capacity of clay refers to the extent to which a clay will expand when wet and retract when dry. Soil that is problematic due to high capacity is known as shrink-swell soil, or expansive soil.-Description:...
, such as Vertisols, recover quickly from compaction where moisture conditions are variable (dry spells shrink the soil, causing it to crack). But clays which do not crack as they dry cannot recover from compaction on their own unless they host ground-dwelling animals such as earthworm
Earthworm
Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...
s — the Cecil soil series
Cecil (soil)
]Originally mapped in Cecil County, Maryland in 1899, more than 10 million acres of the Cecil soil series are now mapped in the Piedmont region of the southeastern United States...
is an example.
Compaction Methods
There are several means of achieving compaction of a material. Some are more appropriate for soil compaction than others, while some techniques are only suitable for particular soils or soils in particular conditions. Some are more suited to compaction of non-soil materials such as asphaltAsphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
. Generally, those that can apply significant amounts of shear as well as compressive stress, are most effective. The available techniques can be classified as:
- Static - a large stress is slowly applied to the soil and then released.
- Impact - the stress is applied by dropping a large mass onto the surface of the soil.
- Vibrating - a stress is applied repeatedly and rapidly via a mechanically driven plate or hammer. Often combined with rolling compaction (see below).
- Gyrating - a static stress is applied and maintained in one direction while the soil is a subjected to a gyratory motion about the axis of static loading. Limited to laboratory applications.
- Rolling - a heavy cylinder is rolled over the surface of the soil. Commonly used on sports pitches. Roller-compactors are often fitted with vibratory devices to enhance their ability.
- Kneading - shear is applied by alternating movement in adjacent positions. An example, combined with rolling compaction, is the 'sheepsfoot' roller used in waste compaction at landfills.
The construction plant available to achieve compaction is extremely varied and is described elsewhere
Road roller
A road roller is a compactor type engineering vehicle used to compact soil, gravel, concrete, or asphalt in the construction of roads and foundations, similar rollers are used also at landfills or in agriculture.In some parts of the world, road rollers are still known colloquially as steam...
.
In construction
Soil compaction is a vital part of the construction process. It is used for support of structural entities such as building foundations, roadways, walkways, and earth retaining structures to name a few. For a given soil type certain properties may deem it more or less desirable to perform adequately for a particular circumstance. In general, the preselected soil should have adequate strength, be relatively incompressible so that future settlement is not significant, be stable against volume change as water content or other factors vary, be durable and safe against deterioration, and possess proper permeability.When an area is to be filled or backfilled the soil is placed in layers called lifts. The ability of the first fill layers to be properly compacted will depend on the condition of the natural material being covered. If poor material is left in place and covered over, it may compress over a long period under the weight of the earth fill, causing settelment cracks in the fill or in any structure supported by the fill. In order to determine if the natural soil will support the first fill layers, an area can be proofrolled. Proofrolling consists of utilizing a piece heavy construction equipment (typically, heavy compaction equipment or hauling equipment) to roll across the fill site and watching for poor areas to be revealed. Poor areas will be indicated by the development of rutting or ground weaving.
To ensure adequate soil compaction is achieved, project specifications will indicate the required soil density or degree of compaction that must be achieved. These specifications are generally recommended by a geotechnical engineer in a geotechnical engineering report.
The soil type - that is, grain-size distributions, shape of the soil grains, specific gravity of soil solids, and amount and type of clay minerals present - has a great influence on the maximum dry unit weight and optimum moisture content. It also has a great influence on how the materials should be compacted in given situations. Compaction is accomplished by use of heavy equipment. In sands and gravels, the equipment usually vibrates, to cause re-orientation of the soil particles into a denser configuration. In silt
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
s and clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
s, a sheepsfoot roller
Road roller
A road roller is a compactor type engineering vehicle used to compact soil, gravel, concrete, or asphalt in the construction of roads and foundations, similar rollers are used also at landfills or in agriculture.In some parts of the world, road rollers are still known colloquially as steam...
is frequently used, to create small zones of intense shear
Shear strength (soil)
Shear strength is a term used in soil mechanics to describe the magnitude of the shear stress that a soil can sustain. The shear resistance of soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts. Due to interlocking, particulate...
ing, which drives air out of the soil.
Determination of adequate compaction is done by determining the in-situ density of the soil and comparing it to the maximum density determined by a laboratory test. The most commonly used laboratory test is called the Proctor compaction test
Proctor compaction test
The Proctor compaction test is a laboratory method of experimentally determining the optimal moisture content at which a given soil type will become most dense and achieve its maximum dry density. The term Proctor is in honor of , who in 1933 showed that the dry density of a soil for a given...
and there are two different methods in obtaining the maximum density. They are the standard Proctor and modified Proctor tests; the modified Proctor is more commonly used. For small dams, the standard Proctor may still be the reference.
In agriculture
Compaction of agricultural soils is a concern to many agricultural soil scientists and farmers, since soil compaction due to heavy field traffic may reduce plant growth. However, it cannot be stated that all compaction reduces plant growth. The topic is complicated, because it involves the response of the plant to the soil structure and the availability of water. Thus, it requires knowledge about the stress distribution in the soil below the applied load, and knowledge about the resulting soil deformation and shearing.Solutions to overcome compaction include tillage
Tillage
Tillage is the agricultural preparation of the soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shovelling, picking, mattock work, hoeing, and raking...
and the zaï
Zai
Zai are Pashtun tribes in Pakistan and Afghanistan. The title of tribe end in Zai and the their clans end in Khel.Some of the Pashtun tribes:* Alizai* Ghilzai* Kakar* Kakazai* Khudiadadzai* Khulozai* Mohamedzai* Noorzai* Omarzai...
-system.
See also
- CompactorCompactorA compactor is a machine or mechanism used to reduce the size of waste material or soil through compaction. A trash compactor is often used by homes and businesses to reduce the volume of trash....
- Consolidation (soil)
- EarthworkEarthworks (archaeology)In archaeology, earthwork is a general term to describe artificial changes in land level. Earthworks are often known colloquially as 'lumps and bumps'. Earthworks can themselves be archaeological features or they can show features beneath the surface...
- Soil structureSoil structureSoil structure is determined by how individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them...
- AerationAerationAeration is the process by which air is circulated through, mixed with or dissolved in a liquid or substance.-Aeration of liquids:-Methods:Aeration of liquids is achieved by:...
- Shear strength (soil)Shear strength (soil)Shear strength is a term used in soil mechanics to describe the magnitude of the shear stress that a soil can sustain. The shear resistance of soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts. Due to interlocking, particulate...