Fluid mechanics

In fluid mechanics, specific weight represents the force exerted by gravity on a unit volume of a fluid. For this reason, units are expressed as force per unit volume (e.g., N/m³ or lbf/ft³). Specific weight can be used as a characteristic property of a fluid.^[2]

Soil mechanics

Specific weight is often used as a property of soil to solve earthwork problems.

In soil mechanics, specific weight may refer to:

Moist unit weight

The unit weight of a soil when void spaces of the soil contain both water and air.

$${\displaystyle \gamma ={\frac {(1+w)G_{s}\gamma _{w}}{1+e}}}$$

where

• γ is the moist unit weight of the material
• γ_w is the unit weight of water
• w is the moisture content of the material
• G_s is the specific gravity of the solid
• e is the void ratio

Dry unit weight

The unit weight of a soil when all void spaces of the soil are completely filled with air, with no water. The formula for dry unit weight is:

$${\displaystyle \gamma _{d}={\frac {G_{s}\gamma _{w}}{1+e}}={\frac {\gamma }{1+w}}}$$

where

• γ is the moist unit weight of the material
• γ_d is the dry unit weight of the material
• γ_w is the unit weight of water
• w is the moisture content of the material
• G_s is the specific gravity of the solid
• e is the void ratio

Saturated unit weight

The unit weight of a soil when all void spaces of the soil are completely filled with water, with no air. The formula for saturated unit weight is:

$${\displaystyle \gamma _{s}={\frac {(G_{s}+e)\gamma _{w}}{1+e}}}$$

where

• γ_s is the saturated unit weight of the material
• γ_w is the unit weight of water
• G_s is the specific gravity of the solid
• e is the void ratio^[3]

Submerged unit weight

The difference between the saturated unit weight and the unit weight of water.^[4] It is often used in the calculation of the effective stress in a soil. The formula for submerged unit weight is:

$${\displaystyle \gamma '=\gamma _{s}-\gamma _{w}}$$

where

• γ′ is the submerged unit weight of the material
• γ_s is the saturated unit weight of the material
• γ_w is the unit weight of water

Civil and mechanical engineering

Specific weight can be used in civil engineering and mechanical engineering to determine the weight of a structure designed to carry certain loads while remaining intact and remaining within limits regarding deformation.