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Terramechanics and off-road mobility
Gravimetric water content of a soil specimen
Mass of water per unit mass of dry solids in a soil specimen, from the wet and oven-dry masses (container excluded, same unit).
- Ref
- TER-30
- Rev
- v1
- Field
- Terramechanics and off-road mobility
- Result
- w [kg/kg]
Gravimetric water content is the mass of water divided by the mass of dry solids, w = (m_wet − m_dry)/m_dry. The denominator is the dry mass, not the wet mass, so w can exceed 1 for very wet or organic soils. The specimen is weighed, oven-dried until its mass stops changing, then weighed again, and the container mass is subtracted from both.
Mass of water per unit mass of dry solids in a soil specimen, from the wet and oven-dry masses (container excluded, same unit). Multiply by 100 for percent. It can exceed one for some soils.
Calculator
- Water content in percent20 %
- Mass of water, m_wet − m_dry0.12 kg
- Water content on a wet basis, (m_wet − m_dry)/m_wet16.67 %
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- The soil holds 20 % water by dry mass, 0.12 kg in a specimen of 0.6 kg dry solids.
- On a wet basis the same sample is 16.7 % water; the two bases differ more as the soil gets wetter, so always state which one a report uses.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (kg/kg) | Local step | Change in result |
|---|---|---|---|---|---|---|
| mdry Oven-dry mass | 0.05 to 0.6 kg | 0.2 | 13.4 | 13.2 | +10% | −54.5 % |
| mwet Wet mass | 0.6 to 5 kg | 0 | 7.333 | 7.333 | +10% | +60 % |
Sweep of Oven-dry mass
| Oven-dry mass (kg) | Water content (kg/kg) |
|---|---|
| 0.05 | 13.4 |
| 0.1187 | 5.063 |
| 0.1875 | 2.84 |
| 0.2562 | 1.81 |
| 0.325 | 1.215 |
| 0.3937 | 0.8286 |
| 0.4625 | 0.5568 |
| 0.5313 | 0.3553 |
| 0.6 | 0.2 |
Sweep one variable
Variables and how each one contributes
Water content
What the equation gives you. Everything else feeds it.
Wet mass
The wet mass of the specimen, in kg, with the container excluded. Raises the water content in direct proportion to the water mass. Raising it by 10 % (to 0.792) moves the result by +60 %, from 0.2 to 0.32 kg/kg.
Oven-dry mass
The oven-dry mass of the specimen, in kg. A smaller dry mass for the same wet mass means more water per unit solids, so the water content rises steeply. Raising it by 10 % (to 0.66) moves the result by −54.5 %, from 0.2 to 0.09091 kg/kg.
Worked example with the values above
- Water mass: m_wet − m_dry = 0.72 − 0.6 = 0.12 kg.
- Divide by the dry mass: 0.12 / 0.6.
- w = 0.2, or 20 %.
Assumptions and limits
- Representative, intact specimens of known mass or volume, with the same units for all masses and densities.
- Solid particles of a single particle density, and fully drained, non-swelling soil.
Common mistakes
- Including the container mass in the masses.
- Entering water content in percent where a fraction is needed.
- Using a particle density that does not represent the soil, which shifts the void ratio and the saturation.
Statistics and uncertainty
Soil index quantities depend on sampling and handling: water content is typically repeatable to 1 % to 2 % of the value, and void ratio to about 0.02, but sampling disturbance or a non-representative specimen can dominate the error.
Where it is used
- Recording the state of a test bed before a plate, shear or wheel test.
- Checking soil data for consistency before fitting strength models.
Maths you need
- Phase diagram: solids, water and air volumes and masses
- Water content: mass of water over mass of dry solids
- Particle density: mass of solids per unit volume of solids