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Terramechanics and off-road mobility
Dry bulk density from field density and water content
Dry bulk density from the moist bulk density ρ and the gravimetric water content w as a fraction of dry mass, measured on the same specimen volume.
- Ref
- TER-31
- Rev
- v1
- Field
- Terramechanics and off-road mobility
- Result
- ρd [kg/m³]
The moist bulk density ρ includes the water mass, so the dry density is the mass of solids per unit total volume: ρd = ρ/(1 + w), with w as a fraction of dry mass. It is the quantity used to compare compaction between samples of different moisture, because it does not change when water is added to a given soil structure.
Dry bulk density from the moist bulk density ρ and the gravimetric water content w as a fraction of dry mass, measured on the same specimen volume.
Calculator
- Water per cubic metre of soil, ρ − ρd320 kg/m³
- Water content in percent20 %
- Dry density as a share of the moist density83.33 %
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 dry density is 1,600 kg/m³, from a moist density of 1,920 kg/m³ with 20 % water.
- Each cubic metre holds 320 kg of water, so a change in water content alone would shift the moist density without changing the soil structure.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (kg/m³) | Local step | Change in result |
|---|---|---|---|---|---|---|
| ρ Moist bulk density | 800 to 2,600 kg/m³ | 666.7 | 2,167 | 1,500 | +10% | +10 % |
| w Water content (fraction of dry mass) | 0 to 1 kg/kg | 960 | 1,920 | 960 | +10% | −1.64 % |
Sweep of Water content (fraction of dry mass)
| Water content (fraction of dry mass) (kg/kg) | Dry bulk density (kg/m³) |
|---|---|
| 0 | 1,920 |
| 0.125 | 1,707 |
| 0.25 | 1,536 |
| 0.375 | 1,396 |
| 0.5 | 1,280 |
| 0.625 | 1,182 |
| 0.75 | 1,097 |
| 0.875 | 1,024 |
| 1 | 960 |
Sweep one variable
Variables and how each one contributes
Dry bulk density
What the equation gives you. Everything else feeds it.
Moist bulk density
The moist bulk density, in kg/m³. Raises the dry density in direct proportion. Raising it by 10 % (to 2,112) moves the result by +10 %, from 1,600 to 1,760 kg/m³.
Water content (fraction of dry mass)
The water content, as a fraction of dry mass. A wetter specimen at the same moist density has less solid in each cubic metre. Raising it by 10 % (to 0.22) moves the result by −1.64 %, from 1,600 to 1,574 kg/m³.
Worked example with the values above
- Divisor: 1 + w = 1 + 0.2 = 1.2.
- ρd = ρ/(1 + w) = 1,920 / 1.2.
- ρd = 1,600 kg/m³.
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