Moisture content measurement is mainly used to perform weight-volume calculations in soils. Also, moisture content is a measure of the shrink-swell and strength characteristics of cohesive soils. In geotechnical work, the result is based on the dry mass of the soil solids, not the total wet mass.
Test applied: ASTM D2216-19, Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass. ASTM lists this as the active edition. The project specification and the purchased standard remain the controlling documents.
What soil moisture content means
The laboratory water content, w, is the mass of water removed from the specimen divided by the dry mass of the soil solids. It is expressed as a percentage. Because the denominator is dry soil mass, a very wet or highly organic soil can have a water content greater than 100%.
w (%) = [(Mwet+c – Mdry+c) / (Mdry+c – Mc)] × 100
Mass of water = Mwet+c – Mdry+c; dry soil-solids mass = Mdry+c – Mc.
| Symbol | Measurement | Check before calculating |
|---|---|---|
| Mc | Mass of the clean, dry container and its lid | Use the same container and lid in every reading. |
| Mwet+c | Mass of the container, lid, and moist specimen before drying | This must be the largest of the three masses. |
| Mdry+c | Mass of the container, lid, and dry specimen after controlled cooling | It must be less than the moist total and greater than the empty container. |
All three readings must use the same mass unit. ASTM D2216 uses SI units. The mass order must be Mwet+c > Mdry+c > Mc. If that order is not satisfied, do not report a result until the entries and laboratory steps have been checked.
Equipment and materials
- Representative disturbed specimen of moist soil or rock
- Clean, corrosion-resistant, oven-safe container with a close-fitting lid
- Calibrated balance with sensitivity suitable for the selected method and specimen mass
- Thermostatically controlled, vented drying oven
- Desiccator or other laboratory-approved controlled cooling arrangement
- Tongs or suitable heat-resistant handling tools and required personal protective equipment
The standard drying condition is 110 ± 5°C. However, that temperature is not suitable for every material. Check the material exceptions below before heating a specimen.
Laboratory sequence
- Identify the specimen and container. Record the sample location or depth when relevant, the container number, and the selected ASTM method.
- Measure the container mass. Weigh the clean, dry container with its lid and record Mc.
- Protect and weigh the moist specimen. Place the representative material in the container, close it, and record Mwet+c promptly so evaporation does not change the reading.
- Dry under the specified condition. Remove or position the lid as required by the laboratory method. Dry at 110 ± 5°C unless the material or governing instruction requires a qualified alternative.
- Use constant mass as the endpoint. An overnight period is common, but time alone does not prove that drying is complete. Continue the dry-cool-weigh cycle until successive readings meet the current standard or laboratory constant-mass criterion.
- Cool without moisture regain. Replace the lid, move the hot container with suitable tools, and cool it in a desiccator or the laboratory’s approved controlled environment. Weigh only after controlled cooling and do not leave the dry specimen exposed to humid air.
- Record the dry total. Weigh the same container, lid, and dry specimen as Mdry+c. Confirm the mass order, calculate the result, and report the required precision and any departure from the standard condition.
Laboratory safety boundary: Hot ovens and containers can cause burns. More importantly, do not oven-dry known or suspected contaminated soil until a competent person has assessed chemical and biological hazards and approved the controls. Some contaminants can react or release hazardous gas when heated. A generic procedure cannot select ventilation, respiratory protection, decontamination, waste handling, or disposal measures for an unknown material.
Worked soil moisture content calculation
Suppose the empty container and lid weigh 32.46 g. The container, lid, and moist soil weigh 86.32 g. After drying to constant mass and controlled cooling, the container, lid, and dry soil weigh 77.18 g.
Mass of water = 86.32 – 77.18 = 9.14 g.
Dry soil-solids mass = 77.18 – 32.46 = 44.72 g.
w = (9.14 / 44.72) × 100 = 20.438…%.
For this example, report 20% under Method A or 20.4% under Method B. Do not choose the reporting method after seeing the result; use the method required by the requesting authority or governing test.
Three-mass soil moisture calculator
Method, sample size, balance, and reporting
ASTM D2216 provides two methods. Method A reports water content to the nearest 1% and is the default/referee method when another method is not specified. Method B reports to the nearest 0.1%. The selected method also controls the required specimen mass and balance sensitivity.
Do not rely on one universal sample mass. A specimen must be large enough to represent the largest particles present, and the exact minimum depends on the method and maximum particle size. Follow the current standard and project specification. ASTM’s public scope also notes that when water content will be used in another mass or density relationship, a specimen up to 200 g is measured with a balance accurate to 0.01 g.
Record the sample and container identification, all three masses, selected method, drying condition, constant-mass check, calculated water content, reporting precision, and any qualified result or departure. For example, moisture content is used to convert wet unit weight to dry unit weight in the Standard Proctor compaction test.
Materials that need special treatment
| Material or condition | Why the normal result can be biased | Laboratory and reporting boundary |
|---|---|---|
| Gypsum or other hydrated compounds | Heating can remove water that is part of the mineral structure, increasing the apparent water content. | Around 60°C or room-temperature desiccation may reduce dehydration when allowed. Report the actual condition and recognize that the result can differ from the 110 ± 5°C value. |
| Highly or fibrous organic soil; peat | The standard temperature can decompose or oxidize part of the solid material. | Use the method required by the requesting authority. ASTM D2974 is an alternate method for peat. Qualify the temperature and method in the report. |
| Marine sediment or material with substantial soluble solids | Dissolved salts can remain after water evaporates and become part of the measured dry mass. | Use special treatment or a qualified definition of water content. ASTM identifies D4542 for marine-sediment information. |
| Soil with cement or other extraneous matter | Reactions or non-soil constituents can make the normal dry-solids assumption invalid. | Follow project-specific treatment and state the definition used for the reported water content. |
| Known or suspected contamination | Heating can cause violent reaction or release hazardous vapor or gas. | Do not proceed without an approved hazard assessment, laboratory safety controls, and waste plan. The oven method may be unsuitable. |
Common errors and their effect
| Error | Likely effect | Check or control |
|---|---|---|
| Moisture lost before the wet mass is recorded | Water content is reported too low. | Seal the sample and weigh promptly. |
| Drying stopped by time instead of constant mass | Residual water makes the dry mass too high and water content too low. | Repeat the dry-cool-weigh cycle to the approved constant-mass criterion. |
| Dry specimen absorbs moisture while cooling | Dry mass increases and calculated water content decreases. | Replace the lid, cool under controlled dry conditions, and weigh promptly. |
| Different lid/container combination | The tare changes, so the derived water or dry-solids mass is wrong. | Keep the labeled container and lid together for every reading. |
| Wet mass ≤ dry mass, or dry mass ≤ container mass | The inputs are physically inconsistent and the result is invalid. | Check transposed readings, units, container identity, and data entry. |
| Wet mass used as the denominator | The reported percentage is not ASTM geotechnical water content. | Divide by dry soil-solids mass, Mdry+c – Mc. |
| Standard temperature applied to sensitive material | Organic decomposition or loss of hydrated water can make the result too high. | Identify the material first and document any qualified drying condition. |
| Specimen too small or unrepresentative | One particle or local wet pocket can dominate the result. | Use the minimum mass required for the method and maximum particle size. |
References
- ASTM D2216-19 active standard record and public scope, ASTM International.
- National Engineering Handbook, Part 645, Appendix B Worksheets, USDA Natural Resources Conservation Service.
- Laboratories: Hazard Recognition and Solutions, U.S. Occupational Safety and Health Administration.