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SOIL STABILIZATION

Introduction

Soil stabilization incorporates the various methods used for modifying the properties of soil to improve its engineering performance. The prime objective of Soil Stabilization is to improve the California Bearing Ratio of in-situ soils by 4 to 6 times. The other prime objective of soil stabilization is to improve on-site materials to create a solid and strong sub-base and base courses. Soil stabilization is mainly used in the construction of roads and airfield pavements where the soil strength or bearing capacity of soil is increased thereby reducing the construction cost and increasing the stability of soil.

Soil stabilization may be done by:

  1. Improving the soil property without using any admixture which include compaction and drainage of soil.
  2. It may also be done by using the admixtures which include mechanical stabilization, stabilization with cement, lime, bitumen and chemicals etc.

Soil Stabilization using admixtures:-

  1. Soil Stabilization with Cement:

The soil stabilized with cement is known as soil cement. The cementing action is believed to be the result of chemical reactions of cement with siliceous soil during hydration reaction. The important factors affecting the soil-cement are nature of soil content, conditions of mixing, compaction, curing and admixtures used.

The appropriate amounts of cement needed for different types of soils may be as follows:

  • Gravels – 5 to 10%
  • Sands – 7 to 12%
  • Silts – 12 to 15%, and
  • Clays – 12 – 20%

The quantity of cement for a compressive strength of 25 to 30 kg/cm2 should normally be sufficient for tropical climate for soil stabilization.

2. Soil Stabilization using Lime:

Slaked lime is very effective in treating heavy plastic clayey soils. Lime may be used alone or in combination with cement, bitumen or fly ash. Sandy soils can also be stabilized with these combinations. Lime has been mainly used for stabilizing the road bases and the subgrade.

Lime changes the nature of the adsorbed layer and provides pozzolanic action. Plasticity index of highly plastic soils are reduced by the addition of lime with soil. There is an increase in the optimum water content and a decrease in the maximum compacted density and he strength and durability of soil increases.

Normally 2 to 8% of lime may be required for coarse grained soils and 5 to 8% of lime may be required for plastic soils. The amount of fly ash as admixture may vary from 8 to 20% of the weight of the soil.

3. Soil Stabilization with Bitumen:

Asphalts and tars are bituminous materials which are used for stabilization of soil, generally for pavement construction. Bituminous materials when added to a soil, it imparts both cohesion and reduced water absorption. Depending upon the above actions and the nature of soils, bitumen stabilization is classified in following four types:

  • Sand bitumen stabilization
  • Soil Bitumen stabilization
  • Water proofed mechanical stabilization, and
  • Oiled earth.

4. Chemical Stabilization of Soil:

Calcium chloride being hygroscopic and deliquescent is used as a water retentive additive in mechanically stabilized soil bases and surfacing. The vapour pressure gets lowered, surface tension increases and rate of evaporation decreases. The freezing point of pure water gets lowered and it results in prevention or reduction of frost heave.

The depressing the electric double layer, the salt reduces the water pick up and thus the loss of strength of fine grained soils. Calcium chloride acts as a soil flocculent and facilitates compaction. Frequent application of calcium chloride may be necessary to make up for the loss of chemical by leaching action. For the salt to be effective, the relative humidity of the atmosphere should be above 30%.

Sodium chloride is the other chemical that can be used for this purpose with a stabilizing action similar to that of calcium chloride.

Sodium silicate is yet another chemical used for this purpose in combination with other chemicals such as calcium chloride, polymers, chrome lignin, alkyl chlorosilanes, siliconites, amines and quarternary ammonium salts, sodium hexametaphosphate, phosphoric acid combined with a wetting agent.

5. Electrical Stabilization of Clayey Soils:

Electrical stabilization of clayey soils is done by method known as electro-osmosis. This is an expensive method of soil stabilization and is mainly used for drainage of cohesive soils.

6. Soil Stabilization by Grouting:

In this method, stabilizers are introduced by injection into the soil. This method is not useful for clayey soils because of their low permeability. This is a costly method for soil stabilization.

This method is suitable for stabilizing buried zones of relatively limited extent. The grouting techniques can be classified as following:

  • Clay grouting
  • Chemical grouting
  • Chrome lignin grouting
  • Polymer grouting, and
  • Bituminous grouting

7. Soil Stabilization by Geotextiles :

Geotextiles are porous fabrics made of synthetic materials such as polyethylene, polyester, nylons and polyvinyl chloride. Woven, non-woven and grid form varieties of geotextiles are available. Geotextiles have a high strength. When properly embedded in soil, it contributes to its stability. It is used in the construction of unpaved roads over soft soils.

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