Unit Weight of Soil

Laboratory Testing

The unit weight of soil (γ) is the weight of soil per unit volume. It represents how heavy a given volume of soil is (links the weight of soil to its volume) and is an essential parameter in geotechnical engineering for analyzing stresses, stability, and settlement in soil structures.

γ = W / V 

Where:

W = total weight of soil (including solids and water)
V = total volume of soil (solids + water + air)

Types of Soil Unit Weight

Depending on the moisture condition, soil unit weight can be expressed as:

Bulk (or Total) Unit Weight (γ): Weight of soil (solids + water) per total volume.
Dry Unit Weight (γ_d): Weight of soil solids per total volume.
Saturated Unit Weight (γ_sat): When all voids are filled with water.
Submerged (or Buoyant) Unit Weight (γ′): Effective weight of soil below the water table (saturated soil minus buoyant force).

Type	Symbol	Definition	Formula	Typical Range
Bulk (Total) Unit Weight	γ	Weight of soil (solids + water) per total volume	γ = (W_s + W_w) / V	15–22 kN/m³
Dry Unit Weight	γ_d	Weight of solids per total volume	γ_d = W_s / V	13–20 kN/m³
Saturated Unit Weight	γ_sat	Soil completely saturated with water	γ_sat = (W_s + W_w) / V (all voids filled)	18–22 kN/m³
Submerged Unit Weight	γ′	Effective weight of saturated soil below water table	γ′ = γ_sat − γ_w	8–12 kN/m³

Where:

W_s = weight of soil solids
W_w = weight of water
V = total volume
γ_w = unit weight of water (≈ 9.81 kN/m³)

The unit weight of soil (γ) is usually determined in the laboratory by preparing a representative soil sample and measuring its weight and volume.

In practice, bulk unit weight is often determined in the field using the sand replacement or nuclear density gauge, while the dry and saturated unit weights are usually derived from laboratory tests combined with moisture content measurements.