Specific weight

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]

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, which is 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

${\displaystyle \gamma }$ is the moist unit weight of the material

${\displaystyle \gamma _{w}}$ is the unit weight of water

w is the moisture content of the material

Gs is the specific gravity of the solid

e is the void ratio

• Dry unit weight, which is 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

${\displaystyle \gamma }$ is the moist unit weight of the material

${\displaystyle \gamma _{d}}$ is the dry unit weight of the material

${\displaystyle \gamma _{w}}$ is the unit weight of water

w is the moisture content of the material

Gs is the specific gravity of the solid

e is the void ratio

• Saturated unit weight, which is 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

${\displaystyle \gamma _{s}}$ is the saturated unit weight of the material

${\displaystyle \gamma _{w}}$ is the unit weight of water

w is the moisture content of the material

Gs is the specific gravity of the solid

e is the void ratio[3]

• Submerged unit weight, which is defined as 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

${\displaystyle \gamma ^{'}}$ is the submerged unit weight of the material

${\displaystyle \gamma _{s}}$ is the saturated unit weight of the material

${\displaystyle \gamma _{w}}$ is the unit weight of water

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.