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.

Mechanical Stabilization

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

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

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.

Bituminous Stabilization

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

Soil bitumen

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.

Sand bitumen

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.

Oil earth

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.

Chemical Stabilization

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
• Polymers
• Chrome Lignin
• Other Chemicals

Thermal Stabilization

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.

Reinforce Earth

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.

A deep foundation is a type of foundation that transfers building loads to the earth farther down from the surface than a shallow foundation does to a subsurface layer or a range of depths. A pile or piling is a vertical structural element of a deep foundation, driven or drilled deep into the ground at the building site.

(a) Classification based on L/d ratio –

• Short Pile (when L/d ratio is <= 10)
• Long Pile (when L/d ratio is > 10)

(b) Classification based on angle of Inclination –

• Vertical Piles (generally to carry vertical loads)
• Batter or Raker Piles (to carry lateral loads)

(c) Classification based on load transfer –

• End bearing Piles (transmit load through bottom tips) also known as Point bearing Piles
• Friction Piles (load is transfer through skin friction between embedded surface of pile and surrounding soil) also termed as Floating Piles
• Combined end bearing and friction Piles (load is transfer through both bottom tips, and skin friction between embedded surface of pile and surrounding soil, that is why it is termed as Combined Piles)

(d) Classification based on Composition –

• Steel Piles (generally load taken upto 400 kN to 1200 kN)
• Timber Piles (generally load taken upto 250 kN)
• Concrete / RCC Piles (pre-stressed pile may take load upto 2000 kN and pre-cast pile may take load upto 1000 kN)
• Composite Piles (upper portion is made up of concrete and lower portion is made up of either timber or steel. difficult to provide a proper joint)
• Sand Piles (bored holes are filled with sand, compacted and covered with concrete cap at the top)

(e) Classification based on method of installation –

• Driven Piles (driven into soil by applying blows from the top)
• Cast-in-situ Piles (hole driven into the ground and then material poured)
• Driven and cast-in-situ Piles (form by driving the casing (closed form bottom end) into ground then filling the material into it and also casing may or may not be removed/ withdrawn after filling the material)

(f) Classification based on use –

These are however classified according to the usage of piles.

• Load bearing Piles
• Tension Piles
• Sheet Piles
• Compaction Piles
• Anchor Piles
• Fender Piles

The method of installation of piles is consider normally in the following types of piles :

• Driven piles
• Driven cast-in-situ piles
• Bored cast-in-situ piles

Procedure installation of piles

Piles are drive into the ground by means of hammers which may be the drop type or single acting or double acting steam hammers or diesel hammers or vibrating type. The weight of the hammer depends on length of piles and type of the pile i.e., whether it is timber, steel, concrete or bored pile etc. In case of a single acting hammer, weight varies from 15 kN to 100 kN and for double acting ; it varies from 3.5 kN to 25 kN.

Sometimes jetting technique are also adopt for pile driving in firm to stiff clays or soils mix with cobbles or boulders. Percussion drilling is also, in some of the cases, adopt e.g., in soils mix with gravel and boulders.

Batter piles are generally drive by partial augering. In this case, a power auger is use to drill the hole for some depth. The incline pile is insert in the hole and drive with hammers to the desire depth.

Functions of piles

The major functions of piles are :

• Resist vertical loads, uplift pressure, horizontal loads.
• Transfer vertical compression loads in the sub-soil.

In building construction various types of soil are use which is describe here.

Soil is a natural aggregate of mineral particles with or without organic content, which can be separate easily by mechanical means such as, agitation in water, frost, wind etc. Rock, on the other side, is also a material of earth crust which supports a building. It is a hard and compact natural aggregate of mineral particles cementing by strong permanent bonds. Knowledge is very essential for achieving safety and economy in foundation design and construction.

TYPES OF SOIL, ENCOUNTER IN CONSTRUCTION OF A BUILDING

• Gravel
• Sand
• Silt
• Clay
• Alluvial
• Black Cotton
• Reclaimed

Gravel ( soil particles varying from 4.75 mm to 80 mm )

The soil particles varying from 4.75 mm to 80 mm are refer as gavel. These are formed due to the disintegration of rocks and are transport by water from their source. Gravel do not exhibit swelling and shrinkage properties on wetting and drying respectively. These provide the best foundation surface, other than rock, for all types of structure. these are cohesion less soil particles.

Sand ( soil particles varying from 0.75 mm to 4.75 mm )

The soil particles varying from 0.75 mm to 4.75 mm are refer as sands. These are the cohesion less particles of silica derive from the disintegration of rocks. These are more permeable but do not allow water to rise up through the capillary action. It provides a good foundation surface provide it is enclose with the help of sheet piles wall etc. Sand is further classified according to particle size as coarse sand ( 2 mm – 4.75 mm ), medium sand ( 0.425 mm – 2 mm ) and fine sand ( 0.075 mm – 0.425 mm )

Silt ( particles varying from 0.002 mm to 0.075 mm )

The soil particles varying from 0.002 mm to 0.075 mm are refer as silt. It is a fine grain soil with little or no plasticity. It is less permeable as compare to sand. The non plastic variety consists of same size particles of quartz and plastic variety consists of appreciable percentage of flaky particles.

Silt exhibits the slight tendency of swelling and shrinkage and therefore, is not regard as good foundation material.

Clay ( particles having sizes less than 0.002 mm )

The soil particles having sizes less than 0.002 mm are refer as clay. These are derive from the chemical decomposition and disintegration of rock constituents. Clay exhibits the plastic property within a moderate range of water content. The permeability is very low and it is also exhibits the properties of swelling, shrinkage and capillary rise of moisture in the soil. The bearing power is very low and hence liable to cause more settlements. It provides cohesion and it cracks on drying because moisture is always present in its pores.

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The technical investigation by which the necessary information regarding various soil properties is obtain to enable the engineer in charge to design a safe and economical foundation is refer as sub surface investigation and Then, the various method of soil exploration are :

The method of soil exploration:

• Inspection
• Probing
• Boring
• Trail pit method

(i) Inspection : It is the first step and should be taken before carrying out the design and construction of the structure. In this, the study is make of all nearby quarries or cuts, resisting structure, all strata expose in the old pits, unlined wells etc. to determine the nature of the soil at various depths. The useful information which can be gather from inspection is to obtain data regarding sulphates, contents in soil, chances of land movements, changes in water table, type of soil whether soft or hard or marshy or water log.

(ii) Probing : In this, a rod about 25 mm in diameter is driven in the ground. It has a point end when it is taken out, the soil which sticks on its sides gives us the idea about the strata available.

(iii) Boring : In this, a hole is bore in the ground for taking soil samples. The exploration of soil below water table is usually very difficult to perform by means of test pits, Therefore, test pits are replaced by boring methods.

Method of boring

(a) Auger boring : It is the device which helps in advancing a bore hole into the ground. This is sample and an efficient method of soil investigation, Auger are use for clayey to silty soils.

Auger boring gives fairly representative disturb samples and a correct idea of different soil layers. The relative stiffness of the soil is indicate from the effort apply to rotate the auger. Auger is operate either by hand or by power. Hand operate augers are suitable for soft soils and up to a depth of 6 meters whereas power operate augers are suitable up to a depth of 9 meters. The most common augers in use are:

• Post hole augers ( for general use )
• Gravel augers ( for gravelly soils )
• Screw augers ( for stiff clay )
• Power or mechanically operate auger ( for greater depths and when boring is require in a comparatively short time )

(b) Wash boring : Wash boring is a quick and inexpensive method of deep boring. It is the best boring where sufficient water is available for the proper working of the water jet plant and also for underground formations consisting of sand, gravel and clayey formations and suitable for obtaining samples of soil as heavier soil particles remain under suspension. Small thickness of important soil layers also remain undetected. The change of stratification can be judge from the rate of progress and color of wash water.

(c) Rotary boring : This process of boring is also know as Mechanical boring. It is use for very deep boring specially when the soil consists of boulders or rock. This is the most fastest method of boring and the bore holes range from 10 to 75 cm.

(d) Percussion boring : In this, a drilling rod is raise up and down alternately in the bore hole. It is use in most of the soils and rocks. It is consider to be the most useful method in penetrating through coarse gravel deposits. They cannot be use for soil consisting of loose sand or fine sand.

Trail Pit Method

(iv) Trail pit method : The method in which a rectangular pit is excavate at the construction site in order to obtain soil samples at different levels is refer as trail pit method or open test pit method.

It is one of the simplest and most common method for investigating the sub-soil in its natural and undisturbed state. This method is limit to small depths up to 3 m only.

This method is very expensive and slow for depths exceeding 3 m and for cuts below water table due to proper bracing of sheet pilling or the use of caissons. If the depth exceeds 6 m, then, boring are prefer.

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