Bulk density is a property of importance in the study of consolidation and other geotechnical aspects of marine and freshwater sediments. It is readily measured by weight/volume or nuclear methods. In addition, several other important geotechnical properties may be computed from the measurement of bulk density.

An empirical relation exists between bulk density (γ) and water content (ω) for water-saturated marine sediments. A polynomial expression has been calculated by regression techniques to fit measurements of these parameters on over 1,500 samples of cohesive marine sediments collected from the Atlantic and Pacific oceans and from other areas. The equation is γ = 2.232 – 0.0129ω + 0.000064ω2.

The range of water contents covered by the measurements is from 30 to 240% by dry weight, and the range of bulk densities from 1.25 g/cm3 to 1.95 g/cm3. Using this equation, the water content can be calculated, with a standard deviation of 2.3, from a measured bulk density.

It also is possible to calculate specific gravity of solids (G) and void ratio (e) directly from water content and bulk density for water-saturated sediments. The relationships are:
G=100γ(100+ωωγ)ρt

and

e=ωγ(100+ωωγ)ρt2

where ρt is the density of pore water at temperature t. The use of these relations may save appreciable time by eliminating or reducing the quantity of laboratory measurements made on sediment cores by geologists and soils engineers. The study of consolidation by the use of sedimentation-compression (e log p′) diagrams may also be greatly facilitated when using nuclear methods of measuring bulk density and deriving water content from these measurements.

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