Soil Stabilization is the process of improving the engineering properties of the soil and thus making it more stable. It is require when the soil available for construction is not suitable for the intend purpose. In its broadest senses, stabilization includes compaction, pre consolidation, drainage and many other such processes. However, the term stabilization is generally restrict to the processes which alter the soil material itself for improvement of its properties. A cementing material or a chemical is add to a natural soil for the purpose of stabilization.
The various methods of soil stabilization and their effects on the engineering properties of the soils.
- Mechanical Stabilization.
- Cement Stabilization.
- Lime Stabilization.
- Bituminous Stabilization.
- Thermal Stabilization.
- Chemical Stabilization.
- Electrical Stabilization.
- Stabilization by Grouting.
- Stabilization by Geo textile and Fabrics.
- Reinforce Earth.
It is the process of improving the properties of the soil by changing its gradation. Two or more types of natural soils are mix to obtain a composite material which is superior to any of its components. To achieve the desire grading, sometimes the soils with coarse particles are add or the soils with fine particles are remove. It is also refer as granular stabilization.
For the purpose of mechanical stabilization, the soils are subdivide into two categories :
- Aggregates ( particles larger than 75 micron )
- Binder ( particles smaller than 75 micron )
Cement Stabilization is do by mixing pulverize soil and Portland cement with water and compacting the mix to attain a strong material. The material obtain by mixing soil and cement is refer as soil cement. The soil cement becomes a hard and durable structure material as the cement hydrates and develops strength.
Types of Soil cement
- Normal Soil Cement
- Plastic Soil Cement
- Cement Modified Cement
Normal Soil Cement
It consists of 5 to 14 % of cement by volume. The quantity of cement mix with soil is sufficient to produce a hard and durable construction material. The quantity of water use should be just sufficient to satisfy hydration requirements of cement and to make the mixture workable.
Plastic Soil Cement
This type of soil cement also contains cement 5 to 14 % by volume, but it has more quantity of water to have wet consistency similar to that of plastering mortar at the time of placement. Plastic soil cement can be place on steep or irregular slopes where it is difficult to use normal road making equipment. It has also be successfully use for water proof lining of canals and reservoirs.
Cement Modified soil
It is a type of soil cement that contains less than 5 % of cement by volume. Cement Modified soil is a semi hardened product of soil and cement. It is quite inferior to the other two types.
Lime stabilization is do by adding lime to a soil. It is useful for stabilization of clayey soils. When lime reacts with soil, there is exchange of cations in the absorb water layer and a decrease in plasticity of the soil occurs. The resulting material is more friable than the original clay, and is, therefore, more suitable as sub grade
Lime is produce by burning of lime stone in kilns. The quality of lime obtain depends upon the parent material and the production process. There are basically 5 types of limes.
- High calcium, quick lime ( Cao )
- Hydrate, high calcium lime ( Ca (OH)2 )
- Dolomitic lime ( CaO + MgO )
- Normal, Hyrate dolomitic lime ( Ca (OH)2 + MgO )
- Pressure hydrate dolomitic lime ( Ca (OH)2 + MgO2 )
The Quick lime is more effective as stabilizer than the hydrate lime; but the latter is more safe and convenient to handle. Generally, the hydrate lime is use. It is also refer as slaked lime.
Bitumen are non aqueous systems of hydrocarbons that are soluble in carbon di-sulfide. Tars are obtain by the destructive distillation of organic materials such as coal. Asphalts are materials in which the primary components are natural or refine petroleum bitumen.
The amount of bitumen require generally varies between 4 to 7 % by weight. The actual amount is determine by trail.
Types of soil bitumen
According to the Highway Research Board of USA, there are four types of soil bitumen.
- Soil bitumen
- Sand bitumen
- Water Proof Clay concrete
- Oil earth
This is water proof, cohesive soil system. The best results are obtain if the soil satisfies the following criteria.
- Passing No. 4 ( 4.76 mm ) Sieve 50%.
- Passing No. 40 ( 0.425 mm ) Sieve 35 to 100 %.
- Passing No. 200 ( 0.074 mm ) Sieve 10 to 50 %.
- Plastic limit less than 18 %.
- Liquid limit less than 40 %.
- The maximum size of the particle should not be greater than one-third the compact thickness of the soil bitumen.
The quantity of bitumen varies from 4 to 7 % of the dry weight.
This is a bitumen stabilize cohesion less soil system. The sand should be free from vegetal matter or lumps of clay. The sand may require filler for its mechanical stability. However, it should not contain more than 25 % minus No. 200 sieve material ( i.e. the material finer than No. 200 sieve ) for dune sands and not more than 12 % in case in other types of sand.
Water Proof Clay Concrete
A soil possessing a good gradation is water proof by a uniform distribution of 1 to 3 % of bitumen in this system. Soils of three different gradations have be recommend. For the three gradations, the percentage passing No. 200 sieve varies between (i) 8 to 12 ; (ii) 10 to 16 and (iii) 13 to 30.
In this system, a soil surface consisting of silt clay material is made water proof by spraying bitumen in two or three applications. Slow or medium curing bitumen or emulsions are use. The bitumen penetrates only a short depth into the soil. The amount of bitumen require is about 5 litres per square meter of the soil surface.
In chemical stabilization, soils are stabilize by adding different chemicals. The main advantage of chemical stabilization is that setting time and curing time can be control. Chemical stabilization is however generally more expensive than other types of stabilization.
The following chemicals have be successfully use.
- Calcium chloride
- Sodium chloride
- Sodium silicate
- Chrome Lignin
- Other Chemicals
It change causes a mark improvement in the properties of the soil. Thermal stabilization is do either by heating the soil or by cooling it.
Heating :- As the soil is heat, its water content decreases. Electric repulsion between clay particles is decrease and the strength of the soil is increase. When the temperature is increase to more than 100 degree celsius, the absorb water is driven off and the strength is further increase.
Freezing :- Cooling causes a small loss of strength of clayey soils due to an increase in inter particle repulsion. However, if the temperature is reduce to the freezing point, the pore water freezes and the soil is stabilize. Ice so form acts as a cementing agent.
Stabilization by grouting
In this method of stabilization, stabilizers are introduce by injection into the soil. As the grouting is always do under pressure, the stabilizers with high viscosity are suitable only for soils with high permeability. This method is not suitable for stabilizing clay because of their very low permeability.
The grouting method is costlier as compare with direct blending methods. The method is suitable for stabilizing buried zones of relatively limit extent, such as pervious stratum below a dam. The method is use to improve the soil that cannot be disturb. An area close to an existing building can be stabilize by this method.
Types of Grouting
- Cement grouting
- Clay grouting
- Chemical grouting
- Chrome lignin grouting
- Polymer grouting
- Bituminous grouting
Stabilization by Geo textile and Fabrics
The soil can be stabilize by introducing Geo textiles and fabrics which are make of synthetic materials, such as polyethylene, polyester, nylon. The Geo textile sheets are manufacture in different thickness ranging from 10 to 300 mils ( 1 mil = 0.0254 mm = 25.4 micron ). The width of the sheet can be up to 10 m. These are available in rolls of length up to about 600 m. Geo textiles are manufacture in different patterns, such as woven, non woven, grid, and hybrid. The woven Geo textiles are make from continuous mono filament or silt film fibers. The non woven Geo textiles are make by use of thermal or chemical bonding of continuous fibers and then press through rollers into relatively thin sheets.
The soil can be stabilize by introducing thin strips in it. In reinforce earth, thin metal strips or strips of wire or Geo synthetics are use as reinforcement to reinforce the soil. The essential feature of the reinforce earth is that friction develops between the reinforcement and the soil. By means of friction, the soil transfer the forces built up in the earth mass to the reinforcement. Thus tension develops in the reinforcements when the soil mass is subject to share stresses under loads.