Soil particle size analysis is a fundamental geotechnical laboratory test that determines the grain size distribution of a soil specimen, providing the basis for soil classification, fill quality control, and foundation design. This article explains the test principle, applicable standards such as ASTM D6913, ASTM D7928, ISO 17892-4, and GB/T 50123, sample requirements, testing procedures, and typical engineering applications.
Technical Overview
Soil particle size analysis, also known as the gradation test or mechanical analysis of soil, is a laboratory procedure used to determine the percentage by mass of particles of different sizes contained in a soil specimen. The results are typically presented as a grain size distribution curve plotted on semi-logarithmic paper, which shows the percentage finer than a given particle diameter across the range from gravel down to clay sizes.
Because the particle size range of natural soils spans several orders of magnitude, the test is generally divided into two complementary procedures. Coarse-grained fractions larger than 0.075 mm are measured by sieve analysis, while fine fractions smaller than 0.075 mm are measured by sedimentation techniques such as hydrometer analysis or the pipette method. For a complete gradation curve covering both coarse and fine fractions, the two methods are combined.
From the distribution curve, key engineering indices are derived, including the effective particle size D10, the median size D30 and D60, the coefficient of uniformity Cu, and the coefficient of curvature Cc. These parameters determine whether a soil is well-graded or poorly-graded and directly support classification systems such as the Unified Soil Classification System (USCS, ASTM D2487) and AASHTO M145.
The test is governed by recognized standards including ASTM D6913 (particle size distribution of soils using sieve analysis), ASTM D7928 (hydrometer analysis), ISO 17892-4 (gradational analysis), BS 1377-2, and the Chinese national geotechnical test standard GB/T 50123. Accredited third-party laboratories routinely perform this test as part of geotechnical investigation programs and construction quality control.
Testing Samples
Disturbed Bulk Soil Samples
Disturbed samples collected from test pits, borings, stockpiles, or borrow areas are the most common sample type. Since the test only requires the mineral grain skeleton, the natural structure and moisture condition of the soil are not critical, making disturbed samples fully acceptable. The sample must be representative of the stratum or material lot being characterized.
Fine-Grained Soils (Silts and Clays)
Fine-grained soils containing a significant fraction finer than 0.075 mm require hydrometer or pipette analysis. Before testing, the specimen is pretreated with a dispersing agent, typically a sodium hexametaphosphate solution, and agitated by boiling or mechanical stirring to break down soil aggregates into individual particles. Organic-rich soils may require additional pretreatment with hydrogen peroxide to remove binding organic matter.
Coarse-Grained Soils (Sands and Gravels)
Sands and gravels are tested primarily by dry sieving through a nested stack of sieves. Wash sieving is applied when cohesive fines are coated on coarse particles, ensuring that the fine fraction passing the 0.075 mm sieve is accurately quantified. The required minimum specimen mass increases with the maximum particle size, following the quantities specified in the applicable standard.
Sample Preparation
- Air-dry or oven-dry the sample to a constant mass, typically at 60 to 110 degrees Celsius depending on the standard and soil type.
- Split or quarter the dried sample using a riffle splitter to obtain a representative test portion of the required mass.
- Record the total dry mass and, where necessary, the mass retained on each sieve in sequence.
- For combined analysis, split the sample at the 0.075 mm sieve and route the undersize fraction to sedimentation testing.
Testing Items
The soil particle size analysis test yields the following principal results:
- Grain size distribution curve expressed as percent passing versus particle diameter.
- Percentage by mass of each size fraction: gravel, sand, silt, and clay.
- Characteristic diameters D10, D30, and D50, D60 read from the distribution curve.
- Coefficient of uniformity, Cu, equal to D60 divided by D10.
- Coefficient of curvature, Cc, equal to D30 squared divided by the product of D10 and D60.
- Soil classification and grading description, for example well-graded gravel (GW), poorly-graded sand (SP), silty clay, or low-plasticity silt, in accordance with the specified classification system.
Testing Methods
Dry and Wet Sieve Analysis
The specimen is placed on the top of a nest of sieves with progressively smaller openings, commonly including 4.75 mm, 2.0 mm, 0.85 mm, 0.425 mm, 0.25 mm, 0.15 mm, and 0.075 mm. The stack is shaken with a mechanical shaker for a standardized period, typically ten minutes. The mass retained on each sieve is weighed, and the cumulative percent passing is calculated. When fines adhere to coarse grains, wash sieving through the 0.075 mm sieve is performed before oven drying and re-sieving.
Hydrometer Analysis
The fine fraction is dispersed in a water column, and a calibrated hydrometer is immersed at timed intervals to measure the suspension density. Based on Stokes' law, larger particles settle faster, so the changing density reflects the progressive settlement of progressively finer particles. Corrections for temperature, deflocculant concentration, and meniscus reading are applied. Readings taken over intervals from about one quarter minute to twenty-four hours allow calculation of the percent finer than diameters ranging from roughly 0.075 mm down to 0.001 mm and below.
Pipette Method
The pipette method is an alternative sedimentation procedure in which small volumes of suspension are withdrawn at a fixed depth and time. The extracted aliquots are dried and weighed to determine the concentration of particles finer than the equivalent Stokes diameter, offering high precision for fine-grained soils and often used in research and soil science applications.
Laser Diffraction Method
Laser diffraction, standardized under ISO 13320, measures particle size distributions from the angular scattering pattern of a laser beam passing through a dispersed suspension. It provides rapid, reproducible, high-resolution results over a broad size range and is increasingly used as a modern complement or alternative to classical sedimentation methods.
Combined Analysis
For soils containing both coarse and fine fractions, sieve analysis and hydrometer or pipette analysis are performed on split portions of the same specimen, and the results are merged into a single continuous distribution curve. This combined approach is specified in ISO 17892-4, GB/T 50123, and ASTM practice for full-range gradation.
Applications
Soil particle size analysis supports a wide range of engineering, construction, and environmental activities:
- Geotechnical site characterization and soil classification for foundation design and bearing capacity evaluation.
- Quality control of engineered fill for embankments, earth dams, road subgrades, and backfill around structures.
- Design of filter and drainage layers, where gradation criteria based on D10, D15, D60, and D85 prevent internal erosion and piping.
- Assessment of soil suitability as construction material, including liquefaction susceptibility screening of loose fine sands.
- Selection and verification of ground improvement methods such as compaction, vibro-compaction, and replacement.
- Agricultural and environmental studies, including permeability estimation, erodibility assessment, and contaminant transport analysis.
- Concrete and asphalt aggregate evaluation, where gradation envelopes govern mix performance.
FAQ
What is the difference between sieve analysis and hydrometer analysis?
Sieve analysis measures particles larger than 0.075 mm by mechanically separating them on woven wire sieves, whereas hydrometer analysis measures particles smaller than 0.075 mm by observing the sedimentation of a dispersed suspension according to Stokes' law. Soils spanning both ranges require a combined test.
Why is a dispersing agent added before hydrometer testing?
Fine clay and silt particles tend to flocculate into larger aggregates in water. The dispersing agent, typically sodium hexametaphosphate, introduces repulsive charges between particles so they settle individually, ensuring the measured distribution reflects true grain sizes rather than aggregate sizes.
How much soil sample is required?
The required mass depends on the maximum particle size. For clean sands, several hundred grams are usually sufficient, while gravels may require tens of kilograms to be representative. Hydrometer analysis typically requires only about 50 to 100 grams of oven-dried fine soil. The applicable standard specifies minimum masses for each size class.
What do Cu and Cc indicate?
The coefficient of uniformity Cu describes the range of particle sizes, and the coefficient of curvature Cc describes the shape of the gradation curve. A gravel or sand is considered well-graded when Cu is greater than 4 or 6 respectively and Cc lies between 1 and 3, indicating a wide, even distribution of sizes that compacts densely.
How long does the test take?
Sieve analysis can be completed within a few hours including drying. Hydrometer analysis requires a suspension that is read over 24 hours or longer, and with sample pretreatment, dispersing, and oven drying, the full combined procedure is usually completed within two to three working days.
Which standards govern the test?
Commonly referenced standards include ASTM D6913 for sieve analysis, ASTM D7928 for hydrometer analysis, ASTM D422 in legacy specifications, ISO 17892-4 for gradational analysis, BS 1377-2, AASHTO T88, and the Chinese standard GB/T 50123. The specifying authority determines which standard applies to a given project.