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.geotechnical engineering When stress is applied that causes densification due to water (or other liquid) being displaced from between the soil grains, then consolidation, not compaction, has occurred.consolidation 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.soil

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.soil scienceagronomy Affected soils become less able to absorb rainfall, thus increasing runoff and erosion.rainfallrunofferosion

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)

Soil compaction is one of the most critical components in the construction of highway embankments, earth dams, foundations and many other engineering structure. Soil must be compacted to their unit weights, increase the strength characteristic which increase the bearing capacity of foundations constructed over them. Soil is compacted by removing air and water from its pore space.

Compaction is a change in soil structure, not just an increase in soil density. Healthy soils have a diversity of pore sizes, while compacted soils have mostly small pores. In general, Soil compaction is defined as the method of mechanically increasing the density of soil. In construction, this is a significant part of the building process. Almost all types of building sites and construction projects utilize mechanical compaction techniques.

During the compaction process soil partials are pushed closer together. This reduces the size of pores, the continuity of pores, and the size and stability of aggregates. Only under severe compaction will aggregates break down.This soil density chart shows properly compacted soil.

Purpose of soil Compaction The principal reason for compacting soil is to reduce subsequent settlement under working loads. Also in a construction material, the significant engineering properties of soil are its shear strength, its compressibility, and its permeability. Compaction of the soil generally increases its shear strength, decreases its compressibility, and decreases its permeabilit

in addition, compaction reduces the voids ratio making it more difficult for water to flow through soil. This is important if the soil is being used to retain water such as would be required for an earth dams. So we conclude that there are four reasons to compact the soil: Increase load-bearing capacity. Prevent soil settlement. Provides stability. Reduce water seepage, swelling and construction.

Types of compaction There are four types of compaction that are commonly used on soil and/or asphalt: Vibration Impact Kneading Pressure

Each of these types is carried out using one of two types of forces: static or vibratory. Static force relies on the weight of a machine to apply downward pressure on soil, thus compressing the soil particles. Adding weights to, or removing them from, the compaction machine can adjust the amount of pressure. Although effective, static compaction is best suited for the upper soil layers. The types of compaction that fall under static are kneading and pressure

Vibratory force, on the other hand, uses mechanically driven force to apply downward pressure in addition to the weight of a machine. The mechanically driven force is an applied vibratory force that rotates the eccentric weight of a piston and spring combination. This is effective in that it not only compacts the top layers, but the deeper layers as well. With vibration, the particles are set in motion and moved closer together to form a high density

Compaction soil types Soil types are classified in three soil groups, with consideration to grain sizes. These types are: Cohesive Granular Cohesive and granular

Cohesive In cohesive soils, such as clay, the particles contain characteristics that make them easily stick together so compaction can be achieved by high impact, which forces the air out of the particles, pushing them together.

Granular Granular soils include sand, gravel, and other particles that typically range in size from to 0.08 inches (0.008 to 0.2 cm). Because granular soils have good water-draining properties, they are able to obtain high density when fully dry or saturated. Granular is best compacted by shaking or vibrating the particles. Any type of vibratory equipment is best suited for this type. Depending on the type of granular soil, different degrees of vibration are required.

Granular and Cohesive Often, soils are a mixture of both granular and cohesive, requiring more precise compaction equipment. Equipment should be chosen on the basis of the soil in the mix that is present in the highest percentage. Some materials, such as asphalt, require both vibration and static pressure to be compacted effectively.

FACTORS AFFECTING COMPACTION IN THE FIELD MOISTURE CONTENT SOIL TYPE LAYER THICKNESS CONTACT PRESSURE SPEED OF ROLLING

MOISTURE CONTENT Proper control of moisture content in soil is necessary for achieving desired density. Maximum density with minimum compacting effort can be achieved by compaction of soil near its OMC (Optimum Moisture Content). If natural moisture content of the soil is less than OMC, calculated amount of water should be added to soil with sprinkler attached to water tanker and mixed with soil by motor grader for uniform moisture content. When soil is too wet, it is required to be dried by aeration to reach up to OMC. Moisture content of the soil is vital to proper compaction

SOIL TYPE Normally, heavy clays, clays and silt offer higher resistance to compaction where as sandy soils and coarse grained or gravelly soils are amenable for easy compaction. The coarse-grained soils yield higher densities in comparison to clays. A well-graded soil can be compacted to higher density.

LAYER THICKNESS The more the thickness of layer of earth subjected to field compaction, the less the energy input per unit weight of soil and hence, less is the compaction under each pass of the roller. Suitable thickness of soil of each layer is necessary to achieve uniform thickness. Layer thickness depends upon type of soil involved and type of roller, its weight and contact pressure of its drums. Normally, mm layer thickness is optimum in the field for achieving homogeneous compaction.

CONTACT PRESSURE Contact pressure depends on the weight of the roller wheel and the contact area. In case of pneumatic roller, the tyre inflation pressure also determines the contact pressure in addition to wheel load. A higher contact pressure increases the dry density and lowers the optimum moisture content

SPEED OF ROLLING Speed of rolling has a very important bearing on the roller output. The greater the speed of rolling, the more the length of embankment that can be compacted in one day. Speed was found to be a significant factor for vibratory rollers because its number of vibrations per minute is not related to its forward speed. Therefore, the slower the speed of travel, the more vibrations at a given point and lesser number of pass required to attain a given density.