GROUND IMPROVEMENT TECHNIQUES FOR HIGHWAYS Guided by, Presented by, Ms.Susan Rose Paul James

INTRODUCTION Transportation - needed for the development of the country. The main mode of transportation is by road or highways. In the process of road development, the alignment of road may have to be fixed through the soils which may not bear the traffic loads . There are various techniques of ground improvement.

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

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 .

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

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 Stabilization Done by mixing soil and cement with water and compacting the mix to attain a strong material. Lime Stabilization Lime stabilization is done by adding lime (2%- 10%) to soil.

Done by a process known as electro-osmosis. Bituminous Stabilization Bituminous stabilization provide water proofing and binding. Chemical Stabilization Stabilization by adding different chemicals. Electrical Stabilization Done by a process known as electro-osmosis. Stabilization by Grouting In this method grouting is done under pressure the stabilizers with high viscosity are suitable only for soils with high permeability.

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

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.

Fig : vertical drains

Sand drains Typically 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 . Sand 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.

STABILIZING TRENCHES Excavation which has a blanket of filter material between 0.5m and 1.00 m thick against its upstream slope and at the bottom of system for collecting and eliminating water. Improves the stability of embankment by providing drainage and replacing weaker material with better material .

Fig : Stabilizing trench

CAPILLARY CUT-OFF In 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 cut­off is of two types. Permeable Capillary Cut-off Impermeable Capillary Cut-off

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

SOIL NAILING 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.

Fig: Soil nail

STONE COLUMNS Done 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. Fig : stone columns

VIBRO COMPACTION For loose sand deposits, the density index can be increased by vibro compaction. 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.

Fig : vibro-compaction

DYNAMIC COMPACTION This involves in increasing the density of soil near the surface by tamping. Density improvement up to 10m is feasible. This method consists of dropping heavy mass of 8 to 40 tonnes known as pounder on the surface from a height 5 to 30m

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.

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 cohesionless soil layers.. Stone column has also been found useful for soft soils.

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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