Soil is the foundation upon which every structure rests, and understanding its granular composition is essential for safe and durable construction. The grain size distribution of a soil sample directly influences its engineering behaviour – permeability, shear strength, compressibility, and compaction characteristics. Among the most fundamental geotechnical tests is the sieve analysis, which separates soil particles by size using a stacked set of sieves with progressively smaller openings. In India, the standard governing this procedure is IS 2720 (Part 4) – 1985, titled “Method of Test for Soil – Grain Size Analysis.” This article covers the sieve analysis procedure – objective, equipment, sample preparation, testing method, calculations, and reporting format. Engineers and technicians involved in geotechnical engineering principles of soil mechanics foundation design and earth structure analysis rely on this test daily to classify soils and make informed design decisions.
Purpose and Significance of Sieve Analysis in Geotechnical Engineering
The primary objective of a sieve analysis, as per IS 2720 (Part 4) – 1985, is the determination of the particle size distribution of fine, coarse, and all-in-aggregates by sieving. This distribution forms the basis for soil classification systems used globally.
Why particle size distribution matters in practice:
- Soil classification – The uniformity coefficient (Cu) and coefficient of curvature (Cc) derived from the gradation curve determine whether a soil is well-graded, poorly graded, or gap-graded.
- Drainage and permeability – Coarse soils drain rapidly and are preferred for backfill. Fine soils retain water and exhibit slow consolidation.
- Compaction control – Well-graded soils achieve higher dry densities than uniform soils under the same compaction effort, making them desirable for embankments.
- Filter design – The relative particle sizes between base soil and filter layer determine whether internal erosion can develop.
Review broader site investigation methods in soil testing for construction comprehensive guide to site investigation and geotechnical analysis covering field and laboratory tests that complement sieve analysis in a geotechnical investigation programme.
Reference Standard and Equipment Requirements
IS 2720 (Part 4) – 1985 is the governing Indian Standard for grain size analysis of soils, prescribing the sieving method and sieve specifications. The required equipment is straightforward but must be in good condition.
Essential equipment:
- Weighing balance – Sensitivity of 0.1 percent of the sample mass, readable to 0.01 g for fine soils and 0.1 g for coarse soils.
- Sieve set – Standard IS sieves from 80 mm down to 75 microns, arranged in descending order. Frame diameter is usually 200 mm or 300 mm.
- Sieve shaker – A mechanical shaker that imparts rotary and tapping motion to the sieve stack for reproducible particle separation.
- Oven – Thermostatically controlled, capable of maintaining 105 to 110 degrees C for drying.
- Wooden mallet and mortar – For breaking clods without crushing individual particles.
The table below summarises the standard sieve sizes used in soil analysis and the corresponding particle fractions they separate.
| Sieve Size | Opening (mm) | Soil Fraction Retained |
|---|---|---|
| 80 mm | 80 | Very coarse gravel |
| 40 mm | 40 | Coarse gravel |
| 20 mm | 20 | Medium gravel |
| 10 mm | 10 | Fine gravel |
| 4.75 mm | 4.75 | Very fine gravel |
| 2.36 mm | 2.36 | Very coarse sand |
| 1.18 mm | 1.18 | Coarse sand |
| 600 mu-m | 0.600 | Medium sand |
| 300 mu-m | 0.300 | Fine sand |
| 150 mu-m | 0.150 | Very fine sand |
| 75 mu-m | 0.075 | Silt and clay (pan) |
The material that passes through the finest sieve (75 mu-m) is collected in a pan at the bottom of the stack. This fraction comprises silt and clay particles that are too small for dry sieving and typically require hydrometer analysis. For more on sieving in soil classification systems, refer to sieve analysis and soil classification.
Sample Preparation and Step-by-Step Testing Procedure
Proper sample preparation is critical for representative results. IS 2720 (Part 4) provides clear instructions on preparing the specimen before sieving.
Sample preparation steps:
- Spread the soil sample in a tray for air drying, or use an oven at 60 degrees C for faster drying – this lower temperature prevents altering clay mineral structure.
- Break any clods present in the air-dried sample using a wooden mallet. This step must be performed carefully so that individual particles are not crushed, which would artificially shift the gradation towards finer sizes.
- Once the sample is free-flowing, quartering or riffle splitting is used to obtain a representative test portion. The specimen mass depends on maximum particle size: for soils up to 4.75 mm, a 500 g sample is typical; for soils containing particles up to 20 mm, a 2 kg sample is required.
Procedure as per IS 2720 (Part 4) – 1985:
- Dry the sample and clean the sieves. The test portion is dried to constant mass at 110 plus or minus 5 degrees C. Clean every sieve before use – clogged apertures skew results towards coarser gradation.
- Arrange and weigh the sieves. Stack sieves in descending order (largest at top, smallest at bottom) with a collecting pan underneath. Weigh each empty sieve and record the tare mass.
- Transfer the sample and sieve. Place the oven-dried sample on the top sieve and secure the stack in the mechanical shaker. Run for not less than 2 minutes. The combined rotary and tapping motion ensures particles smaller than each opening fall through to the next level.
- Weigh the retained material. After sieving, carefully remove each sieve from the top down. Transfer retained material onto a tared pan and record its mass. The pan material represents the fraction finer than 75 mu-m.
- Verify the mass balance. Sum the masses of retained material plus the pan. This total must be within 0.5 percent of the original dry mass. If the discrepancy exceeds this limit, repeat the test.
The sieving technique for soils shares many similarities with aggregate testing. For comparison, see our guide on sieve analysis of aggregates a step by step guide to gradation testing and fineness modulus determination covering the parallel procedure used in concrete technology.
Calculations: Percent Retained, Cumulative Retained, and Percent Finer
Three sequential calculations transform the raw masses into the gradation curve used for classification and design.
Step 1 – Percent retained on each sieve
Percent retained = (Mass retained on sieve / Total dry mass of sample) x 100
This represents the proportion of the sample coarser than the sieve opening but finer than the opening of the sieve immediately above.
Step 2 – Cumulative percent retained
The cumulative percent retained on any sieve is the sum of the percent retained on that sieve and all coarser sieves above it. This indicates the total percentage retained on or above that sieve.
Step 3 – Percent finer
Percent finer = 100 minus Cumulative percent retained
This is the most important value for plotting the gradation curve. It represents the percentage of the sample that passes through each sieve.
A worked example helps illustrate the calculation sequence. Consider a 500 g dry sample of sand:
| Sieve Size | Mass Retained (g) | % Retained | Cumulative % Retained | % Finer |
|---|---|---|---|---|
| 4.75 mm | 10.5 | 2.1 | 2.1 | 97.9 |
| 2.36 mm | 35.0 | 7.0 | 9.1 | 90.9 |
| 1.18 mm | 72.5 | 14.5 | 23.6 | 76.4 |
| 600 mu-m | 105.0 | 21.0 | 44.6 | 55.4 |
| 300 mu-m | 140.0 | 28.0 | 72.6 | 27.4 |
| 150 mu-m | 85.0 | 17.0 | 89.6 | 10.4 |
| 75 mu-m | 40.0 | 8.0 | 97.6 | 2.4 |
| Pan | 12.0 | 2.4 | 100.0 | 0.0 |
| Total | 500.0 | 100.0 | – | – |
From the percent finer data, the engineer determines key gradation parameters: the effective size (D10), the uniformity coefficient (Cu = D60 / D10), and the coefficient of curvature (Cc = D30 squared divided by (D10 x D60)). These values feed into the Unified Soil Classification System (USCS) and the Indian Standard Soil Classification System (ISSCS). Understanding these calculations in the broader context of project economics is valuable – our article on construction economics and value engineering cost escalation analysis value methodology life cycle cost analysis and constructability reviews discusses how material testing decisions affect project budgets and risk allocation.
Reporting Results, Graphical Presentation, and Essential Precautions
IS 2720 (Part 4) requires sieve analysis results to be reported both in tabular form and as a graphical plot. The graph is drawn on semi-logarithmic paper, with sieve openings plotted on the logarithmic horizontal axis (because sizes span several orders of magnitude) and percent finer on the arithmetic vertical axis. This reveals the gradation curve shape at a glance.
Key observations from the gradation curve:
- Steep curve: Indicates a uniformly graded soil where most particles fall within a narrow size range. Such soils have poor compaction characteristics and are prone to liquefaction in seismic conditions.
- Flat, well-distributed curve: Indicates a well-graded soil with particles spanning a wide range of sizes. These soils compact to high densities and have superior engineering properties.
- Plateau or S-shape: A gap-graded soil where one or more intermediate size fractions are missing. These soils may exhibit unpredictable drainage and strength behaviour.
Safety precautions and good laboratory practices:
- Clean every sieve thoroughly before and after use so that no soil particles remain lodged in the apertures. Stuck particles artificially increase the retained mass on that sieve in subsequent tests.
- When weighing the sieve with the retained soil sample, position the sieve concentrically on the balance platform to avoid tilting and inaccurate readings.
- Inspect the electrical connection and mechanical condition of the sieve shaker before each test. A shaker operating at reduced amplitude or uneven speed will not achieve proper particle separation within the specified time.
- Allow the oven-dried sample to cool to room temperature in a desiccator before weighing. Hot samples create convection currents that affect balance readings and absorb moisture from the air as they cool.
- Record all observations immediately in a bound laboratory notebook or digital data sheet. Retrospective entries introduce transcription errors that can propagate through the entire calculation chain.
The data from sieve analysis informs not just soil classification but also structural design. Our article on structural analysis explains how soil bearing capacity and settlement calculations depend on classification established through tests like this one. A soil misclassified due to flawed sieve analysis can lead to inadequate foundation design with serious consequences.
Conclusion
The sieve analysis of soil as per IS 2720 (Part 4) – 1985 is an indispensable laboratory test that provides the particle size distribution data needed for soil classification, compaction control, drainage design, and foundation engineering. From initial drying through sieving and the three-step calculation of percent retained, cumulative retained, and percent finer – each stage contributes to a reliable gradation curve that summarises the soil composition. The semi-logarithmic plot transforms raw masses into an engineering tool: the curve slope tells the geotechnical engineer whether the soil will drain freely, compact densely, or behave unpredictably under load. Adhering to the prescribed precautions ensures trustworthy data. For engineers seeking to connect material behaviour to structural performance, our discussion on qualitative structural analysis explains the reasoning that connects test results to design outcomes.