Soil improvement in its broadest sense is the alteration of any property of a soil to improve its engineering performance. This may be either a temporary process to permit the construction of a facility or may be a permanent measure to improve the performance of the completed facility. The result of an application of a technique may be increased strength, reduced compressibility, reduced permeability, or improved ground water condition.
Need for Stabilization of Earth Roads
An earth road is one whose foundation and wearing surface is composed of solely of the natural soil present originally on the site. Soils can be classified into two categories – cohesion less and cohesive soils. It has been observed that regions that are predominantly clayey do not usually have sandy materials. Clays must be considered as very important and often determining soil component since it has two objectionable qualities that make it the most troublesome of the materials to be dealt with. It swells when subjected to wetting, and shrinks with drying.
Clays and silts are low-grade construction materials, which find use in impervious elements such as cores (dams), cut-offs, they are poorly drained, and they shrink and swell. Also, clays when wet lose all strength; they are highly compressible, producing undesirable settlement as sub-grades of highways. Sands, though, having good drainage properties are also not suitable, as they lack cohesion and spread laterally under vertical loads. Thus, either of the two types alone cannot take the traffic independently. Therefore, combination of the two in certain specific proportions and thorough compaction with or without the use of additives may result in a stable sub-grade. A stabilized material may be considered as a combination of binder-soil and aggregates preferably obtained at or near the site of stabilization, and compacted so that it will remain in its compacted state without detrimental change in shape or volume under the force of traffic and exposure of weather. Several materials have been used as soil stabilizing agents. Of these, the best stabilizer will be the one involving minimum cost and at the same time providing durable effect. The technique is mainly applied in Road construction soil, and is termed as Mechanical Stabilization or Granular Stabilization. The process of mechanical stabilization is used both for base-courses as well as surface-courses. A good mechanically stable base or surfacing usually consists of a mixture of coarse aggregates (gravel, crushed rock, slag, etc.), fine aggregates (natural or crushed stone, sand, etc.), silt and clay, correctly proportioned and fully compacted. The use of correctly proportioned materials is of particular importance in the construction of low-cost roads. The principle of grading soils may be applied to the improvement of sub-grade soils of low bearing capacity, by adding to them materials having particle sizes that are lacking, e.g. sand can added to clay sub-grades and vice versa.
Techniques of Soil Improvement
The various techniques of soil improvement are:-
1 Surface Compaction
2 Drainage Methods
3 Vibration Methods
4 Precompression and consolidation
5 Grouting and Injection
6 Chemical Stabilization
7 Soil Reinforcement
8 Geotextiles and Geomembranes
9 Other Methods
These techniques are briefly described as follows:
1. Surface Compaction
One of the oldest methods of soil densification is compaction. Construction of a new road, a runway, an embankment or any soft or loose site needs a compacted base for laying the structure. If the depth to be densified is less the surface compaction
alone can solve the problem. The usual surface compaction devices are rollers, tampers and rammers. All conventional rollers like smooth wheel, rubber-tyred, sheep foot, vibratory and grid rollers can be used.
2. Drainage Methods
Ground water is one of the most difficult problems in excavation work. The presence of water increases the pore water pressure and decreases the shear strength. Further heavy inflow of water to the excavations is liable to cause erosion or collapse of the sides of open excavations. Certain methods are available to control the ground water and ensure a safe and economical construction scheme.
Common drainage methods are Well-point Systems, Deep- well Drainage, Vacuum Dewatering system, Dewatering by Electro-osmosis etc
3. Vibration Methods
Vibration methods can be effectively used for rapid densification of saturated noncohesive soils. Vibrations and shock waves in loose deposits of such materials cause liquefaction followed by densification accompanying the dissipation of excess pore water pressures. Some of the mostly adopted vibration methods are blasting, Vibrating probe, Vibratory rollers, Vibro-displacement Compaction Piles, Vibrofloatation, Heavy Tamping etc.
4. Pre-compression and Consolidation
This method aims to consolidate the soil before construction. Various techniques adopted are Preloading and Surcharge Fills, Vertical Drains, Dynamic Consolidation, Electro osmotic Consolidation etc.
5. Grouting and Injection
Grouting is a process whereby’ stabilizers, either in the form of suspension or solution are injected into subsurface soil or rock for one or more of the following applications: -Control of ground water during construction
-Void filling to prevent excessive settlement
-Strengthening adjacent foundation soils to protect them against damage during excavation, Pile driving, etc.
-Soil Strengthening to reduce lateral support requirements
-Stabilization of loose sands against Liquefaction
-Reduction of machine foundation vibrations
Grouting is done by Suspension Grouts which include grouting with Soil, Soil-cement Mixes, Cement, Lime, Displacement Grouting and by Solution Grouts using "one shot" or "two shot" systems.
6. Chemical Stabilization
Chemical Stabilization has been widely used in the form of lime, cement, fly ash and the combination of the above is widely used in soil stabilization. Chemical Stabilizations reduce permeability of the soils, improve shear strength, increase bearing capacity, decrease settlement and expedite construction. Chemical Stabilization is used for surface soils more successfully. Mixtures of soils and chemicals are mixed either mechanically in place or by batch process. Some of the chemicals used are Lime, Cement, and Fly Ash etc.
7. Soil Reinforcement
Soil Reinforcement is in the form of a weak soil reinforced by high-strength thin horizontal membranes. A large variety of materials such as rubber, aluminum and thermoplastics have been used successfully.
8. Geotextiles and Geomembranes
Geotextiles are porous fabrics manufactured from synthetic materials, which are primarily petroleum products and others, such as polyester, polyethylene, polypropylene and polyvinyl chloride, nylon, fibreglass and various mixtures of these. Geotextiles are used as separators, filters, Drains, reinforcement, geomembranes etc.
9. Other Methods
Other methods include Thermal methods, Moisture barriers, Prewetting, addition or removal of soils, etc.