GROUND GROUND IMPROVEMENT IMPROVEMENT TECHNIQUESTECHNIQUES

Presented by Presented by MOHAMMED. OSMAN ALI MOHAMMED. OSMAN ALI

KHAN,KHAN,B.Tech., (Civil) Final Year, B.Tech., (Civil) Final Year,

Medak College of Engineering.Medak College of Engineering.

Abstract:Abstract:

Ground Improvement is the most Ground Improvement is the most imaginative field of geotechnical imaginative field of geotechnical engineering. It is a field in which the engineering. It is a field in which the engineer forces the ground to adopt the engineer forces the ground to adopt the projects requirements, by altering the projects requirements, by altering the natural state of the soil, instead of having to natural state of the soil, instead of having to alter the design in response to the grounds alter the design in response to the grounds natural limitations. The results usually natural limitations. The results usually include saving in construction cost ad include saving in construction cost ad reduction of implementation time.reduction of implementation time.

There are number of techniques available for improving the mechanical and engineering properties of the soil.

To improve the strength of the soils, especially in case of granular type of soils, COMPACTION METHODS are found as best methods among all type of techniques. Based on the mechanism applied for compacting the soil, it is sub divided into different methods like dynamic compaction, blasting, vibro techniques… etc.

Introduction:Introduction:

Large Civil Engineering Projects are Large Civil Engineering Projects are being executed in all over the country being executed in all over the country in order to enhance the infrastructure in order to enhance the infrastructure of the country. Thus it is increasingly of the country. Thus it is increasingly important for the engineer to know the important for the engineer to know the degree to which soil properties may be degree to which soil properties may be improved or other alternations that can improved or other alternations that can be thought of for construction of an be thought of for construction of an intended structure at stipulated site.intended structure at stipulated site.

If unsuitable soil conditions are encountered at the site of a proposed structure, one of the following four procedures may be adopted to insure satisfactory performance of the structure.

By pass the unsuitable soil by means of deep foundations extending to a suitable bearing material.

Redesign the structure and it’s foundation for support by the poor soil. This procedure may not be feasible or economical.

Remove the poor material and either treat it to improve and replace it (or) substitute for it with a suitable material.

Treat the soil in place to improve its properties.

Ground Improvement Ground Improvement Techniques:Techniques:

On the basis of mechanism by which they On the basis of mechanism by which they improve the engineering properties of soil, the improve the engineering properties of soil, the most of common of these can be divided into most of common of these can be divided into the following major categories. These arethe following major categories. These are

Densification Techniques.Densification Techniques. Reinforcement Techniques.Reinforcement Techniques. Stabilization Techniques.Stabilization Techniques. Miscellaneous Methods.Miscellaneous Methods.

Apart from the methods listed above, there are Apart from the methods listed above, there are some other simple methods like removal and some other simple methods like removal and replacement of soil. In this paper these are replacement of soil. In this paper these are discussed first before taking up above discussed first before taking up above techniques.techniques.

Removal and Replacement of Removal and Replacement of Soil:Soil:

One of the oldest and simplest soil One of the oldest and simplest soil improvement methods is to simply improvement methods is to simply excavate the unsuitable soil and replace excavate the unsuitable soil and replace them with compacted fill. This method is them with compacted fill. This method is often used when the problem the soil is often used when the problem the soil is that it is too loose. In that case, the that it is too loose. In that case, the same soils used to build the fill, except same soils used to build the fill, except now it has a higher unit weight (because now it has a higher unit weight (because of compaction) and thus has been better of compaction) and thus has been better engineering properties. This is a common engineering properties. This is a common way to remediate problems with way to remediate problems with collapsible soils.collapsible soils.

Removal and replacement is Removal and replacement is

generally practical only above the ground generally practical only above the ground

water table. Earthwork operations water table. Earthwork operations

become more difficult when the soil is become more difficult when the soil is

very wet, even when the free water very wet, even when the free water

pumped out, and thus are generally pumped out, and thus are generally

avoided unless absolutely necessary.avoided unless absolutely necessary.

Pre-compression of Soil:Pre-compression of Soil:

It requires only conventional equipment It requires only conventional equipment earthmoving equipment, which is readily earthmoving equipment, which is readily available. No special or proprietary equipment available. No special or proprietary equipment is needed.is needed.

Any grading contractor can perform the workAny grading contractor can perform the work The results can be effectively monitored by The results can be effectively monitored by

using appropriate instrumentation and ground using appropriate instrumentation and ground level surveys.level surveys.

The method has a long track record of success.The method has a long track record of success.

The cost is comparatively low, so long as The cost is comparatively low, so long as soil for preloading is readily available. soil for preloading is readily available. However, there also are disadvantages.However, there also are disadvantages.

The surcharge fill generally must extend The surcharge fill generally must extend horizontally at least 10m beyond the horizontally at least 10m beyond the perimeter of the planned construction. This perimeter of the planned construction. This may not possible for confined sites.may not possible for confined sites.

The transport of large quantities of soil onto The transport of large quantities of soil onto the sites may not be practical, or may have the sites may not be practical, or may have unacceptable environmental impacts (i.e., unacceptable environmental impacts (i.e., dust, noise, traffic) on the adjacent areas.dust, noise, traffic) on the adjacent areas.

The surcharge must remain in place for The surcharge must remain in place for months or years, thus delay in construction.months or years, thus delay in construction.

Densification Densification Techniques:Techniques:

The strength and stiffness of the soil is The strength and stiffness of the soil is

higher when the particles are packed in a higher when the particles are packed in a

dense configuration than they are dense configuration than they are

packed loosely. As a result, densification packed loosely. As a result, densification

is one of the most effective and is one of the most effective and

commonly used means of improving soil commonly used means of improving soil

characteristics. This can be approaches characteristics. This can be approaches

in following ways.in following ways.

Vibro Techniques:Vibro Techniques:

Vibro techniques use probes that are Vibro techniques use probes that are

vibrated through soil deposit in a grid vibrated through soil deposit in a grid

pattern to densify the soil over the pattern to densify the soil over the

entire area of thickness of the entire area of thickness of the

deposit. These are classified in to deposit. These are classified in to

the following methods. These arethe following methods. These are

1Vibro Compaction:1Vibro Compaction:Vibro Compaction is a method for Vibro Compaction is a method for

compacting deep granular soils by compacting deep granular soils by repeatedly inserting a vibratory probe. It repeatedly inserting a vibratory probe. It is also known as VIBRO DENSIFICATION.is also known as VIBRO DENSIFICATION.

By inserting depth vibrations, the By inserting depth vibrations, the vibrations are produced by rotating a vibrations are produced by rotating a heavy eccentric weight with the help of an heavy eccentric weight with the help of an electrical motor with in the vibrator. The electrical motor with in the vibrator. The vibratory energy is used to rearrange the vibratory energy is used to rearrange the granular particles in a denser state. granular particles in a denser state. Penetration of the vibro is typically aided Penetration of the vibro is typically aided by water jetting at the tip of the probe.by water jetting at the tip of the probe.

The Vibro-Compaction Process

Some of advantages and Some of advantages and disadvantages of this method are disadvantages of this method are given below.given below.

It is often an economical alternative It is often an economical alternative

to deep foundations, especially when to deep foundations, especially when

considering the added liquefaction considering the added liquefaction

protection in seismic ares.protection in seismic ares.

It is most effective in granular soilsIt is most effective in granular soils

It cannot be sued in cohesive soils.It cannot be sued in cohesive soils.

Vibro Flotation:Vibro Flotation: In vibro floatation a torpedo like probe (the In vibro floatation a torpedo like probe (the vibro float) suspended by a crane is used to vibro float) suspended by a crane is used to density a soil deposit. Vibro floats usually 12 to density a soil deposit. Vibro floats usually 12 to 18 inch in diameter and about 10 to 16 ft long, 18 inch in diameter and about 10 to 16 ft long, contain weights mounted eccentrically on a contain weights mounted eccentrically on a central shaft driven by electric or hydraulic central shaft driven by electric or hydraulic power.power.

The vibro float is initially lowered to the The vibro float is initially lowered to the bottom of the deposit by a combination of bottom of the deposit by a combination of vibration and water or air jetting through ports vibration and water or air jetting through ports in its pointed nose cone. The vibro float is then in its pointed nose cone. The vibro float is then incrementally with drawn in 2 to 3 ft intervals at incrementally with drawn in 2 to 3 ft intervals at an over all rate of about 1ft/min to loosen the an over all rate of about 1ft/min to loosen the soil above the vibro float temporarily and aid in soil above the vibro float temporarily and aid in its with drawl. The vibrations produce a its with drawl. The vibrations produce a localized zone of temporary liquefaction that localized zone of temporary liquefaction that causes the soil surrounding the vibro float to causes the soil surrounding the vibro float to densify.densify.

Principle of the techniquePrinciple of the technique

Vibro floating is most effective in clear Vibro floating is most effective in clear granular soils with the contents less than granular soils with the contents less than 20% and clay contents below 3%20% and clay contents below 3%

Vibro flotation has been used successfully to Vibro flotation has been used successfully to density soils to deep [this of up to 115ft]density soils to deep [this of up to 115ft]

Dynamic Compaction:Dynamic Compaction:Dynamic Compaction is normally used under the Dynamic Compaction is normally used under the

following circumstances:following circumstances: To increase in-situ density and this way improve To increase in-situ density and this way improve

the bearing capacity and consolidation the bearing capacity and consolidation characteristics of soils (or waste materials) to characteristics of soils (or waste materials) to allow conventional foundation and surface bed allow conventional foundation and surface bed construction to be carried out. The technique construction to be carried out. The technique typically improves the in-situ soils such that typically improves the in-situ soils such that allowable bearing pressures of up to 250 kpa allowable bearing pressures of up to 250 kpa can be used with foundation settlements of the can be used with foundation settlements of the order of 10 to 20 mm.order of 10 to 20 mm.

To increase in-situ density and in this way To increase in-situ density and in this way improve in-situ permeability and/or reduce improve in-situ permeability and/or reduce liquefaction potential.liquefaction potential.

What soils are suitable:What soils are suitable:

Most soil types can be improved, Most soil types can be improved,

including silts and some clays. The including silts and some clays. The

most commonly treated soils are old most commonly treated soils are old

fills and granular virgin soils. Soils fills and granular virgin soils. Soils

below the water table are routinely below the water table are routinely

treated. However, careful control has treated. However, careful control has

to be used to allow dissipation of to be used to allow dissipation of

excess pore pressures created during excess pore pressures created during

the weight dropping.the weight dropping.

Blasting:Blasting:

Blasting is most effective in loose sands Blasting is most effective in loose sands that contain less than 20% silt and less that contain less than 20% silt and less than 5% clay.than 5% clay.

Although blasting is quite economical, it Although blasting is quite economical, it is limited by several considerations, as it is limited by several considerations, as it produces strong vibrations that may produces strong vibrations that may damage near by structures or produce damage near by structures or produce significant ground movements.significant ground movements.

Reinforcement Reinforcement Techniques:Techniques:

In some cases it is possible to In some cases it is possible to

improve the strength and stiffness of a improve the strength and stiffness of a

existing soils deposit by installing discrete existing soils deposit by installing discrete

inclusions that reinforce the soil. These inclusions that reinforce the soil. These

inclusions may consist of structural inclusions may consist of structural

materials, such as steel, concrete or materials, such as steel, concrete or

timber and geomaterials such as timber and geomaterials such as

densified gravel.densified gravel.

Compaction Piles:Compaction Piles: Compaction piles improve the seismic performance Compaction piles improve the seismic performance

of a soil by three different mechanisms. First the of a soil by three different mechanisms. First the flexural strength of piles themselves provides flexural strength of piles themselves provides resistance to soil movement (reinforcement). resistance to soil movement (reinforcement). Second, the vibrations and displacements produced Second, the vibrations and displacements produced by their installation cause densification. Finally, the by their installation cause densification. Finally, the installation process increses the lateral stress in the installation process increses the lateral stress in the soil surrounding the piles.soil surrounding the piles.

Compaction piles generally densify the soil with in a Compaction piles generally densify the soil with in a distance of 7 to 12 pile diameters and consequently distance of 7 to 12 pile diameters and consequently installed in a grid pattern. Between compaction installed in a grid pattern. Between compaction piles a relative density of up to 75% to 80% are piles a relative density of up to 75% to 80% are usually achieved. Improvement can be obtained usually achieved. Improvement can be obtained with reasonable economy to depth of about 60 ft.with reasonable economy to depth of about 60 ft.

JET JET GROUTINGGROUTING

JET GROUTINGJET GROUTING

In jet grouting the soil is mixed with cement In jet grouting the soil is mixed with cement grount injected horizontally under high grount injected horizontally under high pressure in a previously drilled bore hole.pressure in a previously drilled bore hole.

Jet grouting uses a special pipe equipped Jet grouting uses a special pipe equipped with horizontal jets that inject grout into the with horizontal jets that inject grout into the soil at high pressure. The pipes are first soil at high pressure. The pipes are first inserted to the desired depth, then they are inserted to the desired depth, then they are raised and rotated while the injection is in raised and rotated while the injection is in progress, thus forming a column of treated progress, thus forming a column of treated soil.soil.

Because of high pressure, this method is Because of high pressure, this method is usable on a wide range of soil types.usable on a wide range of soil types.

Electro Osmasis and Electro Electro Osmasis and Electro Chemical Hardening Method:Chemical Hardening Method:

The electroosmasis process can be used to The electroosmasis process can be used to increase the shear strength and reduce the increase the shear strength and reduce the compressibility of soft clayey and silty soils compressibility of soft clayey and silty soils beneath foundation. By introducing an electrolyte beneath foundation. By introducing an electrolyte such as calcium chloride at the anode, the base such as calcium chloride at the anode, the base exchange reaction between the iron anode and exchange reaction between the iron anode and surrounding soil is increased, resulting in the surrounding soil is increased, resulting in the formation of ferric hydroxides which bind the soil formation of ferric hydroxides which bind the soil particles together. However because cost of particles together. However because cost of electric power and wastage of electrodes, electric power and wastage of electrodes, electroosmasis with or without electrochemical electroosmasis with or without electrochemical hardening can be considered only for special hardening can be considered only for special situations where the alternative of piling cannot situations where the alternative of piling cannot be adopted.be adopted.

Conclusion:Conclusion:

Unfavorable soil conditions can frequently be Unfavorable soil conditions can frequently be improved using soil improvement techniques. improved using soil improvement techniques. A variety of soil improvement techniques A variety of soil improvement techniques have been developed. However a suitable have been developed. However a suitable technique has to be adopt according to technique has to be adopt according to necessity of the structure and economy.necessity of the structure and economy.

Mainly soil improvement techniques can be Mainly soil improvement techniques can be divided in to four broad categories; divided in to four broad categories; Densification technique, Reinforcement Densification technique, Reinforcement technique, grouting or mixing technique and technique, grouting or mixing technique and stabilization technique.stabilization technique.

Densification is probably the most commonly Densification is probably the most commonly used soil improvement technique. Most used soil improvement technique. Most densification techniques relay on tendency densification techniques relay on tendency of granular soils to densify when subjected to of granular soils to densify when subjected to vibrations. However there is a possibility of vibrations. However there is a possibility of damaging adjacent structures and pipelines damaging adjacent structures and pipelines due to application of this technique.due to application of this technique.

Reinforcement techniques introduce discrete Reinforcement techniques introduce discrete inclusions that stiffen and strengthen a soil inclusions that stiffen and strengthen a soil deposit. The high stiffness and strength of deposit. The high stiffness and strength of the inclusions also tend to reduce the the inclusions also tend to reduce the stresses imposed on the weaker materials stresses imposed on the weaker materials between the inclusion.between the inclusion.

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