Recommended Practice for the Pulverization of Black Cotton Soils for Lime Stabilisation
1973 Edition

The Overview section of IRC 49 details essential specifications and data tables related to soil pulverization and its influence on soil properties. Pulverization degree is defined by the percentage of soil passing designated IS sieves post-pulverization, exemplified by the following table:

An example (Clause 1.5) demonstrates how pulverization level and lime addition affect soaked CBR and moisture uptake in black cotton soils:

These data highlight the critical role of pulverization and lime treatment in enhancing soil strength and reducing moisture absorption. The samples were cured for 10 days. This foundation is vital for soil stabilization in pavement design.

IRC 49 focuses on the pulverization of black cotton soils targeted for road construction and soil stabilization. The key criteria include the pulverization degree defined by the proportion of soil passing specific IS sieves after pulverization, as outlined here:

The impact of pulverization on soaked CBR and moisture absorption when soils are treated with 3% hydrated lime is illustrated below:

This data confirms that higher pulverization improves soaked CBR and decreases moisture absorption, which are crucial factors for soil stabilization. The curing duration for these tests was 10 days.

The pulverization degree is measured by the portion of soil passing through specific IS 460 sieves after pulverization. According to the standard, all soil must pass through the 25 mm sieve, and at least half must pass the 4.75 mm sieve to ensure sufficient pulverization for strength enhancement. The calculation formula is:

% Passing = [(W1 - Wa) / W1] × 100

Where W1 is the initial weight of the soil sample, and Wa is the weight of soil retained after sieving. Achieving at least 50% passing through the 4.75 mm sieve ensures similar strength regardless of finer soil fractions. The sieving process involves gentle shaking and careful breaking of soil lumps to avoid altering particle size distribution.

IRC 49 specifies the following regarding pulverization methods:

• Pulverization degree is evaluated by the percentage of soil passing designated IS 460 sieves:

• If 50% or more passes the 4.75 mm sieve, the resulting strength does not significantly depend on finer soil fractions.

• Sieving procedure for cohesive soils includes:

  1. Weigh approximately 1 kg of pulverized soil (W1).
  2. Spread on the sieve and shake gently, then weigh retained soil (W2).
  3. Break lumps in retained soil to separate finer particles, shake again, and weigh retained soil (Wa).
  4. Calculate percentage passing using the formula: ((W1 - Wa) / W1) × 100.

This approach ensures proper determination of pulverization degree for effective lime mixing and compaction.

Adding lime before pulverization entails mixing about half of the total required lime content with soil broken down into clods of 5 cm or less and letting it act for 3-4 days prior to the final pulverization. This facilitates better pulverization by promoting lime migration at optimal moisture levels. The remaining lime is applied post-pulverization. Although this method slightly reduces lime efficiency due to carbonation, it ultimately improves soil strength. Moisture control is essential to support lime movement and pulverization. An example shows that higher pulverization combined with 3% hydrated lime treatment enhances soaked CBR and reduces moisture absorption.

Annexure 1 of IRC 49 illustrates how pulverization degree impacts properties of lime-treated black cotton soil. Pulverization is quantified by percentages of soil passing through IS 460 sieves:

Techniques to achieve pulverization include manual labor, power rollers, and heavy or light agricultural machinery. Appropriate moisture contents are 15-30% for heavy machinery and 18-22% for light machinery. Adding approximately 50% of lime prior to pulverization and allowing it to act for a few days enhances pulverization and soil characteristics. Clods should be reduced to sizes of 5 cm or less before lime addition. These guidelines assist in optimizing soil stabilization.

As per IRC 49, the sieving method to measure pulverization degree includes:

• Weigh about 1 kg of pulverized soil sample (W1).
• Place soil on IS 460 sieve and shake gently without breaking lumps unnecessarily.
• Weigh soil retained on sieve (W2), then break lumps to separate finer particles.
• Repeat shaking and weighing retained soil (Wa) until no change occurs.
• Calculate percentage passing as: [(W1 - Wa) / W1] × 100.

The minimum pulverization standard requires 100% passing the 25 mm sieve and at least 50% passing the 4.75 mm sieve. Achieving this ensures sufficient pulverization for effective lime-soil mixing and compaction.