Methods of test for soils, Part 4: Grain size analysis

IS 2720 Part 4: Introduction and Adoption - Key Points

1. Introduction (Clause 5.1.4 & 5.2.4)

• The standard covers grain size analysis of soils by sieve and sedimentation methods.
• Procedure involves:
  • Drying and sieving the soil sample.
  • Weighing fractions retained on each sieve.
  • Calculating percentage finer than each sieve size.

2. Calculations (Clauses 5.1.4 & 5.2.5)

• Percentage finer than a sieve size, ( d ), is calculated as:

[ % \text{Finer} = 100 - \left(\frac{\text{Weight retained on sieve}}{\text{Total sample weight}} \times 100\right) ]

• For sedimentation analysis, Stokes' law is applied to determine particle size from settling velocity.

3. Reporting (Clause 6.1)

• Results reported in tabular form (see Appendix A).
• Grain size distribution curve plotted on semi-log graph:
  • X-axis: Particle size (log scale)
  • Y-axis: Percentage finer (linear scale)

4. Standard Sieve Sizes (mm)

graph LR
  A[Soil Sample] --> B[Drying]
  B --> C[Sieving]
  C --> D[Weight retained on each sieve]
  D --> E[Calculate % finer]
  E --> F[Plot Grain Size Distribution Curve]

This concise framework ensures uniform adoption and reporting of grain size analysis per IS 2720 Part 4.

IS 2720 Part 4 — Grain Size Analysis: Purpose & Scope

Purpose:

• Quantitative determination of grain size distribution in soils.
• Expresses proportions by mass of various particle sizes.
• Helps classify soil into gravel, sand, silt, and clay fractions (IS:1498-1970).
• Used in soil classification, filter design for earth dams, and road construction suitability.

Scope:

• Covers methods for grain size distribution determination.
• Results reported as cumulative % finer vs. particle size on a semi-logarithmic graph.
• Particle size on logarithmic scale (x-axis), % finer on linear scale (y-axis).

Key Specifications & Reporting:

• Use standard sieves with sizes as per IS (e.g., 4.75 mm, 2.8 mm, 0.425 mm, etc.).
• Plot grain size distribution curve (Fig. 10 in IS 2720 Part 4).
• Report results in a pro forma (Appendix A of IS 2720 Part 4).

Typical Grain Size Classification (IS 1498):

Grain Size Distribution Curve:

• Plot % finer vs. particle size (log scale).
• Helps identify soil gradation and uniformity.

graph LR
A[Soil Sample] --> B[Sieving & Sedimentation]
B --> C[Mass % Passing Each Sieve]
C --> D[Plot % Finer vs. Particle Size (log scale)]
D --> E[Grain Size Distribution Curve]
E --> F[Soil Classification & Design Use]

This method ensures uniformity and comparability in soil particle size analysis as per IS standards.

IS 2720 Part 4: Rubber Pestle and Mortar Specifications

Key Points from Clauses 3.1.3 & 4.1.6:

• Rubber pestle and mortar are used for gently rubbing soil samples during sieving to separate particles without breaking them.
• The pestle is rubber-covered to avoid particle damage.
• Used mainly to disaggregate soil clumps during particle size analysis.

Sample Mass for Sieving (Note 1):

Max Weight Retained on Each Sieve (Note 2):

Usage Summary:

• Use rubber pestle and mortar to gently rub soil on sieves.
• Avoid breaking particles; only rub enough to separate aggregates.
• Follow sample mass and max retained weight tables for proper sieving.

flowchart TD
    A[Soil Sample] --> B[Sieving through IS Sieves]
    B --> C{Particles retained on sieve?}
    C -- Yes --> D[Rub with Rubber Pestle in Mortar]
    D --> B
    C -- No --> E[Collect finer fraction]

This ensures proper particle size distribution without damaging soil grains.

IS 2720 Part 4: Balance Requirements for Density Measurement

• Clause 5.2.1.7: Balance must measure mass accurate to 0.01 g for precise density determination.

• Clause 5.2.5 & 5.1.4 (Calculations):
  Density (ρ) is calculated by:
  [ \rho = \frac{W_{\text{air}}}{W_{\text{air}} - W_{\text{water}}} \times \rho_{\text{water}} ]
  Where:

  • ( W_{\text{air}} ) = Weight of sample in air
  • ( W_{\text{water}} ) = Weight of sample in water
  • ( \rho_{\text{water}} ) = Density of water at test temperature (from IS 2720 Part 4 Table)

• Clause 5.3 (Plummet Balance): Alternative method uses a plummet immersed in water, requiring balance sensitivity to detect small weight differences. Ensure balance is calibrated and zeroed before measurements.

Key Specifications:

flowchart LR
    A[Sample Weight in Air] --> B[Measure using Balance (±0.01g)]
    C[Sample Weight in Water] --> B
    B --> D[Calculate Density using formula]
    D --> E[Apply Temperature Correction]
    E --> F[Final Density Result]

Summary: Use a balance with 0.01 g accuracy, measure weights in air and water, apply the formula with temperature-corrected water density for precise soil density determination.

IS 2720 Part 4: Mortar with Rubber Covered Pestle

Key Points from Clauses 3.1.2, 3.1.3 & 4.1.6

• Rubber Pestle and Mortar are used to gently rub soil aggregates during sieving to separate particles without breaking them.
• Sample mass depends on the maximum particle size (see Note 1).
• Sieving involves agitating the sieve so particles roll irregularly; particles are not pushed through.
• Material retained on each sieve is rubbed gently with rubber pestle if needed, then re-sieved.

Sample Mass for Test (Note 1)

Max Weight Retained on Each Sieve (Note 2)

Summary

• Use rubber-covered pestle to avoid particle breakage.
• Follow sample mass and max retained weight limits strictly.
• Agitate sieves for proper particle separation.

flowchart TD
    A[Soil Sample] --> B[Sieving through IS Sieves]
    B --> C{Particles retained?}
    C -->|Yes| D[Rub gently with Rubber Pestle]
    D --> B
    C -->|No| E[Collect finer fractions]

This ensures accurate particle size distribution without altering particle integrity.

Mechanical Sieve Shaker (IS 2720 Part 4)

• Clause 4.1.7: Use of mechanical sieve shaker is optional but recommended for efficiency.

• Operation:

  • Minimum shaking time: 10 minutes.
  • Ensures complete sieving with irregular rolling motion.
  • No particles should be forced through the sieve.
  • After sieving, retained soil fractions are collected and weighed.

• Sieves as per Clause 5.1.1.6:

  • Standard IS sieves: 2 mm, 425 micron, 75 micron, plus receiver.

• Procedure Summary:

  1. Place sample on the nest of sieves.
  2. Shake mechanically for at least 10 minutes.
  3. Collect and weigh soil retained on each sieve.
  4. Repeat if manual sieving is used, rubbing particles gently.

• Key Points:

  • Ensure sieving completeness.
  • Use rubber pestle for manual re-sieving when mechanical shaker is unavailable.
  • Record mass retained on each sieve for grain size distribution.

Typical Grain Size Distribution Table (Example)

flowchart TD
    A[Sample Preparation] --> B[Place Sample on Sieves]
    B --> C[Mechanical Sieve Shaker]
    C --> D{Shake for 10 Minutes}
    D --> E[Collect Retained Soil]
    E --> F[Weigh and Record]
    F --> G[Plot Grain Size Distribution]

This ensures repeatable, accurate particle size analysis per IS 2720 Part 4.

IS 2720 Part 4 — Analysis by Dry Sieving (Clause 4.3.2)

Key Specifications:

• Material: Soil fraction retained on the 75-micron IS sieve.
• Preparation: Material may be over-dried before analysis.
• Procedure: Dry sieve the sample through a series of IS sieves.
• Calculation: Compute cumulative percentage retained on each sieve.

Typical Procedure Summary:

1. Oven-dry the soil sample (usually at 105-110°C).
2. Weigh the total sample accurately.
3. Sieve the sample through IS sieves arranged in descending order (e.g., 4.75 mm, 2.36 mm, 1.18 mm, 600 µm, 300 µm, 150 µm, 75 µm).
4. Weigh the soil retained on each sieve.
5. Calculate percentage retained and cumulative percentage retained.

Key Formula:

[ \text{Percentage retained on a sieve} = \frac{\text{Mass retained on sieve}}{\text{Total mass of sample}} \times 100 ]

[ \text{Cumulative percentage retained} = \sum \text{Percentage retained on all coarser sieves} ]

Common IS Sieve Sizes for Dry Sieving:

Notes:

• Dry sieving is suitable for coarse-grained soils (gravel, sand).
• For particles finer than 75 µm, wet sieving or sedimentation methods are preferred.
• Ensure the sample is dry to prevent clogging of sieve apertures.

flowchart TD
    A[Soil Sample] --> B[Oven Dry at 105-110°C]
    B --> C[Weigh Sample]
    C --> D[Arrange IS Sieves (Descending Size)]
    D --> E[Dry Sieve Sample]

IS 2720 Part 4: Apparatus for Grain Size Analysis

Key apparatus details from clauses 5.1.1, 5.2.1, 5.3.2:

• 5.1.1 Apparatus: Standard sieves with specified mesh sizes, conforming to IS 460 (for sieve dimensions and wire gauge).
• 5.2.1 Apparatus: Mechanical sieve shaker for uniform sieving.
• 5.2.1.5 Stirring apparatus: As per 5.1.1.5, a mechanical stirrer to disperse soil samples uniformly in water before wet sieving.
• 5.3.2 Apparatus: Hydrometer for fine particles analysis, conforming to specified dimensions and calibration per IS 2720 Part 4.

Key specifications:

Formula: Effective Particle Diameter (D) from hydrometer reading

[ D = \sqrt{\frac{30 \eta}{(G_s - G_w) t}} ]

Where:

• (\eta) = viscosity of water
• (G_s) = specific gravity of soil solids
• (G_w) = specific gravity of water (≈1)
• (t) = time in seconds

flowchart LR
  A[Soil Sample] --> B[Mechanical Stirrer]
  B --> C[Wet Sieving with Standard Sieves]
  C --> D[Mechanical Sieve Shaker]
  D --> E[Hydrometer Analysis for Fines]

This setup ensures precise grain size distribution per IS 2720 Part 4.

IS 2720 Part 4: Reagents and Solutions Key Points

• Reagents Quality:

  • All reagents must be of analytical grade (Clauses 5.1.2, 5.2.2).

• Common Reagents Used:

  • Sodium hexametaphosphate (chemically pure)
  • Mixture of Sodium hydroxide (IS: 376-1976) and Sodium carbonate (IS: 296-1974, analytical grade)
  • Other suitable dispersing agents may be used if proven effective (Clause 4.2).

• Sodium Hexametaphosphate Solution Preparation:

  • Refer to Clause 5.1.2.3 and associated notes for detailed preparation steps (Clause 5.2.2.3).

Typical Preparation (Summary)

Notes:

• Use distilled water for all solutions.
• Prepare fresh solutions to maintain effectiveness.
• Follow exact concentrations as per IS 2720 Part 4 for consistency.

flowchart LR
    A[Analytical Grade Reagents] --> B[Sodium Hexametaphosphate]
    A --> C[Sodium Hydroxide + Sodium Carbonate]
    B --> D[Dispersing Agent Solution]
    C --> D
    D --> E[Soil Sample Dispersion]

This ensures reliable soil particle size analysis per IS 2720 Part 4.

Pre-treatment of Soil Samples (IS 2720 Part 4:1985)

• Soluble Salts Check: Determine % soluble salts as per IS 2720 Part 21 (1977).

  • If >1%, wash soil with water carefully to avoid loss of particles.

• Sample Preparation:

  • Take two samples (50-100 g) from air-dried soil passing 4.75 mm sieve.
  • For clay: ~50 g; for sand: ~100 g.
  • Determine moisture content (w) of one sample (IS 2720 Part 2).
  • Weigh other sample (Wa) to 0.01 g accuracy.

• Organic Matter Removal:

  • Add 150 ml hydrogen peroxide to weighed sample in wide-mouth conical flask.
  • Stir gently, cover, leave overnight.
  • Heat gently, avoid frothing overflow, stir periodically.
  • Boil to reduce volume to ~50 ml.
  • Add more peroxide if organic content is high.

• Loss in Mass Calculation (from Clause 5.2.5.1):
  [ \text{Loss in mass} = \frac{W_a - W_b}{W_a} \times 100 ]
  where (W_a) = initial weight before treatment, (W_b) = weight after treatment.

• Pretreatment of Soil Retained on 75 micron Sieve:
  Oven dry and analyze as per Clause 4.3.2.1, pretreatment as above.

Summary Table

flowchart TD
   

IS 2720 Part 4 (1985) — Detailed Apparatus Specifications

This part covers apparatus for grain size analysis by sieving and sedimentation.

Key Apparatus Details:

• Stirring Apparatus (Clause 5.3.2):

  • Stirring paddles: Dimensions as per Fig. 2, typically made from 1.25 mm thick sheet.
  • Baffle rods:
    • Permanent baffle rods fixed inside the container.
    • Removable baffle rods for easy cleaning.
    • Short baffle rods positioned as per the location plan.
  • All dimensions in millimeters.

• Dispersion Cups (Fig. 3):

  • Used for sample dispersion in sedimentation tests.
  • Dimensions and shape standardized for uniformity.

Specifications Summary:

Important:

• All dimensions are in millimeters.
• Apparatus must conform strictly to figures and dimensions to ensure test accuracy.

flowchart LR
    A[Stirring Apparatus] --> B[Stirring Paddles (1.25 mm thick)]
    A --> C[Baffle Rods]
    C --> C1[Permanent]
    C --> C2[Removable]
    C --> C3[Short]
    D[Dispersion Cups] --> E[Standard Dimensions]

For exact dimensions, refer to Fig. 2 and Fig. 3 in IS 2720 Part 4 (1985).

Key Formulas and Specifications for Hydrometer Volume and Calibration

From IS 2720 Part 4 (Clause 5.2.3):

1. Volume of Hydrometer Bulb (Vh)

• Method a: Water Displacement
  [ V_h = \text{Water level before immersion} - \text{Water level after immersion} \quad (\text{in ml}) ] Note: Stem volume submerged is negligible.

• Method b: Mass Method
  [ V_h = \text{Mass of hydrometer in grams} \approx \text{Volume in ml} ]

2. Sectional Area of Measuring Cylinder (A)

[ A = \frac{\text{Volume between two graduations (ml)}}{\text{Distance between graduations (cm)}} ]

3. Effective Depth (HR) Calculation

[ H_R = H_1 + \frac{V_n}{A} \times \frac{1}{h} ]

Where:

• (H_1) = distance from neck of bulb to calibration mark (R_n) (cm)
• (V_n) = volume of hydrometer bulb (ml)
• (A) = sectional area of cylinder (cm²)
• (h) = height of bulb (cm) (distance from neck to bottom)

Note: For asymmetrical bulbs, center of volume is at (h/2).

Calibration Procedure Summary

• Measure distances (H_1), (h), and sectional area (A).
• Calculate (H_R) for each calibration mark (R_n).
• Plot (H_R) vs. (R_n) for smooth calibration curve.

flowchart TD
    A[Measure water level] --> B[Immerse hydrometer]
    B --> C[Measure water level again]
    C --> D[Calculate Vh = difference in water levels]
    E[Weigh hydrometer] --> F[Mass ≈ Volume Vh]
    G[Measure distance between graduations] --> H[Calculate sectional area A]
    I[Measure distances H1, h] --> J[Calculate effective depth HR]
    J --> K[Plot HR vs Rn for calibration

IS 2720 Part 4: Calibration & Adjustment of Instruments

1. Hydrometer Calibration (Clause 5.2.3.2)

• Sectional Area (A) of Cylinder:

[ A = \frac{\text{Volume between two graduations (ml)}}{\text{Distance between graduations (cm)}} ]

• Distances to Record:

  • (R_h): Distance from lowest calibration mark to other major marks.
  • Distance from neck of bulb to nearest calibration mark.
  • Height of bulb (h) (neck to bottom).

• Effective Depth (H_R):

[ H_R = H_1 + \frac{V_n}{A \times h} ]

Where,

• (H_1) = length from neck to graduation (R_n) (cm)

• (V_n) = volume of hydrometer bulb (ml)

• (A) = sectional area of cylinder (cm²)

• (h) = twice the length from neck to centre of volume (cm)

• Plot (H_R) vs (R_n) for smooth calibration curve.

2. Balance Adjustment (Clause 5.3.3.1)

• Use rider weights (0 and 100) to check calibration.
• Adjust zero and range screws to get correct pointer readings.
• Immerse plummet in distilled water; set plummet depth (e.g., 9 cm).
• Mark string and adjust pointer to zero without altering screw adjustments.

3. Sampling Pipette Calibration (Clause 15.1.3.1)

• Fill pipette with distilled water, drain surplus.
• Weigh water volume in pipette and tap to nearest 0.05 ml.
• Repeat 3 times and average volume (V_p).

flowchart TD
    A[Measure cylinder graduations] --> B[Calculate sectional area A]
    B --> C[Measure hydrometer distances (Rh, neck to mark, h)]
    C --> D[Calculate effective depth HR]
    D --> E[Plot HR vs Rn curve]
    E --> F[Use curve for test readings]

Summary:

• Use volume/distance for sectional area.
• Calculate effective depth with bulb volume and sectional area

IS 2720 Part 4: Sedimentation Procedures - Key Formulas & Specifications

1. Particle Diameter Calculation

The diameter ( D ) of soil particles in suspension at time ( t ) (in minutes) is given by:

[ D = \sqrt{\frac{30 \eta}{980 (G - G_i) g H t}} ]

Where:

• ( D ) = particle diameter (mm)
• ( \eta ) = coefficient of viscosity of water (poises) at suspension temperature
• ( G ) = specific gravity of soil fraction (g/cm³)
• ( G_i ) = specific gravity of water (≈ 1 g/cm³)
• ( H ) = height of fall or sampling depth (cm)
• ( t ) = time elapsed before sampling (minutes)
• ( g ) = acceleration due to gravity (980 cm/s²)

2. Sedimentation Sampling Procedure

• Add 25 ml sodium hexametaphosphate to a 500 ml sedimentation tube and dilute to 500 ml.
• Immerse tubes in a constant temperature bath; allow temperature equilibration.
• Shake tubes by inversion, then start timing.
• Lower pipette to 100 ± 1 mm below suspension surface for sampling.
• Take samples carefully within specified timings to avoid disturbance.
• Wash pipette and apparatus after each sample.

3. Hydrometer Readings

• Take readings at 8, 15, 30 minutes; 1, 2, 4 hours; and optionally at 24 hours.
• Record suspension temperature with ±0.5°C accuracy.
• Take hydrometer readings in distilled water at corresponding temperatures for temperature correction ( M_t ).
• Avoid vibration and disturbance during readings.

4. Temperature Correction

• Calculate temperature correction ( M_t ) as difference between hydrometer reading in distilled water at test temperature and calibration temperature.
• Prepare a correction chart for ease of use.

Summary Table for Sampling Times & Readings

flowchart TD
   

IS 2720 Part 4 - Soil Sample Preparation & Handling: Key Points

1. Sample Mass Based on Maximum Particle Size (Clause 3.1.2, Note 1)

2. Max Weight Retained on Each Sieve (Note 2)

3. Chemical Treatment for Dispersion (Clause 4.3.1.1)

• Add 2 g sodium hexametaphosphate or 1 g sodium hydroxide + 1 g sodium carbonate per litre of water to soil.
• Stir thoroughly and soak.
• Wash over nested sieves (finest at bottom: 75 μm IS sieve) until wash water is clear.
• Oven dry fractions at 105-110°C and weigh separately.

4. Sample Preparation Steps (Clause 3.2)

• Separate soil passing and retained on 4.75 mm sieve.
• Obtain ~30 g by riffling for water content test (IS 2720 Part 2).
• For organic matter removal: boil soil suspension with distilled water, add 75 ml hydrogen peroxide, heat until frothing subsides.
• For calcium soils, add ~10 ml HCl after peroxide treatment until acid reaction is confirmed.

Summary Flowchart of Sample Preparation:

flowchart TD
    A[Receive Field Soil Sample] --> B[Prepare as per IS 2720 Part 1