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June, 2002 COMPACTION of soil, whenever and wherever laid, is of utmost importance to ensure good health of the structure built over it throughout the designed life of the structure. Good compaction of soil results in minimizing of its settlement on application of load, increases its density thus increasing its shear strength and decreases its permeability leading to a fall in its water absorption and reduction in its swelling or shrinkage. Whether it is the soil laid beneath a building or that under a floor or that in an embankment, its compaction has a direct effect on the future behavior of the structure built over it. Take the example of an earthen dam. Good compaction of soil laid in layers to raise it to its desired height will make the soil dense, thereby decreasing the seepage- loss of water stored behind the dam. The dam structure itself will have a longer life as water will not be able to make way into it. In case of an embankment, the chances of settlement of soil due to movement of heavy traffic over it will be minimized thus reducing the maintenance cost of the road built over it and making the embankment stand against the ravages of time. Compaction of soil can be done either by expulsion of air entrapped in the soil or by expulsion of water present in its pores or by elastic compression of soil particles. Among these, the elastic compression of soil particles contributes negligibly whatever amount of compaction we may do. Compaction, therefore, will depend upon the expulsion of air and water from the soil. It has been noted that usually no water gets expelled from the voids during compaction. Only a gradual expulsion of water may take place from the pores of saturated cohesive soils under the action of continuously static loads. Thus during the compaction of soils, it is virtually the expulsion of air only that brings desired results and increases the density of soil. How to achieve good results Compaction of soil depends upon a number of factors. Among these the most prominent are a) The moisture content of soil b) Type of soil c) The method of compaction. Compaction of soil is measured in terms of dry density of soil. More is the dry density, more compact the soil is. The moisture content at which maximum dry density is achieved is called Optimum Moisture Content (OMC). Here it must be noted that this OMC is for a particular "compaction effort" applied to the soil. If we increase the compaction effort, the OMC will decrease for this new and increased effort and the density of soil will further increase. Thus there are two variable factors: OMC and compaction effort. More and more density can be achieved by increasing the compaction effort and decreasing the OMC for that effort. It should be borne in mind that the density of soil will increase only if the shear stress produced by the "compaction effort" is more than the shear strength of soil at its existing density and moisture content. The compaction effort should, therefore, be quite adequate. Well-graded coarse soils can be compacted to a high density as compared to fine grained soils. OMC for fine grained soils is more than that for coarse grained soils because finer particles have larger surface area and need more water to wet themselves. Thus for same compaction effort, maximum dry density of fine grained soils shall be lesser than that for coarse grained soils. To achieve good results for a particular soil, a graph should first be prepared between the compaction effort and the OMC for that effort by applying different efforts and noting the OMC for each effort. The desirable as well as possible result should then be chosen as the target to be achieved. This is important because sometimes, the best compaction effort and the OMC may not be possible to achieve at a particular site. How to compact the soils To carry out the compaction of soil in the field, mechanical means are most suitable. A number of types of rollers are available for this purpose. However the type of rollers to be used depends upon the nature of soil to be compacted. For clayey soils, kneading type rollers are most suitable. Pneumatic or sheep foot rollers shall, therefore, prove better for such soils as these rollers penetrate into the soil. 95% of the modified proctor density can be achieved in these soils by using rubber tyred rollers with tyre load of 11 MT. For sandy or cohesion less soils, vibratory rollers are most suitable. Even 100% of modified proctor density can be achieved in these soils by using proper vibratory equipment. If rubber tyred vibratory rollers with 13.5 MT load are applied, even 103 to 104% of modified proctor density is achievable in these soils after making 30 passes of these rollers. Once the thickness of layers of soil to be laid and compacted is decided, the type and weight of rollers can also be decided as the thickness of layer also affects the choice of rollers. Lesser is the thickness of a layer, more is the compaction achieved for a given type of roller. After making right choice of rollers, the number of passes required to achieve the desired density should be observed in the field and followed for each layer to be laid. The vibratory rollers consist of a smooth cylindrical steel drum with one rotating eccentric weight placed at the center of the drum. The rotating eccentric drum produces vibrations causing the roller to bounce against the soil. Vibrations do not allow the top layer of an embankment to achieve density. So the top layer should always be compacted by switching off the vibratory mode of the roller. Compaction performance of a vibratory roller depends upon its frequency, vibrating mass and amplitude. These parameters should therefore be examined while selecting a vibratory roller for the density to be achieved. Checking the density It is very important to check the actual density of soil rather than just passing the rollers over each layer a few times and believing that the soil has been compacted. To determine the maximum dry density of soil in the laboratory, a simple apparatus known as Standard Proctor Test Apparatus and Modified Proctor Test Apparatus may be used. Particulars of these apparatuses are given in Table 1. Sometimes a mould of 15 cm diameter is used instead of 10 cm dia. In such case, the number of blows is increased from 25 to 56. Rest of the test procedure remains the same. As the number of layers is lesser and the energy input is also lesser in case of Standard Proctor Test, it gives lesser values of maximum dry density as compared to modified proctor test. Though the maximum values achieved by each type of test vary from soil to soil, it has been observed that 95% of standard proctor density is equivalent to 88 to 90% of modified proctor density. Standard Proctor Test is adopted for density checking of unimportant structures while the Modified Proctor Test is applied in harder conditions such as runways, sub-grades and embankments. In case of embankments and earthen dams, many builders make such an arrangement that the fleet of dumpers has to pass over the laid layers to reach their location. It is a good and practical method of imparting more density to the laid layers of the soil. Field densities Once the norms have been laid for a particular work with respect to the maximum dry density achieved by Standard Proctor Test or Modified Proctor Test, the field densities can be taken from time to time and compared with the norms. To check the field density, core cutters are used. Samples of compacted soil are drawn by use of core cutters, are weighed, dried to find the moisture content and the dry density achieved is arrived at. It must be noted that an 80% of the Standard Proctor Test density is almost equivalent to soft soil. After all, compaction has to be achieved by expulsion of air only. Anybody can well imagine the little difference in air content of loose soil and compacted soil. A soil having a maximum dry density of 1.85 gram/cubic cm and a water content of 15% shall have an air content of about 10% only and percentage air voids of 4% only. Thus the compaction of soil should be fully taken care of to avoid any trouble at a later stage.
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