Unit Weight of Soil
The unit weight of soil is the ratio of total weight of the soil to the volume of the soil. The unit weight is also called ‘weight density’, of soil equals to its weight per cubic meter and is typically expressed as kN/m3. As we know that the unit weight of soil is generally important for geotechnical design or infrastructure as well as determining earthworks volumes. To find out the unit weight of soil in the field we need to do some experiments i.e sand cone test and nuclear densitometer.
Bulk Unit Weight
The bulk unit weight of soil is the ratio of the absolute or moist weight of soil mass to its volume V.
\[\gamma = \frac{W}{V}\]
Dry Unit Weight
Dry unit weight is the ratio of the solid particles to the total volume.
\[\gamma_{d} = \frac{W}{Vd}\]
Unit Weight of Solids
The unit weight of the solids is the ratio of soil solids to the volume of solids.
\[\gamma_{s} = \frac{Wd}{Vs}\]
Thus, when the dry weight is calculated which is associated with the standard or original volume is called the dry unit weight of the solids and when it is calculated which is associated with the volume of solids then it is called the unit weight of the soil solids. Considering the volume of the solids is not revised, the unit weight of the solid is constant for the given sample soil, at the point where dry unit weight is not constant, in reality, it depends on the volume of the solids.
Saturated Unit Weight
Saturated unit weight is similar to the bulk unit weight when the voids are totally filled with water. It is the ratio of the total saturated soil sample to its total volume.
Submerged Unit Weight
The submerged unit weight is the ratio of the submerged soil solids to the total volume of the soil mass.
\[\gamma^{\prime}=\frac{\left(W_{d}\right)_{s u b}}{V}\]
When the soil solids are submerged in water then the weight of the soil solids is reduced by its tendency to float or buoyancy. The submerged weight is equal to the weight of the soil solids in air minus the weight of water is displaced by soil solids.
\[\gamma^{\prime}=\gamma_{sat}-\gamma_{w}\]
How to find dry unit weight of soil from spt?
| SPT penetrations N value | g(lb/ft³) |
| 0-4 | 70-110 |
| 4-10 | 90-115 |
| 10-30 | 110-130 |
| 30-50 | 110-140 |
| >50 | 130-150 |
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