Geotechnical Engineering

Unit Weight of soil

The ratio of the weight of a material to its volume is its unit weight, sometimes termed specific weight or
weight density. The unit weight of water, Yw, is 9.81 kN/m3 in the SI system and 62.4 lb/ft3 in the English
system. The unit weight of solids, Y s, varies with the mineralogy of the soil particles but is commonly in
the range of 26.0 to 27.0 kN/m3 or 165 to 172 lb/ft3. The total unit weight of a soil (the solids-water-air
system), denoted g, is the ratio of the total weight to the total volume occupied:

The saturated unit weight, denoted g sat, is the total unit weight that would be obtained if the air voids were filled with an equal volume of water (S = 100% and Vw = V). The dry unit weight, denoted g d, is often termed the dry density and has particular importance in field control of soil compaction. It is the ratio of the weight of solids to the total volume:

Note that the dry unit weight matches the weight of a single component—the solids—with the entire volume of solids, water, and air. It does not represent the unit weight of any component or consistent set of components, but rather provides a measure of how much solid material by weight is in the total volume of a container, such as an earthmover or a compaction mold. The buoyant unit weight or effective unit weight, Y’, is equal to the saturated unit weight minus the unit weight of water, Yw:

The buoyant unit weight is sometimes used to directly calculate vertical effective stresses below the water table instead of calculating total stresses and subtracting pore pressures.

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