Methods of Soil Stabilization: Top 8 Methods | Soil Engineering

The following points highlight the eight major methods of soil stabilization. The methods are: 1. Lime-Pozzolana Stabilization 2. Cement Stabilization 3. Soil-Bituminous Stabilization 4. Organic Stabilizers 5. Thermal Stabilization 6. Electrical Stabilization 7. Complex Stabilization Technique 8. Complex Stabilization.

Method # 1. Lime-Pozzolana Stabilization:

Pozzolana is a siliceous material, which, while in itself possessing to cementitious properties will in a finely divided form and in the presence of water, react with calcium hydroxide and form cementitions compounds.

Pozzolana can be naturally occurring volcanic ash or industrial waste such as fly ash, or can be produced by calcining clay. When fly ash is used, the mixture is known as lime-fly ash stabilized mixture, or in abbreviated form LFA stabilized mixture.

Not all soils possess enough quantity of clay minerals with which lime can react to form cementitious products. If, therefore, a pozzolanic material is added to such soils, the stabilization can easily take place.

Silts, sandy soils, gravels, crushed stone and slag are some of the material types where lime-Pozzolana stabilization can be successful. The alluvial silts of a northern India fall into this category of soils which can be stabilized with lime-Pozzolona.

The ratio of lime to Pozzolana depends upon a number of factors and can vary so widely as from 1 : 1 to 1 : 9. The combined quantity of lime-Pozzolana in a mixture may vary from 10 to 25 per cent.

Lime-Pozzolana aggregate mixtures can be used for the superior strength road bases. A layer of this material has great structural strength and behaves more like a semi-rigid pavement.

The use of lime-Pozzolana in our country is still in an early stage, but in view of the problem of disposal of huge quantity of fly ash from thermal plants, this material should be used more and more.

Method # 2. Cement Stabilization:

Portland cement has been used with great success to improve existing gravel roads, as well as to stabilize natural soils. It can be used for base courses and sub-base of all types.

It can be used in granular soils, silty soils and lean clays, but it cannot be used in organic materials which cause delayed setting and reduction in strength. Since addition of cement increases strength of the natural material, it is very often used for base course construction.

Stabilization of soil with cements consists of adding cement to pulverized soil and permitting the mixture to harden by hydration of cement.

The factors that affect the physical properties of soil cement include:

1. Soil type

2. Quantity of cement

3. Degree of pulverization and mixing

4. Time of using

5. Dry density of compacted mixture

A. Soil Type and Quantity of Cement:

Portland Cement Association has collected consider­able amount of information regarding the soil type and quantity of cement required for adequate soil hardening.

The quantity of cement required for stabilization increases as soil plasticity increases. For highly plastic soils as much as 15 to 20 per cent cement by weight of soil is required for hardening of soil.

Sandy soils and gravel mixtures generally are stabilized readily with cement. The amount of cement required to stabilize a granular material depends upon the quantity and quality of fines contained in granular material and final compacted density (void ratio of compacted material). The cement required for sandy soils range between 5 and 12% by weight.

Normally the following requirements are adopted for selection of materials for cement stabilized soil:

1. Maximum liquid limit — 40-45 %

2. Maximum plastic limit — 20%

3. pH value of soil-cement : 12.1 (min.)

4. Maximum content of soluble salts

Sulphates—4%

Chlorides—8%

B. Sandy and Gravelly Soils:

1. Passing maximum size 50 mm sieve—100%

2. Passing maximum size 5 mm sieve—50%

3. Passing maximum size 400 micron sieve—above 15%

4. Passing maximum size 75 micron sieve—below 3%

The materials used for stabilization include sand and gravel, laterite, kankar, brick aggregate, crushed rock or slag or any combination of these. The material should be well graded with uniformity coefficient not less than 5% and capable of producing a well closed surface finish.

Method # 3. Soil-Bituminous Stabilization:

The basic principles of soil-bitumen stabilization as applied to highways and airfield construction are the methods of designing and mixing local soil or aggregates with bituminous materials to form a stable and waterproof base course.

The soil-bitumen base course resist deformation through cementing action of bitumen which binds the soil partners together. The thin coating of bitumen around the soil particles also provides high degree of waterproofing which also resists deformation.

Thus, bitumen may be used:

(i) For binding soil particles together for purpose of supplying cohesion for non-mechanical stabilized granular material.

(ii) For waterproofing, mechanical stabilized granular mixtures or for waterproofing cohesieve soils.

The principle controlling the stabilization of granular material is to provide a thin coating of bitumen around the particles without destroying the natural frictional resistances of particles.

For cohesive soil, the principle is of adding sufficient bitumen to block capillary action within small soil aggregates to avoid moisture changes within these aggregates. Complete water proofing is not desired neither it is necessary.

In most areas where suitable flexible base materials are not available or are expensive, soils can be mixed with bitumen to form a suitable base course at low cost.

Factors Affecting Bitumen Stabilization:

1. Soil Factor:

This requires proper selection of the best available soil, for which lab tests are to be conducted in different soil types and construction materials. This data will help to determine thickness of treatment proportioning of material and construction procedure. A mechanical soil analysis showing the grain size will give answer whether soil is suitable for soil-bitumen stabilization or not.

2. Bitumen Type:

Selection of type, grade and amount of bituminous material to be mixed is next step in lab investigations. Generally, asphalts and tars are equally suitable. Rapid curing cut back or emulsified asphalts produce best results when mixed with extremely sandy soil and those containing minimum clay and silt particles.

Slow curing cut-back asphalts having gas oil or heavy residuals penetrate clay soils more rapidly than other types of asphaltic materials. They perform better when mixed with soils having plastic index more than 10 or silts and clay content of more than 30%.

Climatic conditions also affect the grade of material. In warmer climates a heavier grade can be used than for cooler climates. In warm weather rapid-curing cut-back should be used.

3. Bitumen Content:

i. Fine-grained soils — 4-8%

ii. Sands — 4-10%

iii. Gravel and sand gravel — 2-6%

4. Moisture Content and Mixing:

To produce a homogeneous mass with minimum mixing effort, the soil should contain some moisture for cut-back asphalts or tar and 10% or more for emulsified asphalts otherwise the emulsions break immediately upon contact with soil. The reason for moisture is that it acts as a carrier for the asphalt and is an aid to mixing.

In case of cut-backs the mixing process has influence on performance of bituminous waterproofing agents. Excess mixing can reduce waterproofing.

5. Aeration and Compaction:

After mixing is complete the next step is aeration and compaction. The presence of moisture and varieties result in low density and too low stability. Therefore, mixture should be aerated to remove moisture upto three-fourths of optimum and reduce volatiles from 65 to 75% in cut-back. Aeration is done by manipulating the mixture in a manner that permits the evaporation of moisture and volatiles.

The moisture content at the time of compaction should be such so as to give optimum field content for maximum density.

Method # 4. Organic Stabilizers:

A number of organic stabilizers have been used for chemical stabilization of soils.

Some of them are:

1. Sodium silicate

2. Lignin

3. Resins

4. Molasses

Sodium silicate reacts in aqueous situation with soluble calcium salts, forming insoluble and gelatinous calcium silicates.

Calcium needed for the reaction can either be present in the soil itself or be added in aqueous solutions. The amount of chemical needed may vary from one to ten per cent.

The natural binding material that holds together the fibres in wood is lignin.

Lignin is a major by-product in paper manufacturing industry. Lignin is available from the paper manufacturing process in a water solution known as calcium lignosulphonic acid. Calcium lignosulphate is the constituent which is used as a road binder.

About 0.5 to 1 per cent by weight of dry soil is used for stabilization.

Stabilization is by the cementing bond that develops between the soil particles due to the presence of binder. The material also closes the voids and thus reduces penetration of water through the layer. It retards the rate of evaporation of water and arrests loss of moisture.

Natural or processed resins can also be used for soil stabilization. Vinsol resin and resin or derivatives of resin are commonly used. Resins are wood products. Resin treated soils reduce water absorption, facilitate compaction and increase the stability of the treated mixtures.

Their drawback is their susceptibility for microbiological attack by bacteria and fungi, but this handicap can be identified and surmounted. A quantity of 1 to 3 per cent by weight of soils is normally sufficient.

Molasses is a waste product from the process of manufacturing sugar from sugar cane. A thick syrupy liquid, it is hygroscopic and can be used as a dust-palliative and as a binder for incorporation during compaction.

It is easily knocked out by rainwater. If water is prevented from entering the mixture, as by means of an impermeable bituminous surfacing, it can be expected to last long.

Method # 5. Thermal Stabilization:

(a) Heating:

One of the methods of reducing the plasticity of highly clayey soils is heat treatment. This is one of the oldest methods of soil stabilization which has been used extensively by the Australian agencies in making pathways.

For heat treatment of soil in situ, a travelling furnace capable of handling large amounts of soil at temperature over 500°C is employed.

The heat treatment of pulverized black cotton soil to reduce its plasticity in laboratory was studied in India. These studies show that the soil becomes non-plastic after being heated to about 500°C.

Depending upon the method of burning these will always be a mixture of pulverized soil and clods after heat treatment which gives a good gradation. The CBR values of this material after 4 days soaking would be of the order of 110 to 140 per cent.

There is also a possibility of further improving the heat treated soil with cement or bitumen. To increase its reactivity with cement, some raw clayey soil may have to be added to heat treated soil. The optimum clay content to be added can be determined in the laboratory based on compressive strength tests.

(b) Freezing:

Cooling causes a small loss of strength of clayey soil due to an increase in interparticle repulsion. However, if the temperature is reduced to the freezing point, the pure water freezes and the soil is stabilized.

Ice so formed acts as a cementing agent. Water in cohesionless soils freezes at about 0°C. However, in cohesive soils, water may freeze at a much lower temperature.

The strength of the soil increases as more and more water freezes. This method of stabilization is very costly.

This method is used only in some special cases. It has been successfully used to solidify soils beneath foundations. This method is commonly used when advancing tunnels or shafts through loose silt or fine sand.

Freezing may cause serious trouble to adjacent structures if the freezing front penetrates these areas. It may cause excessive heaving.

Method # 6. Electrical Stabilization:

Electrical stabilization of clayey soils is done by a process known as electro-osmosis. As a direct current (D.C.) is passed through a clayey soil, pure water migrate to the negative electrode (cathode).

It occurs because of the attraction of positive ions (cations) that are present in water towards cathode.

The strength of the soil is considerably increased due to removal of water. Electro-osmosis is an expensive method, and is mainly used for drainage of cohesive soils. Incidentally, the properties of the soil are also improved.

Method # 7. Complex Stabilization Technique:

(a) Grouting:

Stabilizers are introduced by injection into the soil. As the grouting is always done under pressure, the stabilizers with high viscosity are suitable only for soils with high permeability. This method is not suitable for stabilizing clays because of their very low permeability.

It is suitable for stabilizing buried zones of relatively limited extent, such as a pervious stratum below a dam. The method is used to improve the soil that cannot be disturbed. An area close to an existing building can be stabilized by this method.

(b) By Geotextile and Fabrics:

The soil can be stabilized by introducing geotextiles and fabrics which are made of synthetic materials, such as polythene, polyester, nylon. These sheets are quite permeable. Their permeability is comparable to that of fine sand to coarse sand.

These are quite strong and durable. These are not affected by even hostile soil environment. Major functions of geotextile are – Separation, filtering, draining, and reinforcement for strengthening the soil and as reinforcement in retaining walls.

(c) Reinforced Earth:

The soil can be stabilized by introducing thin strips in it. In reinforced earth, thin metal strips or strips of wire or geo-synthetics are used as reinforcement to reinforce the soil. The essential feature is that friction develops between the reinforcement and the soil. By means of friction, the soil transfers the forces built up in the earth mass to the reinforcement.

Thus, tension develops in the reinforcement when the soil mass is subjected to shear stresses under loads.

Method # 8. Complex Stabilization:

It is defined as the method of stabilization with more than one stabilizer. Difficult soils such as organic soils, highly plastic clays and soils with easy soluble salts require more than one stabilizer for their effective treatment. Complex stabilization involves the use of binding material and surface active additives or electrolytes.

At present the following combinations are considered the best:

(i) Cement + Calcium Chloride + Lime

(ii) Cement + Bituminous emulsions

(iii) Cut-back + Lime

(iv) Cement + Naphtha Soap