Different Methods of Soil Stabilization | Civil Engineering Optional Notes for UPSC PDF Download

Soil Stabilization

The process of improving the engineering properties of the soil by using different admixtures is called soil stabilization. To summarise, the engineering properties of soil are modified to achieve more stable soil. The stabilized soil may form the subgrade or wearing a layer of the road.

Some of the objectives of soil stabilization are as follows:-

1. Poor-quality soils can be substituted with aggregates with better engineering properties.
2. Strengthening the soil and increasing its bearing capacity.
3. Waterproofing is used to protect natural or man-made structures.
4. To promote the use of waste geomaterials in the construction of buildings.
5. To improve permeability properties.
6. To improve undesirable soil properties like excessive swelling or shrinkage, high plasticity, and so on.
7. To make use of low-quality local materials.

Some of the materials used for soil stabilization process are:-

• Different gradations of soil.
• Aggregates of various grades.
• Lime, Cement and Fly Ash
• Polymers, both synthetic and natural.
• Solid municipal, mining, and industrial waste.
• Geomaterials, which include geogrids and geo blankets.

Soil Stabilization Methods

As we know, the purpose of soil stabilization is to enhance the engineering properties of the soil. This can be done with the help of various soil stabilization techniques. Different types of methods that can be used for soil stabilization techniques are:-

Mechanical Stabilization of Soil

• This category includes physical procedures such as compacting or tamping with machinery such as rollers or rammers. Heavy machinery capable of applying a large compressive force is required to carry out this procedure. This usually consists of a massive compactor or, in rare cases, a crane with falling weight.
• The soils stabilized by mechanical soil stabilization are frequently rocky and sandy soils. Because of advancements in other types of soil stabilization, mechanical stabilization is rarely the sole stabilization method.
• While mechanical soil stabilization is not widely used, it does have benefits. One advantage is that the soil particles are physically transformed, which means that no chemical changes occur that will eventually wear away. Because of the physical modification, the method is known to be time-consuming, difficult, and expensive.

Advantages

The advantages of mechanical stabilization of soils are:-

• Environmentally friendly.
• Permanent.

Disadvantages

Apart from the advantages, some disadvantages of the mechanical stabilization of soils are:-

• Expensive.
• Labour intensive.
• This is no longer a popular method.

Factors Affecting the Mechanical Stabilization of Soils

The mechanical stability of mixed soil depends upon the following variables:

• Gradation- The gradation of the mixed soil should be such that the voids created by the coarser particles are filled with finer particles, resulting in a high density.
• Soil Properties- Soils with a higher liquid limit and plasticity index are generally suitable for use as soil binders for surfacing. These soils are more cohesive and have a higher moisture retention capacity. They provide a better seal against surface water movement downward.
• Compaction- The mechanical stability of the stabilized soil mass is determined by the amount of compaction achieved in the field. Compaction is typically performed at or near the optimum moisture content.
• Mechanical Strength of the Aggregates- The mixed soil becomes more stable when the aggregates used are of high strength. However, if the mixture is properly designed and compacted, even low-strength aggregates can provide good mechanical stability.
• Mineral Composition- The mechanical stability of mixed soil is entirely dependent on the mineral composition. As a result, the minerals must be weather resistant.

Chemical Stabilization of Soil

As the name says, the chemical stabilization of soil depends upon the chemical reaction that occurs between the stabilizer used and the soil particles. Some of the materials used include cement, lime, magnesium chloride, bitumen emulsion and fly ash.

Cement or Lime Stabilization of Soil

• Soil stabilization with cement and lime is a popular method of soil stabilization. This soil stabilization method involves mixing lime or cement into the soil to increase its strength and resistance.
• The percentage of lime or cement mixed into the soil varies according to the qualities of the native soil. The more plastic the soil, the more lime or cement is usually added. Because lime and cement are both used as binders, they are frequently combined.
• Although soil stabilization with cement and lime is a common technique, it is most commonly used on paved roads. Typically, treating unpaved roads with cement is prohibitively expensive.
• The cement or lime stabilization of soil is frequently influenced by geography. Because some areas have easy access to lime while others do not, making the cement more cost-effective.
• Soil stabilization with lime or cement works by binding all of the soil's particles together, thereby increasing the soil's strength. Because this method of soil stabilization requires the addition of cement or lime to the soil, nearly all soil types are compatible with it.
• Soil testing is required to ensure that the correct amount of additives are used. If only a small amount of additive is used, the soil will not reach the required strength. If more is applied, the soil may shrink or crack.

Advantages

Some of the advantages of soil stabilization with cement and lime are:-

• Permanent and long-lasting.
• It is compatible with the majority of soil types.
• This method of soil stabilization is tested and proven.
• Reduces the moisture content of the soil.

Disadvantages

Apart from the advantages, some disadvantages of cement and lime stabilization of soil are:-

• Expensive.
• Possible health risks.
• Only applies to paved roads.
• Comprehensive soil testing is required prior to application.

Soil Stabilization by Fly Ash

• The product used in fly ash soil stabilization differs from that used in lime or cement soil stabilization, but the method is similar. Fly ash is a coal byproduct used in coal-fired power plants. 
• Fly ash is typically stored in landfills or ponds, but it can also be used as a bonding agent to strengthen the soil. Fly ash is commonly used to treat the sub-base of paved roads in the same way that cement and lime stabilization are used for soil stabilization as the sub-base of paved roads.
• Because fly ash is a dry additive, one significant advantage is that it can be used on a regular basis in the soil above optimum moisture levels. The powder will absorb moisture and reduce the soil's moisture content.
• Soil stabilization by fly ash is typically less expensive than cement and lime stabilization of soil. Fly ash is a byproduct of various manufacturing processes. Fly ash has the distinct advantage of being a dry additive, which is especially beneficial in high-moisture soils. The disadvantage of being a dry additive is that it is difficult to apply and poses a health risk to workers due to the powder's inhalation risk.
• The curing time is another factor to consider when using fly ash. While the cure period varies according to the fly ash and soil, it is typically longer than that of cement or lime-treated soil.

Advantages

Some advantages of soil stabilization by fly ash are as follows:-

• Long-lasting, permanent.
• Most soil types are compatible.
• Various types for various applications.
• More affordable than cement or lime stabilization of soil.
• Useful in high moisture soils.

Disadvantages

• Some of the disadvantages of soil stabilization by fly ash are:-
• Based on the availability of coal-fired power plants.
• Labour intensive method.
• Health concerns.

Polymer Stabilization

• The addition of polymers to soils to improve their physical and engineering properties is known as polymer stabilization of soil. Polymers tend to increase soil strength through their interaction with clayey particles in the soil. Many of the polymers currently in use improve the soil water retention and shear strength. There are two types of polymers used for soil stabilization: biopolymers and synthetic polymers. Biopolymers are less harmful to the environment than other chemical soil stabilizers.
• Polymers function on the basis of a long repeating molecule, which means soil particles bond to the molecule. Polymers are extremely compatible with all types of soil, from sandy to clay. The size of a polymer's molecule affects its performance on any specific soil because the soil particle must fit into the molecule. The polymer's molecule size is especially important because a molecule that is too large may not allow a small clay particle to fit properly, or vice versa with a small molecule and a large sand particle.
• Polymers are versatile soil stabilizers, but not all of them are appropriate for fine soils. The lifespan and properties of polymers may vary depending on their properties. Traditional treatments, on the other hand, endure for 1 to 3 years with minimal upkeep. The majority of polymer applications result in an impermeable surface that is impervious for the duration of the application. However, just as lifespan varies, so does water performance.

Advantages

Some advantages of polymer stabilization of soils are:-

• Simple to use.
• Impermeable.
• Compatible with all types of soils.

Disadvantages

Apart from the advantages, the polymer stabilization of soil has some disadvantages. They are as follows:-

• Not always good for the environment.
• Lifespan is limited.
• Each application necessitates a large number of products.

Soil Stabilization using Geotextiles

• Geotextiles are materials that are applied to soil to help with soil stability, erosion control, and drainage. Geotextiles come in a range of sizes and thicknesses. Some are woven, while others are thick plastic extrusions up to four inches thick. Geotextiles are mostly used to prevent soil from eroding. They protect the top layer of soil from being moved by rain or water.
• Geotextiles can be used for road construction in specific locations and for specific purposes. Thicker extrusion geotextiles are frequently required for road construction to provide the required strength. Geotextiles come in a variety of shapes and sizes and can be used on almost any soil type. 
• Because of their high cost, they are rarely used for road construction or large, heavy-load applications. Because of their mechanical nature, geotextiles are typically labour-intensive, as labourers must manually lay them out.

Advantages

Some advantages of soil stabilization using geotextiles are:-

• Long-term, permanent
• Most soil types are compatible with it.
• Various types for different applications.

Disadvantages

• Time-consuming
• Labour intensive
• Expensive

Bituminous Soil Stabilization
Bituminous materials such as tar and asphalt are commonly used in Bituminous Soil Stabilization. Bituminous stabilization improves soil stability while decreasing water absorption quality. Because of this behaviour, the soil does not erode easily, keeping the surface intact for a longer period of time. For better results, combinations of soil, sand, oil, and other materials are used in addition to bitumen.

Some Other Methods of Soil Stabilization
Despite the above-mentioned soil stabilization methods, there are numerous other methods that are sometimes used if the above methods are not applicable to the situation.

Other methods of soil stabilization include:-
Mass Stabilization Method

• Mass stabilization is a shallow to deep stabilization method that can stabilize the entire volume of soft soil to a specified depth. The technique is still relatively new, but it is well suited for the stabilization of high moisture content soils such as clay, silt, organic soils and contaminated sediments.
• The soil mass can be blended using an excavator-mounted mixing tool with unique shuttles pneumatically delivering the binder to the mixing tool's head and into the mix zone, or by self-injection of the binder into a rotating auger or mixing head and the soil.
• Mass stabilization is a cost-effective solution for ground improvement in site remediation, especially with high water content and large quantities of contaminants.

Deep Mixing Method
​​The deep mixing method involves stabilizing soils at great depths. It is an in-situ ground modification technology in which a wet or dry binder is injected into the ground and mechanically or rotary mixed with in-situ soft soils (clay, peat, or organic soils).​​

Grouting Method
In the grouting method of soil stabilization, stabilizers are injected into the soil for stabilizing the soil. This method is ineffective for clayey soils as they have low permeability. This is an expensive method of soil stabilization. Grouting method is appropriate for stabilizing buried zones of relatively limited size.
Electrical Method
Electrical stabilization is the process of passing an electrical current through a soil mass (under consideration) in order to migrate charged particles in the soil and change its properties, resulting in overall soil stabilization. This soil stabilization method is primarily used for weak clayey soils. As we all know, weak clayey soil necessitates high-strength treatment. They also have a low hydraulic conductivity, which prevents the use of chemical grout via traditional hydraulic means.