Soil Particle Analysis: Testing Methods, Standards, and Applications

Soil particle analysis is a fundamental geotechnical and environmental laboratory test that determines the particle size distribution of a soil sample, expressed as the mass percentages of gravel, sand, silt, and clay. Performed in accordance with internationally recognized standards such as ASTM D6913, ASTM D7928, and ISO 11277, this analysis provides essential data for soil classification, foundation design, construction quality control, and agricultural management. This article explains the technical principles, sample requirements, testing procedures, and typical applications of soil particle analysis in professional laboratory practice.

Technical Overview

Soil particle analysis, also referred to as particle size distribution (PSD) analysis or gradation analysis, is the process of quantifying the proportions by mass of particles of different sizes within a soil sample. Because particle size exerts a dominant influence on engineering behavior, including permeability, compressibility, shear strength, and compaction characteristics, PSD data form the basis of nearly every soil classification system in use today.

In the Unified Soil Classification System (USCS) used with ASTM D2487, particles larger than 75 mm are classified as cobbles, 4.75 mm to 75 mm as gravel, 0.075 mm to 4.75 mm as sand, and finer than 0.075 mm as fines, with fines further divided into silt and clay at the 0.002 mm boundary. Agricultural systems such as the USDA texture classification use slightly different boundaries, with sand spanning 0.05 mm to 2 mm, silt from 0.002 mm to 0.05 mm, and clay below 0.002 mm. Regardless of the classification framework, the laboratory objective is the same: to produce an accurate gradation curve and derive the characteristic parameters that engineers and scientists rely upon.

Results are conventionally plotted on a semi-logarithmic curve of percent passing versus particle diameter, from which key indices such as the effective size (D10), the median size (D50), and the coarse sizes D30 and D60 are read. These values allow calculation of the uniformity coefficient (Cu = D60/D10) and the coefficient of curvature (Cc = D30²/(D10 × D60)), which together describe whether a soil is well graded, poorly graded, or gap graded. The applicable standards include ASTM D6913 (sieve analysis), ASTM D7928 (hydrometer analysis), ISO 11277 (sedimentation-based determination for mineral soils), ISO 13320 (laser diffraction), BS 1377-2, and GB/T 50123 for geotechnical testing method compliance.

Testing Samples

Fine-Grained Soil Samples

Silts and clays, in which the majority of particles are smaller than the 0.075 mm (No. 200) sieve opening, are typically tested by sedimentation methods. A representative air-dried specimen of approximately 100 g to 150 g is sufficient, oven-dried to determine moisture content, and then dispersed in a calibrated suspension. Multiple specimens may be tested in parallel to verify repeatability of the clay and silt fractions.

Coarse-Grained Soil Samples

Sands and gravels are analyzed by sieving, and the required specimen mass increases with the maximum particle size. As a general rule guided by ASTM D6913, a clean fine sand requires roughly 100 g to 500 g, while materials containing particles up to 19 mm may require several kilograms to remain statistically representative. Under-sampling coarse fractions is one of the most common sources of error in gradation reporting.

Organic and Calcareous Soils

Soils containing significant organic matter, gypsum, or calcium carbonate require pretreatment before sedimentation. Organic matter is oxidized with hydrogen peroxide, and carbonates may be removed with dilute hydrochloric acid where appropriate, as specified in ISO 11277. Without pretreatment, cementing agents cause particles to flocculate and settle as aggregates, which distorts the reported fines distribution.

Sample Preparation and Reduction

All samples are air-dried, weighed, and reduced by cone-and-quartering or riffle splitting to obtain representative test portions. Dispersing agents, most commonly a sodium hexametaphosphate solution, are added to fine-grained specimens and agitated, typically with a mechanical stirrer or end-over-end shaker, to ensure complete deflocculation before sedimentation readings begin.

Testing Items

Depending on the specified standard and project requirements, a comprehensive soil particle analysis report typically includes the following deliverables:

• Full particle size distribution curve (percent passing versus grain diameter, semi-logarithmic plot)
• Mass percentages of gravel, sand, silt, and clay fractions
• Percent passing the 0.075 mm (No. 200) sieve, reported as fines content
• Characteristic particle sizes: D10, D30, D50, and D60
• Uniformity coefficient (Cu) and coefficient of curvature (Cc)
• Soil classification and group symbol per ASTM D2487 (USCS) or AASHTO M145
• USDA soil texture class designation for agricultural and land-use applications
• Consolidated sieve and hydrometer data tables with correction factors applied

Testing Methods

Sieve Analysis (Coarse Fraction)

Sieve analysis covers particles from approximately 0.075 mm up to 125 mm and follows ASTM D6913 or, for construction aggregates, ASTM C136. The dried specimen is placed on a nested stack of standard sieves, commonly including the 9.5 mm, 4.75 mm (No. 4), 2.0 mm (No. 10), 0.425 mm (No. 40), and 0.075 mm (No. 200) openings, and shaken in a mechanical shaker for a standardized period. For soils in which fines coat the coarser grains, a wet-washing procedure is performed: the specimen is washed over the No. 200 sieve, the retained coarse fraction is oven-dried and re-sieved, and the washed fines are accounted for by mass difference. Sieve openings and calibration conform to ISO 3310 or equivalent national specifications.

Hydrometer Analysis (Fine Fraction)

The hydrometer method, standardized in ASTM D7928, BS 1377-2, and GB/T 50123, determines the distribution of particles finer than 0.075 mm based on Stokes' law, which relates the terminal settling velocity of a spherical particle to the square of its diameter. The dispersed suspension is placed in a sedimentation cylinder, and a calibrated hydrometer (type 151H or 152H) is read at prescribed time intervals, often including 2 minutes, 5 minutes, 15 minutes, 30 minutes, 1 hour, 2 hours, 4 hours, and 24 hours. Corrections for temperature, meniscus, and the density of the dispersing solution are applied before the equivalent particle diameters and percentages in suspension are computed.

Pipette Method

The pipette method, the reference procedure in ISO 11277 and widely used in soil science, withdraws small suspension aliquots at a fixed depth and predetermined settling times. Each aliquot is oven-dried and weighed, directly yielding the mass of particles finer than the calculated equivalent diameter. The pipette method offers excellent precision for the silt and clay ranges and is frequently used to validate faster instrumental techniques.

Laser Diffraction Method

Laser diffraction, performed per ISO 13320, measures the angular scattering pattern of a dispersed soil suspension and converts it to a volume-based size distribution using Mie or Fraunhofer optical models. Modern instruments cover roughly 0.01 μm to 3500 μm in a single measurement within minutes. Because the result is volume-based rather than mass-based, and the optical model requires assumptions about particle shape and refractive index, laser diffraction results should be correlated and validated against sieve or sedimentation data before being used for compliance purposes.

Combined (Composite) Analysis

Most natural soils span both the sieve and sedimentation ranges, so a combined analysis is required. The specimen is first washed on the No. 200 sieve; the retained fraction is dry-sieved, while the washed fines are collected, dispersed, and analyzed by hydrometer or pipette. The two data sets are merged, accounting for the mass split at 0.075 mm, to produce a single continuous gradation curve suitable for full classification.

Applications

Soil particle analysis supports decision-making across a wide range of engineering, agricultural, and environmental disciplines:

• Geotechnical engineering: foundation bearing capacity assessment, settlement and consolidation analysis, drainage and filter design, slope stability evaluation, and seismic liquefaction screening, where fines content below the No. 200 sieve is a key input.
• Transportation and earthworks: subgrade and embankment fill acceptance, base and subbase course gradation control, and verification of borrow-material suitability for road and rail construction.
• Construction materials: quality control of natural and crushed aggregates, blending calculations for engineered gradations, and sand suitability screening for mortar and concrete production.
• Agriculture and land management: USDA texture classification for irrigation planning, water-holding capacity estimation, nutrient management, and erodibility assessment of surface soils.
• Environmental site assessment: evaluation of contaminant adsorption and transport potential, landfill liner and cover characterization, and remediation treatability studies.
• Scientific research: sedimentological studies, Quaternary and paleoenvironmental reconstruction, and geotechnical research programs requiring high-resolution gradation data.

FAQ

What is soil particle analysis?

Soil particle analysis is a laboratory test that measures the distribution of particle sizes in a soil sample by mass, dividing it into gravel, sand, silt, and clay fractions. The resulting gradation curve and derived indices are used to classify the soil and predict its engineering and hydrological behavior.

Which standards govern soil particle analysis?

The most commonly cited standards are ASTM D6913 for sieve analysis, ASTM D7928 for hydrometer analysis, ASTM D2487 for USCS classification, ISO 11277 for mineral soil determination by sedimentation, ISO 13320 for laser diffraction, and BS 1377-2 or GB/T 50123 for regional geotechnical compliance. Projects may also reference AASHTO T 88 for highway applications.

How much soil sample is required for testing?

Fine-grained soils generally require 100 g to 500 g, while sands and gravels may require 1 kg to 10 kg or more depending on the maximum particle size, following the minimum-mass tables in ASTM D6913. Accredited third-party laboratories can advise on required quantities when samples are submitted.

When is sieve analysis used versus hydrometer analysis?

Sieve analysis is used for particles larger than 0.075 mm, while hydrometer or pipette sedimentation analysis is used for particles finer than 0.075 mm. Soils containing both coarse and fine fractions are tested by a combined procedure that merges both data sets into one gradation curve.

What do the uniformity coefficient and coefficient of curvature indicate?

The uniformity coefficient (Cu = D60/D10) describes the spread of particle sizes, and the coefficient of curvature (Cc = D30²/(D10 × D60)) describes the shape of the gradation curve between the coarse and fine fractions. Together they determine whether a soil is classified as well graded (SW/GW) or poorly graded (SP/GP) under ASTM D2487, which influences compaction density and drainage performance.

How long does a soil particle analysis take?

Sieve analysis is typically completed within one working day. Hydrometer analysis requires dispersion, pretreatment where needed, and sedimentation readings extending up to 24 hours or longer, so a full combined analysis is usually reported within three to five working days after sample receipt.

Why is a dispersing agent added during sedimentation testing?

Fine soil particles naturally attract each other and form aggregates (floccules) that settle faster than individual grains. A sodium hexametaphosphate solution neutralizes the surface charges so that each particle settles independently, which is essential for accurate silt and clay fraction determination.

Can organic or calcareous soils be analyzed?

Yes. Organic matter is removed by hydrogen peroxide oxidation and, where necessary, carbonates are dissolved with dilute acid before sedimentation, as detailed in ISO 11277. These pretreatments prevent aggregation and ensure the reported distribution reflects true primary particle sizes.