BITUMINOUS STABILISATION AND TYPES

Bituminous Stabilisation

Bitumens are non-aqueous system of hydrocarbon that are soluble in carbon di-sulphide.Bitumen stabilisation is generally done with asphalt as binder.In any inorganic soil which can be mixed with asphalt is suitable for bituminous stabilisation. In cohesionless soils, asphalt binds the soil particles together and thus serves as a bonding or cementing agent.In cohesive soils, asphalt protects the soil by plugging its voids and water proofing it.It helps the cohesive soil to maintain low moisture content and to increase the bearing capacity.The amount of bitumen required generally varies between 4-7% by weight.The actual amount is determined by trail.

There are four types of soil bituminous stabilisation:

1. Soil Bitumen
2. Sand Bitumen
3. Oiled Earth
4. Water Proofed Clay Concrete

1. Soil Bitumen 

Soil bitumen is a water proof, cohesive soil system.The best results are obtained if the soil satisfies the following criteria:

• Plastic Limit Less than 18%
• Liquid Limit Less than 40%
• The maximum size of the particle should not be greater than one-third the compacted thickness of the soil-bitumen.

The quantity of bitumen varies from 4 - 7% of the dry weight.

2. Sand Bitumen

This bitumen stabilised cohesionless 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 for dune sand and not more than 12% in case in other types of sand.The amount of bitumen varies from 4 - 10%.

3. Oiled Earth

In this system, a soil surface consisting of silt-clay material is made water proof by spraying bitumen in two or three applications.The bitumen penetrates only a short depth into the soil.The amount of bitumen required is about 5 litres per square meter of the soil surface.

4. Water Proofed Clay Concrete

A soil possessing a good gradation is water proofed by a uniform distribution of 1 - 3% of bitumen in this system.Soils of three different gradations have been recommended.  

Factors Affecting Bituminous Stabilisation

1. Types Of Soil

Bituminous stabilisation is very effective in stabilising sandy soils having little or no fines.

2. Amount Of Asphalt

The quality of bitumen-stabilised soil improves with the amount of asphalt upto a certain limit.

3. Mixing

The quality of bitumen-stabilised soil improve if the soil is mixed thorough.

4. Compaction

The dry density of bitumen soil depends upon the amount and type of compaction.It also depend upon volatile content.

Chemical Soil Stabilisation

In chemical soil stabilisation, soils are stabilised by adding different chemicals.These chemicals react with the soil which in return cause its structure to be changed.The chemical seal the space between particles, leaving no room for water to penetrate through.Among the most common chemicals that are used for soil stabilization include sodium chloride, calcium chloride and sodium silicate.The main advantages of chemical stabilisation is that setting time and curing time can be controlled. Chemical stabilisation is however generally more expansive than other types of stabilisation. The following chemicals have been successfully used:

1. Calcium Chloride
2. Sodium Chloride
3. Sodium Silicate
4. Chrome Lignin
5. Polymers
6. Other Chemicals

1. Calcium Chloride

Calcium chloride is an inorganic compound, a salt with the chemical formula CaCl2.When calcium chloride is added to soil it causes colloidal reaction and amend the characteristics of soil water.When calcium chloride is dissolved it reduces the loss of moisture from the soil.It also reduces the chances of frost heave, as the freezing point of water is lowered.This method is very effective for stabilisation of silty and clayey soil which lose strength with an increase in water content.

2. Sodium Chloride

Sodium chloride is commonly known as salt with the chemical name NaCl.Sodium chloride is mixed with the soil either by the mix-in place method or by the plant-mix method.It should not be applied directly to the surface.When sodium chloride is added to the soil, crystallisation occurs in the pores of the soil and it forms a dense hard mat with the stabilised surface.

3. Sodium Silicate

Sodium silicate are colorless glassy or crystalline solids or white powders.Sodium silicates, as well as other alkali silicates have been successfully used for soil stabilisation.Sodium silicate gives strength to soil when it react with it and make the soil impervious.This method of stabilisation is relatively inexpensive but its long term stability is doubtful.

4. Chrome Lignin

Lignin is an organic substance binding the cells,fibers and vessels which constitute wood and the lignified elements of plants.Chrome lignin is formed from black liquor obtained during sulphite paper manufacture.Sodium bicarbonate or potassium bicarbonate is added to sulphite liquor to form chrome lignin.It slowly polymerises into brown gel.

5. Polymers

Polymers are long-chained molecules formed by polymerising of certain organic chemicals called monomers.Polymers may be natural or synthetic.When a polymer is added to a soil reaction takes place.Polymers mainly affect the aggregation and strength of soils through their interactions with fine clay particle.Coating of adsorbed polymers on clays can increase their steric stabilisation by preventing clay particles from approaching each other as closely.

6. Other Chemicals

Apart from the above mentioned chemicals some other chemical are also used for soil stabilisation which are as fallows:

 

A. Some water proofer such as alkyl chloro silanes, siliconates amines and quaternary ammonium salts have been used for water proofing of soils.

B. Dispersant such as sodium hexa-metaphosphate, are used to increase electrical repulsion and to cause dispersed structure.

C. Coagulating chemicals such as calcium chloride and ferric chloride have been used to increase the electrical attraction and to form flocculated structure in order to improve the permeability of the soil.

Advantages Of Chemical Soil Stabilisation

• It gives more strength to the soils.
• It improves the permeability of soil.
• It this method the setting time and curing time can be controlled.

Disadvantages Of Chemical Soil Stabilisation

• Require extra experienced labor.
• Chemicals should not be used directly to the surface.
• This method of stabilisation is more expensive relatively to other methods.

LIME STABILISATION - METHOD & BENEFITS

 Lime Stabilisation

Lime stabilisation is done by adding lime to the soil.It is very useful for stabilisation of clayey soils.When lime reacts with soil, there is exchange of cations in the adsorbed 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 subgrade.Lime stabilisation improve the density and bearing capacity of the soil.Lime-soil stabilisation is useful to construct sub-base and base course for pavement.Lime treated soil is more suitable for warm regions where temperature is very high and for colder regions it is not suitable.

Quick lime is more effective as stabiliser than the hydrated lime, but the latter is more safe and convenient to handle.Generally the hydrated lime is used.It is known as slaked lime.Lime stabilisation is not effective for sandy soils because these soils can be stabilised in combination with clay, fly-ash or other pozzolanic materials.The ratio of fly ash to lime varies between 3 to 5.The fly ash is used about 10 - 20 % of the soil weight.

Method Of Mixing Lime Into Soil

The method of mixing lime into soils for stabilisation involves following steps:

• The soil which need stabilisation is scarified and pulverized by suitable equipment.

• Now add some amount of lime to the pulverized soil either the powder or slurry form and then mixed with suitable equipment.

• If lime powder is used then water is sprayed all over the soil.

• After spraying the water leave this mixture for 1 to 4 days. Lime-soil reaction is slow process and it required some time.

• Once the lime-soil reaction take place then spread the soil to required grade and compact it with rollers.

• When compaction is completed and the soil achieve the required dry density, the compacted lime-soil layer is allowed for curing for 1 week. 

• Finally after 1 week of curing the field tests are conducted to check water content and maximum dry density of compacted soil.

Factor Affecting Lime Soil Stabilisation

The following are the factors which affecting lime-soil stabilisation:

1. Soil Type
2. Lime Type
3. Lime Content
4. Compaction
5. Additives
6. Curing 

Benefits Of Lime Soil Stabilisation

• It is easy to mix with soil.
• It reduces the plastic properties of the soil when wetted.
• It sets slowly, the time interval between mixing and compacting is not critical.
• Cost of stabilisation is reasonably low.

WHAT IS SOIL SUCTION?

Soil Suction

Soil suction is considered as the summation of matric suction (ψ_{m) and osmotic suction (π).}

Soil suction is defined as the state of the soil when it is under reduced pressure.It is measured in terms of the height of the water column (h) suspended in the soil.

Soil Suction Formula

Soil suction can be represented as common logarithm of the height of the water column (h).The value of 'h' is substituted in centimeters.

 

Pf = log10 (h)

For example: The height of the water column is 100cm.Hence, the Pf value is given by:

 _{Pf = log10 (h)}

     _{= log10 (100)}

     _{= log10 (10^2)}

     _{= 2 x log10}

     _{= 2}

_{Similarly, for a height of column = 1 cm.}

Hence the Pf value is given by:

 Pf  = log10 (h)

 Pf   = log10 (1)

 Pf   = 0

Soil suction can also be represented in atm, kPa, kg/cm2 and bar. 

FACTOR AFFECTING SOIL SUCTION

The suction in soils depends mainly on the following factors:

1. Particle Size

In general the smaller the particle size, the greater is the soil suction.The soils with fine particles have a large number of small pores with small radii of menisci. It results in large capillary rise and hence greater suction.

2. Water Content

 Water content is also known natural moisture content, is the ratio of the weight of water to the weight of the solids in a given mass of soil.As the water content of a soil decreases, the soil suction increases and it attains the maximum value when the soil is dry.

3. Soil Structure 

Soil structure describes the arrangement of the solid parts of the soil and of the pore space located between them.The soil structure governs the size of interstices in the soils.As the soil suction depends upon the size of interstices, a charge in the soil structure affects the soil suction.

4. Denseness of Soil

As the denseness of a soil increases, generally soil suction increases.When the soil is loose, with a low density the pores are of large radius and the soil suction is low.

5. Temperature

A rise in temperature causes a reduction in surface tension (Ts) of the water.Consequently, the soil suction decreases as the temperature increases.

6. Dissolved Salts

The surface tension of water increases with an increase in impurities, such as salt.

7. Angle of Contact

The angle of contact between water and soil particles depends upon the mineralogical composition of soils.As the angle of contact (𝛉) increases, the soil suction decreases.The soil suction is maximum when the angle of contact is zero.

8. Wetting Cycle

As discussed in the preceding section, for the same water content, the soil suction is greater during the drying cycle than in the wetting cycle.