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Dynamic Soil Compaction Methods

written by: Jayant R Row • edited by: Lamar Stonecypher • updated: 9/10/2010

Dynamic soil compaction methods are used to compact soils that cannot be directly accessed. This may be due to high water tables, or may even be due to the difficulty to excavate and store the overburden for reaching the soil. There are a number of techniques used for soil compaction remediation.

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    The Necessity for Soil Compaction

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    Soil compaction remediation is necessary to increase the density of soils. This gives the soil higher strength to bear loads and decreases its permeability so that the ingress of water is halted. Water can be damaging to foundation structures and can also rise into a structure through capillary action, which in turn can damage floors, reinforcement bars, and other structural elements. Soil compaction helps to increase the bearing capacity of soils which in turn can vastly reduce foundation costs.

    On the other hand, soil compaction in agricultural soils plays a part in reducing plant growth and not allowing nutrients to reach the roots. Gardeners are aware of this and use aerators to reduce compaction for better lawn growth.

    Image Source: Wikimedia : Compaction by ramming

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    Soil Classification and Its Effect on Compaction

    Soils are either cohesive, granular, or both. Cohesive soils like clay are generally very fine grained and can be compacted very easily. Granular soils have high drainage properties and are fully compacted when fully dry or fully saturated. Soils that have both small and large granular particles need to be analyzed properly before deciding on any compaction method. Laboratory techniques are available and have been standardized for soil analysis. Granular soils are best compacted by vibratory equipment whereas cohesive soils respond better to ramming and pressure. Standards set for soil compaction can also be measured through various laboratory methods.

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    Soil Compaction and the Various Methods Used For This

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    The ability of soil to be compacted depends on its nature. This includes the size of the grains, the distribution of the various sizes, and the level of the water table. Various types of soil compaction methods are in use by civil engineers throughout the world and the techniques are constantly evolving. Soil compaction is achieved by expelling air and some part of the water in the soil.

    The need to compact soil was realized by engineers years ago. Before the advent of machinery, the first soil compaction method used was done using animals like horses and mules to pull loaded carts over areas needing compaction. Manual methods for compaction using hand compactors made of wood or metal are also used. It is quite common in developing countries to see a pair of laborers lifting and releasing huge logs of wood with flat cut faces to achieve compaction.

    Dynamic soil compaction can be achieved by static or vibratory methods by impact, pressure, kneading, or vibration. Static methods use the weight of a machine or loads to compact soil. Such methods would normally affect only upper soil layers and are more in use for roads and other light structures where the load required to be borne by the soil is not very great. Modern day engineers have the advantage of adding vibratory pressures to such static loads with the use of simple devices using eccentric forces on moving parts. For compacting soil or fill in the plinth of a building, vibratory plate compactors are frequently used.

    Rammers are used to compact soil by high impact blows at a certain frequency. These machines would have rates of 500 vibrations per minute to deliver these blows through heavy weights. Rollers are of course a familiar site on most construction and road sites, and while flatbed rollers are used for the top layer compaction or for asphalt, some rollers may have vibratory mechanisms or extensions called sheep feet, which increase the depth to which the compaction is felt.

    Piling is another method used to compact soil, though most engineers associate piling with structures that go down to rock level for them to build foundations on. One of these techniques is to use micro-piling. This technique is used in cohesive soils where the water tables are very high. Boring is done, quite often by hand, to drill holes into the earth to a depth of 10 to 12 feet. These bores may be just 2 inches in diameter. These bores are then filled up with granular material like sand or metal. This gives the subsoil water a path to come up. Such bores are done at very close intervals of about a foot or to apart. After this operation the entire area is loaded with heavy static weights and the soil is compressed after the water in it comes to the surface. This water is let out and removed to allow the soil to consolidate. Drainage is effective even by putting a layer of metal over the entire area and making drains on the periphery which can be dewatered.

    Dewatering is quite often associated with soil compaction, and the use of high vacuum is used to remove the water and alternate with dynamic compaction that can increase bearing capacities of the soil by over fifty percent.

    Image Source : Wikimedia : Roller

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    References

    Contractor's Depot - Soil compaction equipment

    RitchieWiki.com - Soil compaction

    scribd.com - Soil compaction

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