To determine the specific gravity of soil, weigh a pycnometer empty, with dry soil, with soil and water, and with water only. The four masses give the mass of dry solids and the mass of an equal volume of water. The result, Gs, is dimensionless.
The purpose of the test is to determine the specific gravity of the soil fraction covered by the selected method. The value is needed in calculations of soil properties such as void ratio and degree of saturation. It is also used in hydrometer and compaction calculations.
What Is Specific Gravity of Soil Solids?
Specific gravity, Gs, is the ratio of the density of the soil solids to the density of water at a stated reference temperature. It refers to the solid grains, not the soil mass including air and water in the voids.
Therefore, specific gravity has no unit. Particle density does have a unit and may be found from ρs = Gsρw. At 20°C, ASTM D854-23 uses the density of water at 20°C when calculating soil-solids density.
This test is not an aggregate absorption or bulk-specific-gravity test. Soil solids passing the selected sieve and aggregate retained on that sieve require different methods and definitions.
Apparatus
- Clean, calibrated pycnometer with its matching cap or stopper
- Balance suitable for the required precision
- Drying oven and desiccator when the oven-dry method is specified
- Distilled or de-aired water
- Thermometer and constant-temperature or insulated setup
- Vacuum system, water aspirator, or approved heating arrangement for removing air
- Funnel, wash bottle, and tools needed by the selected standard
The original procedure used water, alcohol, and ether for cleaning. Follow the laboratory procedure and safety data sheets. Use a volatile solvent only when it is approved for the equipment and the work is properly ventilated.
Procedure for Specific Gravity of Soil by Pycnometer
- Clean and dry the pycnometer. Fit its cap or stopper exactly as it will be used during the test.
- Weigh the empty pycnometer and record the mass as W1.
- Prepare a representative soil specimen by the method required in the project specification. The original 1 L pycnometer procedure used about 200 g of oven-dried soil passing the 4.75 mm sieve; current standards may specify a different mass and preparation.
- Cool an oven-dried specimen in a desiccator. Transfer it without loss, fit the cap, and record the pycnometer plus dry soil as W2.
- Add enough distilled or de-aired water to cover the soil. Wet the complete specimen and allow the required soaking time.
- Remove entrapped air by the vacuum, heating, or combined procedure permitted by the selected standard. Agitate as required until air removal is complete.
- Add thermally equilibrated water to the calibration mark or fill position. Avoid splashing soil above the water line.
- Bring the pycnometer and contents to a uniform temperature. Adjust the water level, dry the outside, and record the pycnometer plus soil and water as W3.
- Empty and thoroughly clean the pycnometer. Fill it with water only to the same mark at the same temperature.
- Dry the outside, fit the same cap or stopper, and record the pycnometer plus water as W4.
Specific Gravity Formula
Equivalent form: Gs,T = Ms / [Ms + W4 – W3]
- W1 = mass of empty pycnometer and cap, g
- W2 = mass of pycnometer, cap, and dry soil, g
- W3 = mass of pycnometer, soil, and water at test temperature T, g
- W4 = mass of pycnometer and water only at the same temperature, g
- Ms = W2 – W1 = mass of oven-dry soil solids, g
- (W2 – W1) – (W3 – W4) = mass of water displaced by the soil solids, g
If a reference-temperature correction is required, use the factor from the governing method:
where K = ρw,T / ρw,ref
Observation Table
| Observation | Trial 1 | Trial 2 | Trial 3 |
|---|---|---|---|
| Test temperature, °C | |||
| Empty pycnometer, W1 (g) | |||
| Pycnometer + dry soil, W2 (g) | |||
| Pycnometer + soil + water, W3 (g) | |||
| Pycnometer + water, W4 (g) | |||
| Dry soil mass, W2 – W1 (g) | |||
| Displaced-water mass (g) | |||
| Specific gravity at test temperature | |||
| Corrected/reported specific gravity |
Soil Specific Gravity Calculator
Enter all four masses in the same unit. Grams are shown because they are normally used in the laboratory, but the ratio is unchanged if one consistent mass unit is used. Leave K at 1.0000 when no separate temperature correction is being applied.
Dry soil mass: 200.00 g
Displaced-water mass: 75.00 g
Gs at test temperature: 2.6667
Corrected Gs: 2.6667
Worked Example
Suppose W1 = 650.00 g, W2 = 850.00 g, W3 = 1540.00 g, and W4 = 1415.00 g. The dry soil mass is:
W2 – W1 = 850.00 – 650.00 = 200.00 g
The soil replaces 75.00 g of water:
(W2 – W1) – (W3 – W4) = 200.00 – (1540.00 – 1415.00) = 75.00 g
Therefore:
Gs = 200.00 / 75.00 = 2.6667
The unit grams cancels, so the final specific gravity is dimensionless. If the governing procedure requires K = 0.9988, the corrected value would be 0.9988 × 2.6667 = 2.6635.
Common Errors and Their Effect
| Error | Likely effect | Control |
|---|---|---|
| Entrapped air remains in the slurry | Usually makes Gs too low | Use de-aired water and the complete specified de-airing cycle with agitation |
| W3 and W4 are measured at different temperatures | Changes the water mass at the fixed volume | Reach thermal equilibrium and record temperature for each mass |
| Water is not at the same mark | Changes W3 or W4 directly | Read the meniscus at eye level and use the same stopper position |
| Soil is lost during transfer or boiling | Breaks the four-mass balance | Use a funnel, rinse particles into the bottle, and prevent boil-over |
| Outside of bottle is wet or touched while warm | Adds mass or disturbs temperature | Dry the exterior and handle near the rim |
| Soluble, floating, or highly organic material is treated as ordinary mineral soil | Water displacement may not represent the intended solids | Use the alternate method or qualified reporting required by the specification |
How to Interpret the Result
Many mineral soils fall roughly between 2.6 and 2.9. Quartz-rich sand is commonly near 2.65. However, this is only a reasonableness check, not an acceptance range. Organic matter can reduce the value, while iron-rich or heavy minerals can raise it.
A result outside the expected range may be real, or it may indicate trapped air, incomplete drying, temperature mismatch, a fill-mark error, or soil loss. Review the sample description and the raw masses before repeating the test.
Specific gravity is often used with soil moisture content and dry density. It is also used to plot the zero-air-voids line for a Standard Proctor compaction test.
Pycnometer and Density Bottle Are Not Interchangeable Procedures
| Point | Larger traditional pycnometer | Small density bottle or volumetric pycnometer |
|---|---|---|
| Typical teaching use | Larger oven-dry sample, often used for sandy or coarser fine fractions | Small sample, commonly used for fine soil particles |
| Mass calculation | Often recorded as W1, W2, W3, and W4 | May use the same mass balance with different symbols |
| Precision and calibration | Depends on the named procedure and apparatus | Smaller mass makes balance resolution and calibration especially important |
| Rule | Use the apparatus, sample preparation, air-removal method, temperature control, and reporting rules from one governing standard. | |
What to Report
- Sample identification and the tested particle-size fraction
- Standard designation, edition, and method used
- Moist or oven-dry specimen preparation, as applicable
- Pycnometer identification and calibration basis
- W1, W2, W3, W4, test temperature, and any correction factor
- Individual determinations, repeatability check, and reported average
- Any excluded fraction, unusual material, alternate fluid, or deviation
References
- ASTM D854-23, Standard Test Methods for Specific Gravity of Soil Solids by the Water Displacement Method.
- ISO 17892-3:2015, Geotechnical investigation and testing – Laboratory testing of soil – Part 3: Determination of particle density.
- BIS catalogue record for IS 2720 (Part 3/Sec 1):1980, Fine Grained Soils.
- FHWA NHI-05-037, Geotechnical Aspects of Pavements, Chapter 5.
- AASHTO re:source, Pycnometers, Calibrations, and Insulated Containers.