The process of improving the engineering properties of soils is called soil stabilization. It is the simplest method of soil stabilization. By soil stabilization, the soil becomes more stable by the reduction in the permeability and compressibility and by the increase in shear strength. With the increase in soil stabilization properties the bearing capacity of soil is increased significantly.
The various method of soil stabilization includes:
- Mechanical Stabilization
- Cement Stabilization
- Lime Stabilization
- Bitumen Stabilization
- Stabilization by Geo-textiles
In this method changing the gradation of virgin soil stabilizes the soil. To do this two or more types of soils are mixed to attain a desired property to suit a particular construction.
For mechanical stabilization, the soils are grouped into two categories. They comprise aggregates and binders. Aggregates are sands and gravels and binders are silts and clays. When mixed together in a definite proportion, a soil possessing required internal friction and cohesion is obtained. When properly placed and compacted, the material becomes mechanically stable.
The various factors, which affect the mechanical stability of a mixed soil, comprise strength of aggregate, mineral composition, gradation, and plasticity characteristics.
Mechanical stabilization is generally used to improve the sub-grade of low bearing capacity. It is extensively used in the construction of bases, sub-bases and surfacing of the roads.
In cement stabilization pulverized soils and cement in suitable proportion are mixed with water and the resulting mixture is compacted by compacting equipment such as rollers. The material obtained by mixing cement and soil is called soil-cement. When cement hydrates and develops strength the soil cement becomes a hard and durable structural material.
Factors affecting Soil cement – The various factors, which affect soil cement, are:
- Type of Soil
- Quality of cement
- Quantity of water
- Mixing, compaction and curing
The suitable materials must be pulverisable. They, in general, comprise granular materials with sufficient fines. Such material requires less cement. In materials, which contain deficiency in fines require more cement but they are also fall under suitable materials.
The cement requirement depends upon the gradation of the soil. A well graded soil requires about 5 % cement, whereas a poorly graded, uniform soil may require about 9 % cement. Non-plastic silts require about 10 % cement, whereas plastic clays may need about 13 % cement. The actual quantity required shall have to be ascertained by carrying out laboratory tests.
Water requirement should be sufficient for cement hydration and good workability. Generally the amount of water ascertained by compaction requirement is adequate for cement hydration as well. Potable water is most desirable.
In the soil cement, the ingredients are mixed thoroughly in order to make it more stable. Otherwise non-homogeneous weak product results. After it is compacted well as in the case of soil, addition of admixtures such as lime, calcium chloride, fly ash, sodium carbonate, sodium sulfate reduces the cement requirement.
In lime stabilization the soil is stabilized by adding lime. By this method clayey soil is stabilized well. When lime is added to soil, it reacts with the soil and cations are exchanged in the diffused double layer. As a result plasticity is reduced significantly and the resulting material becomes more friable than the original clay. Consequently the material is therefore more suitable as sub grade. The amount of lime required for stabilization varies between 2 to 10 %. For a rough guide the following amount of lime may be used.
For clayey gravel having less than 50 % of silt-clay fraction 2 to 5 % of lime may be required. If the silt-clay fraction in soil exceeds more than 50 %, 5 to 10 % of lime may be used. For heavy clay the amount of lime required is about l0 %. The lime stabilization is not suitable for stabilization of sands.
Bituminous stabilization is generally done with asphalt as binder. Asphalt acts as a binder for coarse-grained soil. In cohesive soil asphalt protects the soil by plugging its void and water proofing it. It helps the cohesive soil to maintain low moisture content and to increase bearing capacity. There are four types of Bituminous Stabilization. They include:
- Soil Bitumen
- Sand Bitumen
- Water Proof Clay Concrete
- Oiled Bitumen
Soil Bitumen – The soil bitumen is used to stabilize clay soil. The stabilized soil becomes water- proof. The quantity of bitumen required varies from 4 to 7 % of dry weight.
Sand Bitumen – This is bitumen stabilized cohesionless soil system. The sand should be free from vegetable matter or lumps of clay. The sand should not contain more than 25 % minus 200-sieve material for dune sands and not more than 12 % in case of other types of sand. The amount of bitumen required varies from 4 to 10 %.
Water – proofed Clay Concrete – It is water proofed soil made by adding 1 to 3 % of bitumen. In general, three gradations are in use to successfully stabilize the soil. The percentages passing 75 micron sieve are specified as:
- 8 to 12 %
- 10 to 16 %
- 13 to 30 %
Oiled Bitumen – It is a bitumen treated silty clay material. The material is made waterproof by spraying bitumen in two or three applications. Slow or medium curing bitumen or emulsions are used. The bitumen penetrates only a short depth into the soil. The amount of bitumen required is about 5 liters per square meter of the soil surface.
Stabilization by Geo – Textiles
Geo-textiles are fabrics made of synthetic materials, such as polyethylene, polyester, nylon, polyvinyl chloride and so on. They are manufactured in varieties of ways in the woven, non-woven or grid-form. A geo-textile has high tensile strength. When properly embedded in the soil, it contributes to its stability. It is used in the construction of unpaved roads over soft soils. The geo-textile is laid over the soil. Aggregates are laid directly over the geo-textile. When traffic passes over the road, the geo-textile deforms and its strength is mobilized. The more a geo-textile deforms, the greater the load it can carry.