Characterizing Density

Measuring density is a simple method of measuring the mass, measuring the volume, then calculating the density as mass divided by volume.

Dry density, wet density, and grain density can all have geotechnical utility. All three are calculated by dividing mass by volume, but use different techniques for determining which mass and which volume to use in the calculation. Dry density is determined by drying the sample before weighing. Wet density assumes that all available pore space is filled with water, so is calculated by measuring the porosity and dry mass, then adding the mass of water that will fill in the pore volume:
mwater = Vpore x ρwater

For grain density, only the solid mass is included and pore space is neglected, so dry mass is divided by the volume of only the solid material. This is calculated by subtracting the pore volume from the total volume:
Vsolid = Vtotal − Vpore

The total volume of the material is determined by either dumping a loose material (such as gravels, soils, and aggregates) into a pycnometer filled with air or water, or by preparing an intact rock to a right cylinder of measurable dimensions.

Density can also be measured in-situ through digging a test pit, weighing the material removed, and filling the with a water, sand, or another material to measure the volume, or through driving a cylinder of fixed volume into soil to remove material for weighing.

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