Soil stabilization by admixture

Admixture stabilization is the improvement of soil strength, durability and volume stability by mixing a suitable additive (admixture) into the soil and compacting the mixture. The additive changes the soil properties by chemical reaction, cementing action or by filling voids.

Common admixtures and their action:

• Cement: hydration forms a cementing gel that binds particles. Best for sands, gravels and low-plasticity silts (soil-cement, 5-12% cement by weight).
• Lime: cation exchange and flocculation reduce plasticity; pozzolanic reaction gives long-term strength. Best for clays (usually 3-8% lime).
• Fly ash: a pozzolana, used with lime or cement to give extra strength.
• Bitumen: waterproofs and binds granular or fine-grained soils.
• Chemicals (calcium chloride, sodium silicate): reduce water loss or bind particles.

Procedure: pulverise the soil, spread the admixture, mix with water to optimum moisture content (OMC), compact to maximum dry density, and cure.

Results: higher strength and CBR, lower plasticity, swelling and compressibility, and better resistance to water. It is widely used in subgrades, road bases and under light foundations.