Stabilization of soil, in a broad sense, incorporates the various methods employed for modifying the properties of a soil to improve its engineering performance. In stabilization the property of an existing soil is altered by improving the shear strength of the soil according to the desired requirements so as to meet the constructional standard. Stabilization of soil is being used for a variety of engineering works, the most common application being in the construction of road and air field pavement, where the construction cost by making best use of locally available materials.
Different methods of stabilization of soil
Although stabilization of soil is the process of modification but it can be attained by different methods according to the need. The methods of stabilization of soil can be grouped into two different types. These are as follows:
- Method of modification in which the property of soil can be improved without adding any admixture, and
- Method of modification in which the property of soil is improved by adding an admixture.
The methods of compaction and drainage come under the method of modification is in which the property of the soil is altered without adding admixture whereas mechanical stabilization, cement stabilization, lime stabilization, bitumen stabilization and chemical stabilization falls under the category of methods in which stabilization is done without the addition of admixture.
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A brief descriptions of the above mentioned of stabilization of soil methods are given below:
1.Mechanical stabilization – Mechanical stabilization is a combination of two operations. In the first operation the composition of the soil is changed by addition or removal of certain constituents, whereas in the second operation the densification or compaction of the soil is done. The prime purpose of the mechanical stabilization is to have a soil that can provide effective resistant to deformation and displacement of the soil under loading. The mechanical stabilization has been largely used in the construction of cheap roads. If the soil from one source does not meet the gradation and plasticity requirements of a job, it becomes necessary to mix materials from more sources for obtaining the desired mixture. The blending of materials can be carried out by making trial combinations.
Proper compaction plays a very important role in stabilization. Compaction has a great effect on soil properties, such as strength and stress-strain characteristics, permeability, compression, swelling and water absorption. The properties of a soil under compaction depend upon the water content, amount of compaction and the type of compaction. Compared to coarse grained soils, the properties of fine grained soils are affected to a greater extent by the placement conditions.
2.Cement stabilization – The stabilization of soil with cement is known as soil cement. The cementing action is believed to be the result of chemical reaction of cement with the siliceous soil during hydration. The binding action of individual particles through cement may be possible only in coarse-grained soils. In fine grained, cohesive soils, only some of the particles can be expected to have cement bonds, and the rest will be bonded through natural cohesive. The important factors affecting soil cement are such as nature of the soil, cement content, condition of mixing, compaction and curing and admixtures.
4.Lime stabilization – Lime stabilization is a very effective method of stabilization for heavy, plastic clay soil, more over sandy soils can also be stabilized or stabilization of soil can be done with this method. In lime stabilization lime may be used alone, or in combination with cement, bitumen or fly ash. The addition of lime in soil exhibits two reactions:
- Alteration in the nature of the absorbed layer through base exchange phenomenon, and
- ii) Cementing or pozzolanic action. Generally 2 % – 8% of lime may be required for coarse grained soils, and 5% to 10% for plastic soils. The amount of fly ash as admixture may vary from 8% to 20% of the soil weight.
5.Bitumen stabilization – Asphalt and tars are the bitumen materials which are used for stabilization of soil, generally for pavement construction. These materials are normally too viscous of incorporated directly with soil. The fluidity of asphalt is increased by heating, emulsifying, to a soil impart cohesion or binding action and reduced water absorption. Thus either the binding action or the water proofing action or both the actions, may be utilised for stabilization. Depending upon these actions and the nature of soils, bitumen stabilization is classified under the following four types:
- Sand bitumen
- Soil bitumen
- Water proofed mechanical stabilization, and
- Oiled earth
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6.Chemical stabilization – There are many chemicals which are used for stabilization of soil. Some of the most commonly used chemicals which found great importance in chemical stabilization are as follows:
- Calcium chloride – Calcium chloride acts as soil flocculent. It facilitates compaction and usually causes a slight increase in the compacted density.
- Sodium chloride – The stabilization action of sodium chloride is somewhat similar to that of calcium chloride, but it has not been widely used.
- Sodium silicate – The sodium silicate solution in water, known as water glass, in combination with other chemicals, such as calcium chloride, is used as an injection for stabilization of soil as deep deposition of soil. The two chemicals reacts and precipitate in the form of an insoluble silica-gel within the soil pores making the soil impervious to water and increasing its shearing strength.