GROUND IMPROVEMENT TECHNIQUES IN HIGHWAY CONSTRUCTION

1. INTRODUCTION

Transportation is one of the fields which play an important role in the development of a country. There are many means of transportation such as road, railway, airways etc... But the main mode of transportation is always 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. This seminar deals with such ground improvement techniques such as soil stabilization methods, use of geotextiles, vertical sand drains, stabilizing trenches, capillary cutoff, soil nailing, vibro-compaction , vibro-floatation, vibro-replacement, dynamic compaction, stone columns etc…

2. METHODS FOR GROUND IMPROVEMENT

1. Soil stabilization
2. Vertical drains
3. Stabilization trenches
4. Capillary cut-off
5. Soil nailing
6. Vibro compaction
7. Dynamic compaction

2.1 Soil Stabilization

Stabilization is the process of blending and mixing materials with soil to improve certain properties of soil. The process may include the blending of soil to achieve a desired gradation or the mixing of commercially available additives that may alter the gradation, texture or plasticity, or act as a binder for the cementation of soil. It improves the stability or bearing capacity of soil by use of controlled compaction or by the addition of suitable admixtures or stabilizers

The methods of soil stabilization are:

1. Mechanical Stabilization
2. Additive stabilization
3. Stabilization with Portland cement
4. Stabilization with lime
5. Stabilization with bitumen
6. Stabilization by geotextile
7. Reinforced earth
8. Using bio-enzymes

2.1.1 Mechanical Stabilization

It is accomplished by mixing or blending of soils of two or more gradation to obtain a material meeting the required specification. It may take place at the construction site , a central plant ,or a borrow area. The blended material is compacted to required densities by conventional means.

2.1.2 Additive Stabilization

It is obtained by addition of proper percentage of cement , lime , flyash , bitumen or combination of these materials to the soil. The selection of the type and determination of the percentage of additive to be used is dependent upon the soil classification and the degree of improvement in soil quality desired. It is usually used to modify properties such as gradation, workability, and plasticity.

2.1.3 Stabilization with Portland Cement

Portland cement can be used either to modify and improve the quality of soil or to transform the soil into a cemented mass with increased strength and durability. The amount of cement used will depend upon the soil to be modified or stabilized. It is done by mixing soil and cement with water and compacting the mix to attain a strong material.

2.1.4 Stabilization with Lime

Lime stabilization is done by adding lime(2-10%) to soil. In general a lime treated fine grained soil exhibit decreased plasticity , improved workability and reduced volume changecharacteristics. The properties of lime soil mixtures are dependent on many variables like soil type, lime type, lime percentage and curing conditions.

Many type of lime have been successfully used as soil stabilizing agents for many years. The most commonly used products are hydrated high calcium lime, monohydrated dolomitic lime, calcitic quick lime and dolomitic quick lime.

2.1.5 Stabilization with Bitumen

Stabilization of soil and aggregates with asphalt differs greatly from cement and lime stabilization. This basic mechanism involved in asphalt stabilization of fine grained soil is a water proofing phenomenon. Soil particles or soil agglomerates are coated with asphalt that prevents or slows the penetration of water which could normally result in decrease in soil strength in addition asphalt stabilization can improve durability characteristics by making the soil resistant to the detrimental effects of water such as volume. In non cohesive materials such as sands and gravel ,crushed gravel and crushed stone , two basic mechanisms are active water proofing and adhesion.

2.1.6 Stabilization with Reinforced Earth

Soil can be stabilized by introducing thin strips in to it as reinforcements.Current advances in new construction material have resulted in soil retention systems through reinforced earth construction which offer a new application for flood control and stream bank protection control measures. Most conventional applications of reinforced earth systems have primarily involved earthwork slope stabilization or roadway projects. However, reinforced earth systems offer numerous advantages to conventional stream bank stabilization techniques which include: structural reinforcement of the earthen stream bank, ease of construction with normal fill operations, low costs, and an inert mattress that becomes an integral matrix with the earthen bank. Geogrids are typically manufactured of high density polyethylene (HDPE) resins that resist elongation when subjected to high load for long periods of time. The geogrids can carry large tensile load applied in one or two directions, and their open structure interlocks with the natural earthen fill material. The geogrid installation results in a structurally stable embankment which reacts similar to a gravity wall. In addition, the geogrid constructed stream banks can provide a steeper than normal slope face which can be naturally landscaped and provides an aesthetic alternative to conventional stream bank revetments.

2.1.7 Stabilization with Bio-Enzymes

Recently several environmental friendly enzymes have come into the market such as fujibeton, terrazyme, and renolith etc..use of this products indicates minimization , elimination of the use of aggregates and is referred to as aggregate free pavement technology

2.1.7.1 Fujibeton As A Soil Stabilizing Agent

It is a material developed in japan. It is a climatically stable material and suitable for stabilization of all types of soil. The product is an inorganic polymer that chemically binds with all compounds, where blended with ordinary Portland cement in I to 3% by weight. The balanced mix is called fujibeton mix which is used for stabilization of soil that improves the engineering properties of soil

2.1.7.2 Terrazyme As A Soil Stabilizing Agent

it is a natural, non toxic, environmentally safe, bio enzyme, produt that, improves engineering qualities of soil reduses, ruts and pathioles resulting in more durable and longer lasting roads. The function of terrazyme is to minimize absorbed water in the soil for maximum compaction which decreases the swelling capacity of the soil particles and reduces permeability .the application enhances weather resistance.

The advantages of Terrazyme technology are:-

1. Considerable improvement in soil CBR.

2. Minimum lose of gravel due to erosion or abrasion by the traffic preserving original transverse section of slopes.

3. Impediment of wide spread occurances of dust from loose fine material on the road surface.

Case study of the two roads built by pwd of Maharashtra revealed that the use of terrazyme resulted in overall cost saving in the range of 18-26%.

2.1.7.3 Soil Cement Renolith Stabilization Technique

Renolith is a polymer based chemical, which is environmental friendly and which facilitates the bonding of soil particles and soil cement. Renolith has high modulus of elasticity and can disperse the wheel loads very efficiently.it gives strong and durable base, reduseedmaintainance and dust free environment.

2.1.8 Use Of Geo-Textiles

Jute geotextiles is a kind of natural technical textile laid in or on soil to improve the engineering properties. It is made out of yarns obtained from the jute plant. It has high moisture absorption capacity. It has excellent dropability, high tensile strength.

2.2 Vertical Drains

Vertical drains are artificially created drainage paths which can be installed by one of several methods and which can have a variety drains along with precompression has the sole purpose of shortening the drainage path (distance to the drainage boundary) of the pore water, thereby accelerating the rate of primary consolidation. They act as free drainage 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 drain is a convenient technique for stabilization of soft and compressible soil.

There are two types of vertical drains-sand drains and sand wicks.

2.2.1 Sand Drains

Typically 200-500 mm in dia , They are formed by infilling sand into a hole in the ground. The hole is formed by driving , jetting or augering. The typical spacing is 1.5-6.0

2.2.2 Sand Wicks

Sand wicks are improved techniques of sand drains. A small diameter hole is made by driving mandrel or by boring. Then cylindrical bags with sand is lowered into this.

Fig. 1 Vertical drains

2.3 Stabilization Trenches

The trenches are excavated to provide good drainage. Excavation which has a filter material between 0.5m to 1m thick against its upstream slope and at the bottom of system for collecting and eliminating water.

These trenches improve the stability of embankment by providing drainage and replacing weaker material with better material.

Fig. 2 Stabilization trenches

2.4 Capillary Cutoff

In some cases capillary water accumulates and saturates the subsurface layers which results in failure. To arrest this capillary rise capillary cutoff has to be provided. There are two types of capillary cutoffs

1. Permeable capillary cutoff

2. Impermeable capillary cut off

2.4.1 Permeable Capillary Cutoff

Here in this case a layer of granular materials is provided which has a thickness higher than the capillary rise so that water cannot rise above the cutoff layer.

Fig. 3 Cross section of pavement showing permeable capillary cutoff

2.4.2 Impermeable Capillary Cutoff

An impermeable capillary cutoff is prepared by inserting bituminous layer in place of permeable blanket.

Fig. 4 Cross section of pavement showing impermeable capillary cutoff

2.5 Soil Nailing

The basic concept of soil nailing is to reinforce and strengthen the existing ground by installing closely placed steel bars, called nails into a slope or excavation as construction proceeds from the” top down”. this process creates a reinforced section that is internally stable and able to retain the ground mass. It is usually done in earth retention structure that combines reinforcements and shortcrete to support excavation , hill sides, embankments steeping etc..

The nails must have bending stress. The tension developed in nails provides resisting forces which stabilizes thesoil mass.

Fig. 5 Soil nailing

2.6 Vibro Compaction and Vibro Replacement

The vibrostone column tecqniqe is one of the most widely used ground improvement process in the world. Although its potential for improving irish sites has yet to be fully exploited. Historically the system has been used increasingly to reinforce soft cohesive soils and mixed fills

Vibro floatation is performed with a vibrating poker device which can penetrate to the required treatment depth under the action of its own vibrations. The penetration of poker is followed by the construction of a stone column. The hole created by the poker is filled with inert crushed stone or gravel (or approved constructionwaste ) and is compacted in stages from the base of the hole upwards. Alternating the stone may be fed from a rigid mounted hopper through a permanent delivery tube along the side of the poker, which bends inwards and allows the stone to exit at the poker tip. By remaining in the ground during column construction the poker cases its own hole and hence is suited to ground with a high water table or running sand conditions.

Fig. 6 Vibro compaction

2.7 Dynamic Compaction

During the past 10 to 15 years dynamic compaction has become an increasingly popular technique for densifiying uncontrolled fills and loose natural deposits. Dynamic compaction is highly cost effective in urban areas with relatively high land values. The procedure involves dropping a heavy steel weight from a height of up to 80 feet over the land surface to be compacted. An undesirable feature of dynamic compaction however is the generation of ground vibration caused by the dropping weight.

Ground vibrations can be potentially damaging to adjacent structures in addition to being annoying to people. It is therefore important that vibration monitoring be potentially damaging to adjacent structures in addition to being annoying to people. It is therefore important that vibration monitoring be performed whenever dynamic compaction is performed in close proximity to structures.

Fig. 7 Dynamic compaction

3. CONCLUSION

While constructing highways different ground conditions are encountered. Considering all factors a suitable ground improvement technique has to be done. Ground improvement technique has to be done. Ground improvement technique has to be done. Ground improvement techniques have be extensively used by developed countries.

Lime stabilization is suitable for expansive soil like black cotton soil. In bitumen stabilization water proofing can be achived.

4. REFERENCES

1. Colin j. serridge “Some application of ground improvement techniques in the urban environment”, IAEG (2006)

2. Mayne,P.W.,”ground vibrations during dynamic commpactioon”,presented at ASCE soil dynamics commitie convention in detroit,MIoctober,
3. Sharma.S.C, (1994), "Ground Improvement Techniques", Vol. 22, No: 12, Indian Highways, New Delhi, pp. 3-4.
4. Soil stabilization for pvements,head quarters department of the army and the airforce,Washington DC,
5. Prefabricated vertical drains,Research Development and technology, TURNER-FAIRBANK Highway research center, report no:FHWA/RD-S6/168
6. Arora.K.R, (2003), "Soil Mechanics and Foundation Engineering", Standard Publishers Distributors, Delhi.