Standard Test Method for Particle-Size Analysis of Soils

Monday July-07 2025  12:02:20

Standard test method for particle-size analysis of soils mainly include screening method and sedimentation method. The screening method is suitable for sand and gravel soils with a particle size greater than 0.075 mm, and a set of standard ASTM soil sieve s are used to classify particles of different particle sizes through mechanical vibration; while the sedimentation method is used for silt and clay with a particle size less than 0.075 mm. 

In actual analysis, a combination of the two methods is often used to obtain a complete particle size distribution curve. For high-precision requirements, a laser particle size analyzer can also be used for rapid analysis.Standard test methods for particle-size analysis of soil is mainly used to determine the distribution of particles of different particle sizes in the soil. It is an important basic test in soil classification, engineering property evaluation and agronomic research.

Common standard methods for particle-size analysis of soils

Method category	Test name	Applicable scope	Mainly based on standards
Sieving method (dry sieving/wet sieving)	Soil sieving test	Coarse-grained soil (sand, gravel)	GB/T 50123-2019 (China) ASTM D6913 (USA)
Sedimentation method	Hydrometer method	Fine-grained soil (silt, clay)	GB/T 50123-2019
Laser particle size analysis method	Laser diffraction method	Full particle size, high precision	ISO 13320 (International) GB/T 19077 (China)

Introduction to main test methods

1. Sieving method

Soil particles are mechanically sieved using ASTM soil sieves of different apertures. The mass of soil particles on each sieve layer is weighed to determine the distribution ratio of particles of different particle sizes. It is divided into dry sieving and wet sieving: dry sieving is suitable for dry particles (such as sand and gravel) that are not sticky or agglomerated; wet sieving is used for samples with agglomeration or fine particles, which need to be dispersed with water.

Scope of application: Mainly used for analyzing coarse-grained soil with a particle size greater than 0.075mm, such as sand, gravel, etc.

Main instruments: standard ASTM soil sieve (aperture is usually 75μm-100mm), vibrating screen machine, etc.

Steps: Pass the air-dried soil sample through a 2mm ASTM soil sieve to remove organic matter, impurities, etc. Select a standard ASTM soil sieve (common aperture: 2mm, 0.85mm, 0.425mm, 0.075mm), weigh a certain mass of soil sample (usually 300-1000g), pour it into the top sieve (with the largest aperture), and put it into the vibrating screen machine for 10-15 minutes. Weigh the mass of each sieve layer and the soil sample at the bottom of the sieve in turn, and calculate the percentage of particle content in each particle size range.

Note: The soil sample needs to be fully air-dried to avoid caking; during the screening process, ensure that the ASTM soil sieve is sealed to prevent particles from being missed.

2. Sedimentation method (applicable to fine-grained soil)

Based on Stokes' law, that is, the sedimentation rate of particles in liquid is proportional to the square of the particle size. The particle size is calculated by measuring the sedimentation time and sedimentation distance of particles in water.

Scope of application: Applicable to fine-grained soil with a particle size less than 0.075mm, such as silt, clay, etc.

Main instruments: densitometer (such as hydrometer), measuring cylinder (1000mL), stirrer, constant temperature water tank, etc.

Test steps (taking the densitometer method as an example): Weigh 100g of air-dried soil sample, pour it into a beaker, add appropriate amount of dispersant (such as sodium hexametaphosphate) and water, and boil for 30 minutes to disperse the soil particles. Place the densitometer in the measuring cylinder at a predetermined time (such as 1, 2, 5, 15, 30, 60, 1440 minutes) and read the density value of the soil suspension. Calculate the content of particles of different particle sizes based on the densitometer reading and sedimentation time.

Note: The water temperature should be kept constant during the test (the error should not exceed ±1℃); the amount of dispersant should be adjusted according to the properties of the soil sample to avoid over-dispersion or insufficient dispersion.

3. Laser diffraction method (modern rapid analysis method)

When the laser beam is irradiated on the soil particles, diffraction occurs, and the angle of the diffracted light is related to the particle size. By measuring the intensity distribution of the diffracted light, the particle size distribution is calculated using the Mie Scattering Theory.

Scope of application: It can be used to analyze soil particles with a wide range of particle sizes (usually 0.01μm-2000μm), including coarse-grained soil and fine-grained soil.

Main instruments: laser particle size analyzer, sample preparation system (such as ultrasonic disperser, stirrer), etc.

Test steps: Weigh a small amount of soil sample (about 0.1-1g), add dispersant and water, and ultrasonically disperse for 10-15 minutes to prepare a uniform soil suspension. Inject the soil suspension into the sample pool of the laser particle size analyzer and start the instrument for measurement. The instrument automatically analyzes the diffraction data to generate a particle size distribution curve and statistical parameters (such as average particle size, sorting coefficient, etc.).

Note: The soil sample needs to be fully dispersed to avoid particle agglomeration; the laser particle size analyzer needs to be calibrated regularly to ensure measurement accuracy.

Method selection and comparison

Method	Advantages	Disadvantages	Applicable scenarios
Screening method	Simple operation, intuitive results, suitable for accurate analysis of coarse-grained soil	Fine-grained soil (<0.075mm) cannot be analyzed, and the screening process takes a long time	Engineering detection of coarse-grained soil such as sand and gravel.
Sedimentation method	Applicable to fine-grained soil analysis, can provide detailed data on particle size distribution	Long test cycle, greatly affected by factors such as temperature and dispersant	Geological survey of fine-grained soil such as silt and clay
Laser diffraction method	Fast measurement speed, wide particle size range, high degree of automation, good repeatability	Strict requirements for soil sample dispersion, unable to distinguish particle shape	Scientific research, environmental monitoring and rapid engineering detection

How to choose and combine them?

Typically, for soil samples that contain both coarse and fine particles, a combination of sieving and sedimentation is used:

Sieving separates the coarse particles (sand and gravel).

Sedimentation then analyzes the fine soil that passes through the finest ASTM soil sieve (such as a 0.075 mm sieve) to determine the content of silt and clay.

Finally, all the data is combined and plotted into a soil particle size cumulative distribution curve, which visually shows the proportion of particles of different sizes in the soil.

Basic basis for selecting methods

Soil type	Particle size range	Recommended method	Description
Coarse-grained soil (sand, gravel)	>0.075mm	Sieving method (dry/wet sieving)	Intuitive, classic, suitable for soil with distinct particles
Fine-grained soil (silt, clay)	<0.075mm	Densimeter method (sedimentation method)	Determine particle size by particle settling velocity
Mixed soil with full particle size range	0.002mm～60mm	Sieving method + sedimentation method combined	Screening coarse particles, and measuring fine particles with densimeter
Requires high-precision scientific research analysis	0.01μm～3mm	Laser particle size analysis method	Fast, high resolution

In actual applications, it is necessary to select appropriate methods or combinations based on sample characteristics (such as clay content, organic matter content), analysis purpose (such as engineering grading, soil classification) and equipment conditions, and optimize operating parameters (such as dispersant concentration, ultrasonic time) through preliminary tests when necessary to reduce errors.