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INTRODUCTION Transportation - needed for the development of the country. The main mode of transportation is by road or highways. When the soil on a project site cannot support the loads, ground improvement to increase strength, reduce compressibility, and enhance performance under applied loadings may be required. There are various techniques of ground improvement.

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METHODS FOR GROUND IMPROVEMENT Soil stabilization Vertical drains Capillary cut-off Soil nailing Stone columns Vibro compaction Dynamic compaction

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SOIL STABILIZATION Improvement of stability or bearing capacity of soil by use of controlled compaction or by the addition of suitable admixtures or stabilizers .There are various methods of soil stabilization .

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The methods of soil stabilization are: Mechanical Stabilization Soil-cement Stabilization Soil-lime Stabilization Soil-bitumen Stabilization Chemical Stabilization Electrical Stabilization Stabilization by grouting Stabilization by geotextile and fabrics Reinforced earth Using bioenzymes

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Mechanical Stabilization Mechanical Stabilization Process of improving the properties of soil by changing its gradation. Two or more natural soils are mixed to obtain a composite material. Cement StabilizationCement Stabilization Done by mixing soil and cement with water and compacting the mix to attain a strong material. Lime StabilizationLime Stabilization Lime stabilization is done by adding lime (2%-10%) to soil. 6

Bituminous StabilizationBituminous StabilizationBBituminous stabilization provide water proofing and binding. Chemical StabilizationChemical StabilizationStabilizationStabilization by adding different chemicals. Electrical StabilizationElectrical StabilizationDDone by a process known as electro-osmosis.

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Stabilization by Geotextiles and FabricsStabilization by Geotextiles and FabricsGeotextile which have very high tensile strength can be used as reinforcement for strengthening soil. Reinforced EarthReinforced EarthSoil can be stabilized by introducing thin strips in to it . Stabilization using Bio-EnzymesStabilization using Bio-EnzymesBio-enzyme stabilization is a newer technique for strengthening of sub grade soil. Terra Zyme is one of the largely used bioenzymes.

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VERTICAL DRAINS Act as free draining water channel. surrounded by a thin filter jacket which prevents the surrounding soil from entering the core. A vertical sand drain accelerates the rate of consolidation. Installation of vertical sand drains is a convenient technique for stabilization of soft and compressible soil . There are two types of vertical drains - sand drains and sand wicks.

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10Fig : vertical drains

Sand drainsSand drainsTypically 200-500 mm in dia.Formed by infilling sand in to a hole in the ground.Hole formed by driving ,jetting or augering.Typical spacing 1.5 - 6.0 .

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Sand wicksSand wicks Sand wicks are improved technique of sand drains. A small diameter hole is made by driving mandrel or by boring. Then cylindrical bag with sand is lowered into this. A wick drain is usually about, 4 inches wide, 1/8-inch thick which acts as a high-permeability conduit for water to flow out of the soil and to the surface. Most common application of wick drains is for accelerating the settlement rate of compressible soils.

CAPILLARY CUT-OFFIn some cases capillary water accumulates and saturates the subsurface layers which results in failures.To arrest this capillary rise, capillary cut-off has to be provided.Capillary cut off is of two types. Permeable Capillary Cut-off Impermeable Capillary Cut-off

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Permeable Capillary Cut-off A layer of granular material is provided which has a thickness higher than the capillary rise so that water cannot rise above the cut-off layer

Fig:-Cross-Section of pavement showing Permeable capillary cut-off

Impermeable Capillary Cut-off An impermeable capillary cut-off is prepared by inserting bituminous layer in place of permeable blanket.

15Fig: Cross-Section of pavement showing impermeable capillary cut-off

SOIL NAILING This ground reinforcement process uses steel tendons which are drilled and grouted into the soil to create a composite mass. Earth retention structure that combines reinforcements and shortcrete to support excavations , hillside , embankment steeping etc… The nails must have bending stress. The tension developed in nails provides resisting forces which stabilize the soil mass.

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HOLES DRILLED THROUGH MACHINES

18Fig: Soil nail

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SOIL NAILING

STONE COLUMNSDone to provide adequate support for relatively light foundation. The method consists of forming vertical holes in ground which are filled with compacted crushed stone, gravel and sand or a mixture.The soil-column matrix results in an overall mass having a high shear strength and a low compressibility.

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VIBRO COMPACTION For loose sand deposits, the density can be increased by vibro compaction. The action of the vibrator, usually accompanied by water jetting, reduces the inter-granular forces between the soil particles, allowing them to move into a denser configuration, typically achieving a relative density of 70 to 85 percent. This process employs a depth vibrator suspended from crane Compaction of sand can be achieved up to distance of 2.5m from axis of vibrator. Compaction can be carried out to significant depths up to 12m.

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PROCESS OF VIBRO COMPACTION

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DYNAMIC COMPACTION Dynamic Compaction is the dropping of heavy weights on the ground surface to densify soils at depth. It involves the repeated dropping of large steel tampers by means of crawler cranes. Tampers typically range from 6 to 20 tons and are dropped from heights of about 40 to 80 feet.

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Fig :Dynamic compaction Equipment

CONCLUSIONS While constructing highways different ground conditions are encountered. Considering all factors a suitable ground improvement technique has to be done. Ground improvement techniques have been extensively used by developed countries. Lime stabilization is suitable for expansive soil like black cotton soils. In bitumen stabilization optimum content of bitumen vanes from 4 to 6 % soil. Electrical and Chemical stabilization are expensive compared to others. Stabilization by grouting is suitable only for soils with high permeability. Stabilization by Terra Zyme is most effective for fine grained soil.

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Provision of capillary cut-off is effective for expansive soils.For soft and compressible soils provision of sand drains and sand wicks are found to be effective.Dynamic compaction is found suitable for soft, loose cohesion less soil layers.. Stone column has also been found useful for soft soils.

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CONCLUSIONS CONTINUE……..

References Sanjay Kr Singh & Pradeep Kr Gupta, (2002), "Critical review of ground improvement techniques for highways", Vol 30, No: 8, Indian Highways, NewDelhi, pp. 5-13. Gaulkar.M.P, (1999), "Construction of roads III black cotton soils", Vol 27, No: 3, Indian Highways, New Delhi, pp. 37-43. Sharma.S.C, (1994), "Ground Improvement Techniques", Vol. 22, No: 12, Indian Highways, New Delhi, pp. 3-4. (1994), "Application of Geogrids in Highway Engineering", Special report: 12, Indian Road Congress, New Delhi, pp. 59-61. Biju.P.B, Kuncheria.P.Issac, (2003), "Studies Stabilization Using TerraZyme for Pavement (M.Tech Thesis Report). on Soil Subgrade" Sharma.V.M. et.al, (2000), "Soil Nailing" ,Vol:28, No: 1, Indian Highways, New Delhi, pp. 46-52. Arora.K.R, (2003), "Soil Mechanics and Foundation Engineering", Standard Publishers Distributors, Delhi.

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