Bulk Unit Weight Explained

The unit weight of soil is a fundamental property in geotechnical engineering. It represents the weight of the soil mass per unit of its volume. There are different types of unit weight depending on whether water is present in the soil pores.

Understanding Bulk Unit Weight

The bulk unit weight, often denoted by $\gamma$ or $\gamma_b$, is defined as the total weight of a soil mass divided by its total volume. The total weight includes the weight of both soil solids and any water present in the pores. The total volume includes the volume of solids, water, and air voids.

Mathematically, the bulk unit weight is expressed as:

\(\gamma_b = \frac{\text{Total Weight (W)}}{\text{Total Volume (V)}}\)

The provided correct answer text, "The total weight of a soil mass per unit of its total volume", is the definition of bulk unit weight.

Bulk Unit Weight of Dry Soil

When soil is completely dry, there is no water present in the pores. In this specific case, the total weight of the soil mass is equal to the weight of the soil solids ($W_s$). The total volume ($V$) remains the same (volume of solids + volume of air voids). The bulk unit weight of dry soil is also known as the dry unit weight, denoted by $\gamma_d$.

For dry soil:

• Total Weight ($W$) = Weight of Solids ($W_s$)
• Total Volume ($V$) = Volume of Solids ($V_s$) + Volume of Air Voids ($V_a$)

Thus, the bulk unit weight of dry soil (which is the dry unit weight) is:

\(\gamma_d = \frac{\text{Weight of Solids (W_s)}}{\text{Total Volume (V)}}\)

While the definition "The total weight of a soil mass per unit of its total volume" (Option 1 text) is the general definition of bulk unit weight, when applied to dry soil, the 'Total Weight' is equal to the 'Weight of Solids'. Therefore, this definition implicitly covers the case of dry soil, although Option 3 ("Weight of solids per unit of its total volume of the soil mass") is a more specific definition for the dry unit weight itself.

Analyzing the Options

1. The total weight of a soil mass per unit of its total volume: This is the definition of bulk unit weight ($\gamma_b$). For dry soil, the total weight equals the weight of solids, so this definition applies.
2. Mass of solids per unit of total volume of the soil mass: This is the definition of dry density ($\rho_d$), which uses mass instead of weight.
3. Weight of solids per unit of its total volume of the soil mass: This is the definition of dry unit weight ($\gamma_d$), which is specifically the bulk unit weight when the soil is dry.
4. Unit weight of soil solids per unit volume of solids: This is the definition of the unit weight of solids ($\gamma_s$), which only considers the volume of the solid particles themselves, not the total volume of the soil mass.

Based on the provided correct answer text, the solution focuses on the general definition of bulk unit weight as applicable to dry soil, where total weight becomes weight of solids.