25-07-2012, 12:28 PM
Soil compaction
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INTRODUCTION
Soil compaction is the process to increase the soil (ground) density in order to make use the ground surface for development, i.e. building, road, etc. The volume of void space is reduced by applying high loads over a small area to force the air out of an unsaturated soil mass.Compaction has been an issue in greens management since there were golfers. Compaction reduces water infiltration, reduces the ability of roots to penetrate the soil and limits gas exchange between the soil surface and the atmosphere. To reduce compaction, the soil particles need to be "loosened-up" to provide more air-filled pore spaces. What happens to air spaces when someone steps on the soil or a piece of equipment drives over the soil? The soil compacts again. Without some type of support for the traffic mixed throughout the green, the green will rapidly revert back to its compacted state following aeration. Unless, as Caranci and Mills have shown, the compacted greens are vertidrained and the holes filled with sand. The low compaction sand in the holes may not compact rapidly because the surrounding high compaction soil supports foot and vehicular traffic. In addition, some superintendents have begun to aerify frequently in the summer using smaller 1/4 inch times and leaving the holes open. The area will slowly compact as traffic pushes the holes closed. But, the holes give the soil some elasticity and enhances gas exchange with the atmosphere.
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.
OBJECTIVE OF COMPACTION
• Increasing the bearing capacity of foundations;
• Decreasing the undesirable settlement of structures;
• Control undesirable volume changes;
• Reduction in hydraulic conductivity;
• Increasing the stability of slopes.
In general, soil densification includes compaction and consolidation. Compaction is one kind of densification that is realized by rearrangement of soil particles without outflow of water. It is realized by application of mechanic energy. It does not involve fluid flow, but with moisture changing altering.
Consolidation is another kind of densification with fluid flow away. Consolidation is primarily for clayey soils. Water is squeezed out from its pores under load.
SOIL COMPACTION
To review some basics of soil mechanics, compaction is the process by which a mass of soil consisting of solid soil particles, air, and water is reduced in volume by the momentary application of loads, such as rolling, tamping, or vibration. Compaction involves an expulsion of air without a significant change in the amount of water in the soil mass. Thus, the moisture content of the soil, which is defined as the ratio of the Weight of water to the weight of dry soil particles, is normally the same for loose, uncompacted soil as for the same soil after compaction . Since the amount of air is reduced without change in the amount of water in the soil mass, the degree of saturation (the ratio of the volume of water to the combined volume of air and water) increases. When used as a construction material, the significant engineering properties of Soils are its shear strength, its compressibility, and its permeability. Compaction of the soil generally increases its shear strength, decreases its compressibility, and decreases its permeability.
SOIL CLASSIFICATIONS
Considering soil compaction, the two broad classifications are cohesive soils and cohesionless, or noncohesive , soils. Cohesive soils are those that contain sufficient quantities of silt or clay to render soil mass virtually impermeable when properly compacted. Such soils are all varieties of clays, silts, and silty or clayey sands and gravels. By contrast, cohesionless soils are the relatively clean sands and gravels, which remain pervious even when well-compacted.
SOILS AND COMPACTION
An important characteristic of cohesive soils is that compaction improves their shear strength and compressibility properties. Such characteristics follow the principles stated by R.R. Proctor in 1933. The most recognizable development of his theory was a test now known as the “Standard Proctor,” which is used to estimate the maximum density of soils. Today, there are several laboratory compaction standards many construction methods to compact cohesive soils; however, the effect of the soil’s water content on the resulting dry density is similar for all methods. For each compaction procedure, there is an optimum moisture content, which results in the greatest dry density or state of compactness. At every other moisture content, the resulting dry density is less than this maximum. The adjacent figure, which represents this principle, shows two moisture-density curves (a Standard Proctor Curve and a Modified Proctor Curve) for different amounts of compactive effort on the same soil.