Soil stabilization factors; mechanical; cement-soil base

Factors affecting soil stabilisation

The success of stabilisation depends on:

1. Type of soil - its grain size, plasticity index, mineralogy and organic content decide the suitable method (cement for sands and gravel, lime for plastic clays).
2. Type and amount of stabiliser - cement 4-8 percent, lime 3-8 percent, bitumen 4-7 percent; too little gives no strength gain and too much is costly or causes cracking.
3. Mixing efficiency - uniform pulverisation and mixing of soil and stabiliser; clods must be broken (below 25 mm).
4. Moisture content and compaction - optimum moisture and high density are needed; delay between mixing and compaction (more than 2 hours for cement) reduces strength.
5. Curing and time - adequate moist curing for 7 days or more; temperature (cement and lime reactions are slow below 10 C).
6. Presence of harmful materials - sulphates, organic matter and salts retard cement and lime reactions.
7. Climate and drainage - freezing, wetting-drying and water access affect durability.
8. Cost and availability of the stabiliser and equipment.

Mechanical method of soil stabilisation

Mechanical stabilisation improves the soil by changing its gradation and density only, without a chemical binder.

• Two or more soils are blended (for example, a coarse gravel lacking fines with a clayey soil lacking coarse particles) to get a well-graded mix with the right proportions of gravel, sand and fines (plasticity index 4-10 percent).
• The mix is spread at optimum moisture content and compacted with rollers to high density (above 98 percent of modified Proctor).
• Used for gravel roads, sub-base and base of low-volume roads, and as the first step before chemical stabilisation. It is cheap but depends on a good proportion of materials.

Construction of cement soil stabilised road base

Soil-cement (cement-stabilised) base is a mix of soil, cement and water, compacted to give a strong, durable layer (IRC:SP:89, IS:4332).

Materials: granular or sandy soil (PI below 20, fines below 50 percent, maximum size 40 mm; clay and organic soil unsuitable); ordinary Portland cement 4-8 percent by dry weight (decided by laboratory trial: 7-day UCS 1.7-3 MPa); clean water.

Equipment: pulveriser (rotavator) or grader, cement spreader (or bags), water tanker, rollers (vibratory 8-10 t, pneumatic tyre), motor grader, curing sprayer.

Construction procedure

1. Prepare the subgrade, trim and compact; set line and levels.
2. Spread the soil in a uniform layer and pulverise to break clods.
3. Spread the cement in a uniform layer at the specified rate (by bags at fixed spacing or by mechanical spreader).
4. Mix dry cement and soil with rotavator or grader until colour is uniform; add water to the optimum moisture content and mix again.
5. Compact immediately (within 2 hours of adding water) to 98 percent of modified Proctor density, using a vibratory roller and then a pneumatic roller; shape with the grader.
6. Cure for 7 days with moist cover or a bituminous prime coat (bitumen emulsion) to retain moisture; keep traffic off.
7. Provide the bituminous surface or the next layer after curing.
8. Control: cement content (titration), moisture and density, UCS cubes, thickness and level.