IRC 49 (1973) provides recommended practices for pulverizing black cotton soils to optimize lime stabilization in road construction. It addresses the challenges posed by the expansive nature of black cotton soils and details methods to achieve effective pulverization, which is critical for uniform lime mixing and improved soil strength. This standard is essential for engineers working on sub-base preparation in black cotton soil regions to enhance pavement performance and durability.
Overview
IRC 49 (1973) provides recommended practices for pulverizing black cotton soils to optimize lime stabilization in road construction. It addresses the challenges posed by the expansive nature of black cotton soils and details methods to achieve effective pulverization, which is critical for uniform lime mixing and improved soil strength. This standard is essential for engineers working on sub-base preparation in black cotton soil regions to enhance pavement performance and durability.
Audience
Contents
Structure
The Introduction of IRC 49 provides key specifications and tables related to soil pulverisation and its effect on soil properties. The Degree of Pulverisation is defined by the percentage of soil passing specific IS sieves after pulverisation, as shown in the table:
| Sieve designation (IS : 460) | Per cent by weight of soil passing the sieve after pulverisation |
|---|---|
| 25 mm | 100 |
| 4.75 mm | 50 |
Additionally, an illustrative example (Clause 1.5) shows how the degree of pulverisation and lime addition affect the soaked CBR and moisture absorption of black cotton soil:
| Sample No. | % Passing 25 mm | % Passing 4.75 mm | % Passing 2 mm | Soaked CBR | Moisture Absorption (%) |
|---|---|---|---|---|---|
| 1 | 100 | - | - | 2.4 | 27.5 |
| 2 | 100 | 50 | 15 | 14.2 | 26.3 |
| 3 | 100 | 100 | 30 | 14.3 | 26.9 |
| 4 | 100 | 100 | 100 | 14.7 | 25.3 |
These tables highlight the importance of pulverisation and lime treatment in improving soil strength and reducing moisture absorption. The curing period for these samples was 10 days. This information is foundational for soil stabilization in pavement design.
Sources: TABLE: 3. DEGREE OF PULVERISATION, Clause 1.5: TABLE: ILLUSTRATIVE EXAMPLE OF THE EFFECT OF DEGREE OF PULVERISATION AND PERCENTAGE ADDITION OF LIME ON PROPERTIES OF BLACK COTTON SOIL
The scope of IRC 49 primarily addresses the pulverisation of black cotton soils for road construction and soil stabilization. Key specifications include the degree of pulverisation defined by the percentage of soil passing specific IS sieves after pulverisation, as per the following table:
| Sieve designation (IS : 460) | Per cent by weight of soil passing the sieve after pulverisation |
|---|---|
| 25 mm | 100 |
| 4.75 mm | 50 |
Additionally, the effect of pulverisation on soaked CBR and moisture absorption when treated with 3% hydrated lime is illustrated below:
| Sample No. | % Passing 25 mm | % Passing 4.75 mm | % Passing 2 mm | Soaked CBR | Moisture Absorption (%) |
|---|---|---|---|---|---|
| 1 | 100 | - | - | 2.4 | 27.5 |
| 2 | 100 | 50 | 15 | 14.2 | 26.3 |
| 3 | 100 | 100 | 30 | 14.3 | 26.9 |
| 4 | 100 | 100 | 100 | 14.7 | 25.3 |
This demonstrates that increasing pulverisation improves soaked CBR and reduces moisture absorption, critical for soil stabilization. The curing period for samples is 10 days (Clause 4.75).
Sources: Clause None: TABLE: 3. DEGREE OF PULVERISATION, Clause 1.5: TABLE: ILLUSTRATIVE EXAMPLE OF THE EFFECT OF DEGREE OF PULVERISATION AND PERCENTAGE ADDITION OF LIME ON PROPERTIES OF BLACK COTTON SOIL, Clause 4.75
The Degree of Pulverisation in soil is assessed by the percentage of soil passing specific IS 460 sieves after pulverisation. As per the provided table, 100% of soil should pass the 25 mm sieve, and at least 50% should pass the 4.75 mm sieve. This ensures adequate pulverisation for strength development (Clause 3). The key formula to calculate the percentage passing is:
% Passing = [(W1 - Wa) / W1] × 100
where W1 = initial weight of soil sample, Wa = weight of soil retained after complete sieving.
It is noted that if at least 50% passes the 4.75 mm sieve, the strength attained is practically the same regardless of finer fractions (Clause 4.75). The method involves gentle shaking and breaking lumps carefully to avoid altering particle size distribution.
Sources: Clause 3, Clause 4.75, TABLE: 3. DEGREE OF PULVERISATION
The key specifications for Methods of Pulverisation per IRC 49 are as follows:
| Sieve designation (IS : 460) | % by weight of soil passing after pulverisation |
|---|---|
| 25 mm | 100 |
| 4.75 mm | 50 |
As per Clause 4.75, if at least 50% of soil passes the 4.75 mm sieve, the strength attained is practically the same regardless of finer fractions.
Method of Sieving for Cohesive Soils:
This ensures proper assessment of pulverisation degree for soil-lime mixing and compaction.
Sources: Clause None: 3. DEGREE OF PULVERISATION, Clause 4.75, Clause None: Method of Sieving for Cohesive Soils
Prior addition of lime to aid pulverisation involves adding about 50% of the required lime concentration to soil already broken down to clods of 5 cm or less, then allowing it to work for 3-4 days before final pulverisation, as per Clause 5.1 and 5.3. This helps achieve better pulverisation by aiding lime migration at suitable moisture content. The remaining lime is added after pulverisation (Clause 5.2). The process slightly reduces lime effectiveness due to carbonation but improves overall soil strength.
An illustrative table (Clause 1.5) shows the effect of pulverisation degree and lime addition on soaked CBR and moisture absorption of black cotton soil treated with 3% hydrated lime:
| Sample No. | % Passing IS Sieve 25 mm | % Passing IS Sieve 4.75 mm | % Passing IS Sieve 2 mm | Soaked CBR | Moisture Absorption (%) |
|---|---|---|---|---|---|
| 1 | 100 | - | - | 2.4 | 27.5 |
| 2 | 100 | 50 | 15 | 14.2 | 26.3 |
| 3 | 100 | 100 | 30 | 14.3 | 26.9 |
| 4 | 100 | 100 | 100 | 14.7 | 25.3 |
This demonstrates improved CBR with increased pulverisation and lime treatment. Moisture content should be controlled to aid lime migration and pulverisation (Clauses 4.3.3, 4.4.2).
Sources: Clause 5.1, Clause 5.2, Clause 5.3, Clause 1.5, Clause 4.3.3, Clause 4.4.2
The degree of pulverisation significantly affects lime-mixed black cotton soil properties, as illustrated in IRC 49 Annexure 1. Pulverisation is measured by the percentage of soil passing specific IS 460 sieves after pulverisation, for example:
| Sieve designation (IS : 460) | % by weight of soil passing after pulverisation |
|---|---|
| 25 mm | 100 |
| 4.75 mm | 50 |
Methods to achieve pulverisation include manual labour, power rollers, heavy and light agricultural machinery, and other variations (Clauses 4.1 to 4.5). Moisture content is critical, typically 15-30% for heavy machinery and 18-22% for light machinery.
Adding lime prior to pulverisation (about 50% of total lime) and allowing it to act for 3-4 days improves pulverisation and soil properties (Clause 5). Clods should be broken down to 5 cm or less before lime addition.
These guidelines help optimize soil stabilization by controlling pulverisation and lime content.
Sources: Clause 3.1, Table: 3. DEGREE OF PULVERISATION, Clauses 4.1 to 4.5, Clauses 5.1 to 5.3
The Method of Sieving for Degree of Pulverisation as per IRC 49 involves the following key steps and specifications:
The key sieve designations and minimum passing percentages after pulverisation are:
| Sieve designation (IS : 460) | Per cent by weight of soil passing the sieve after pulverisation |
|---|---|
| 25 mm | 100 |
| 4.75 mm | 50 |
As per Clause 4.75, if at least 50% of soil passes the 4.75 mm sieve, the strength attained is practically the same regardless of finer fractions.
This method ensures adequate pulverisation for lime-soil mixing and compaction.
Sources: Clause 4.75, TABLE: 3. DEGREE OF PULVERISATION
Frequently Asked
For effective lime stabilization of black cotton soils, the soil must be in a reasonably pulverised state at the time of lime addition. Black cotton soils are hard when dry and sticky when wet, making uniform pulverization challenging but essential. Proper pulverization ensures uniform mixing and influences the amount of lime required to achieve the desired strength gain. Therefore, thorough pulverization to break down soil lumps and achieve a friable texture is recommended before lime application.
Sources: Clause 1.3
Different methods of pulverisation are suitable for varying moisture conditions as per IRC 49:
Manual Labour (Clause 4.1): Best used in dry conditions, typically summer months, when soil is dry enough to be loosened and broken manually.
Power Rollers (Clause 4.2): Applicable only when soil is dry; clods broken to max 5 cm and rolled repeatedly.
Heavy Agricultural Machinery (Clause 4.3): Most effective when soil moisture is between 15-30%. Below 15%, soil is too dry; above 30%, soil becomes sticky and machinery bogs down.
Light Agricultural Machinery (Clause 4.4): Suitable for moisture content range of 18-22%. Less economical but more readily available.
Other Variations (Clause 4.5): Soil can be dried to about 15-20% moisture before pulverisation using heavy wooden logs and ploughs.
Thus, dry soil suits manual and power roller methods, moderate moisture (15-30%) suits heavy machinery, and slightly narrower moisture range (18-22%) suits light machinery.
Sources: Clause 4.1, Clause 4.2, Clause 4.3, Clause 4.4, Clause 4.5
The degree of pulverisation significantly affects the soaked CBR and strength of lime-treated black cotton soil. As per the illustrative example in Clause 1.5, increasing pulverisation (finer soil particles passing smaller sieves) improves soaked CBR values markedly. For instance, with 3% lime and compaction at 1.5 gm/cc, soaked CBR increases from 2.4 (no pulverisation) to about 14.7 when 100% passes through 2 mm sieve. This shows that finer pulverisation enhances lime-soil reaction and strength gain. Moisture absorption slightly decreases with finer pulverisation, indicating better soil-lime bonding. Hence, thorough pulverisation to reduce clod size (preferably below 5 cm before lime addition as per Clause 5.3) is essential for effective lime stabilization and strength improvement. Methods for achieving this include manual, power rollers, and agricultural machinery (Clauses 4.1 to 4.5).
Sources: Clause 1.5, Clause 5.3, Clauses 4.1 to 4.5
Yes, lime can be added prior to pulverisation to improve the process. As per IRC 49 Clause 5.1, adding a small quantity of lime to soil partially broken down to small clods and allowing it to act for 3-4 days before final pulverisation helps achieve better pulverisation. The moisture content should be adequate to assist lime migration. Clause 5.2 recommends tentatively adding about 50% of the required lime concentration before pulverisation and the remaining after pulverisation. Also, per Clause 5.3, clay clods should be broken down to 5 cm size or smaller before this pre-pulverisation lime addition.
Sources: Clause 5.1, Clause 5.2, Clause 5.3
IRC 49 suggests several equipment and procedures for pulverizing black cotton soils at embankment sites:
Manual Labour (Clause 4.1): Remove vegetation, loosen dry soil crust (~20 cm depth) with crow-bars, break clods with pick-axes or rammers, or use a country plough pulled by bullocks. Alternatively, wet and dry soil repeatedly to disintegrate clods.
Power Rollers (Clause 4.2): For dry soil, break clods to max 5 cm, spread on subgrade, and pass an 8-tonne smooth wheeled roller about 8 times with frequent raking.
Heavy Agricultural Machinery (Clause 4.3): Use tractors (~110 HP) with mould board ploughs (40 cm depth), disc harrows (5 discs, 70 cm dia), and off-set harrows (18 discs, 50 cm dia). Plough, then harrow multiple times to break clods. Ideal moisture content: 15-30%.
Light Agricultural Machinery (Clause 4.4): Use smaller tractors (~50 HP) with smaller ploughs and harrows. Effective at 18-22% moisture, requiring more passes.
Other Methods (Clause 4.5): Excavate, dry soil to 15-20% moisture, spread in layers (~25 cm), break clods by dragging heavy wooden logs, then turn soil with off-set plough before lime addition.
These methods ensure thorough pulverization to clod sizes ≤5 cm before lime stabilization.
Sources: Clause 4.1, Clause 4.2, Clause 4.3, Clause 4.4, Clause 4.5
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