16-07-2011, 04:52 PM
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1. INTRODUCTION
Large civil engineering projects are being executed in all over the country in order to enhance the infrastructure of the country. Infrastructure facilities have to be often built at sites where the soil conditions are not ideal. The insitu soil characteristics of a construction site are different from those desired, and almost always far from ideal for a designed need. With increased urban development, site with favorable foundation conditions became depleted. At times the civil engineer has been forced to construct structures at site selected for reasons other than soil conditions. Thus it is increasingly important for the engineer to know the degree to which soil properties may be improved or other alterations that can be thought of for construction of an 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.
Rigid foundations such as piling present a solution but these are often expensive. In such circumstances, ground improvement using different techniques offers a proved and economic solution. At present a variety of soil improvement techniques are available for making soil to bear any type of structure on it and also for mitigation of seismic hazards. The costs of these methods vary widely and the conditions under which they can be used are influenced by nature and proximity of structures and construction facilities.
On the basis of mechanism by which they improve the engineering properties of soil, the most of common of these can be divided into the following major categories. These are
• Densification techniques.
• Reinforcement techniques.
• Stabilization techniques.
• Miscellaneous methods.
The important ground improvement techniques for the highways are discussed in this paper.
From the methods listed above, there are some other simple methods like removal and replacement of soil. In this paper these are discussed first before taking up above techniques.
2. REMOVAL AND REPLACEMENT OF SOIL:
One of the oldest and simplest soil improvement methods is to simply excavate the unsuitable soil and replace them with compacted fill. This method is often used when the problem the soil is that it is too loose. In that case, the same soils used to build the fill, except now it has a higher unit weight (because of compaction) and thus has been better engineering properties. This is a common way to remediate problems with collapsible soils.
Removal also may be available option when the excavated soils have other problems, such as contamination or excessive organics, and need to hauled away. This method can be expensive because of the hauling costs and the need for imported soils to replace those that were excavated. It also can be difficult to find a suitable disposal site for the excavated soils.
Removal and replacement is generally practical only above the ground water table. Earthwork operations become more difficult when the soil is very wet, even when the free water pumped out, and thus are generally avoided unless absolutely necessary.
3. PRECOMPRESSION OF SOIL:
Another old and simple method of improving soils is to cover them with a temporary surcharge fill as shown in figure. This method is called precompression, preloading, or surcharging. It is especially useful in soft clayey and silty soils because the static weight of the fill causes them to consolidate, thus improving both settlement and strength properties. Once the desired properties have been obtained, the surcharge is removed and construction proceeds on improved site.
Pre-compression has the following advantages
• It requires only conventional equipment earthmoving equipment, which is readily available. No special or proprietary equipment is needed.
• Any grading contractor can perform the work.
• The results can be effectively monitored by using appropriate instrumentation and ground level surveys.
• The method has a long track record of success.
• The cost is comparatively low, so long as soil for preloading is readily available.
However, there also are disadvantages
• The surcharge fill generally must extend horizontally at least 10m beyond the perimeter of the planned construction. This may not possible for confined sites.
• The transport of large quantities of soil onto the sites may not be practical, or may have unacceptable environmental impacts (i.e., dust, noise, traffic) on the adjacent areas.
• The surcharge must remain in place for months or years, thus delay in construction.
1. INTRODUCTION
Large civil engineering projects are being executed in all over the country in order to enhance the infrastructure of the country. Infrastructure facilities have to be often built at sites where the soil conditions are not ideal. The insitu soil characteristics of a construction site are different from those desired, and almost always far from ideal for a designed need. With increased urban development, site with favorable foundation conditions became depleted. At times the civil engineer has been forced to construct structures at site selected for reasons other than soil conditions. Thus it is increasingly important for the engineer to know the degree to which soil properties may be improved or other alterations that can be thought of for construction of an 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.
Rigid foundations such as piling present a solution but these are often expensive. In such circumstances, ground improvement using different techniques offers a proved and economic solution. At present a variety of soil improvement techniques are available for making soil to bear any type of structure on it and also for mitigation of seismic hazards. The costs of these methods vary widely and the conditions under which they can be used are influenced by nature and proximity of structures and construction facilities.
On the basis of mechanism by which they improve the engineering properties of soil, the most of common of these can be divided into the following major categories. These are
• Densification techniques.
• Reinforcement techniques.
• Stabilization techniques.
• Miscellaneous methods.
The important ground improvement techniques for the highways are discussed in this paper.
From the methods listed above, there are some other simple methods like removal and replacement of soil. In this paper these are discussed first before taking up above techniques.
2. REMOVAL AND REPLACEMENT OF SOIL:
One of the oldest and simplest soil improvement methods is to simply excavate the unsuitable soil and replace them with compacted fill. This method is often used when the problem the soil is that it is too loose. In that case, the same soils used to build the fill, except now it has a higher unit weight (because of compaction) and thus has been better engineering properties. This is a common way to remediate problems with collapsible soils.
Removal also may be available option when the excavated soils have other problems, such as contamination or excessive organics, and need to hauled away. This method can be expensive because of the hauling costs and the need for imported soils to replace those that were excavated. It also can be difficult to find a suitable disposal site for the excavated soils.
Removal and replacement is generally practical only above the ground water table. Earthwork operations become more difficult when the soil is very wet, even when the free water pumped out, and thus are generally avoided unless absolutely necessary.
3. PRECOMPRESSION OF SOIL:
Another old and simple method of improving soils is to cover them with a temporary surcharge fill as shown in figure. This method is called precompression, preloading, or surcharging. It is especially useful in soft clayey and silty soils because the static weight of the fill causes them to consolidate, thus improving both settlement and strength properties. Once the desired properties have been obtained, the surcharge is removed and construction proceeds on improved site.
Pre-compression has the following advantages
• It requires only conventional equipment earthmoving equipment, which is readily available. No special or proprietary equipment is needed.
• Any grading contractor can perform the work.
• The results can be effectively monitored by using appropriate instrumentation and ground level surveys.
• The method has a long track record of success.
• The cost is comparatively low, so long as soil for preloading is readily available.
However, there also are disadvantages
• The surcharge fill generally must extend horizontally at least 10m beyond the perimeter of the planned construction. This may not possible for confined sites.
• The transport of large quantities of soil onto the sites may not be practical, or may have unacceptable environmental impacts (i.e., dust, noise, traffic) on the adjacent areas.
• The surcharge must remain in place for months or years, thus delay in construction.
