Unit weight, also known as bulk density, is one of the fundamental engineering properties of soil. It is defined as the weight of soil per unit volume and is used to evaluate the condition and compaction of soil in the field.

The bulk or wet unit weight of soil is given by:

$$\gamma_t = \frac{W}{V}$$

where,

• $W$ = total weight of the soil sample,
• $V$ = total volume of the soil sample,
• $\gamma_t$ = bulk or wet unit weight of the soil.

The dry unit weight of soil is related to the bulk unit weight and water content by:

$$\gamma_d = \frac{\gamma_t}{1+w}$$

where,

• $\gamma_d$ = dry unit weight of the soil,
• $\gamma_t$ = bulk or wet unit weight of the soil,
• $w$ = water content of the soil expressed in decimal form.

Importance of Unit Weight

Soil consists of solid particles, water, and air occupying the void spaces. The amount of compaction and the proportion of these phases influence the unit weight of the soil.

The determination of unit weight is important because it helps in:

• Assessing the degree of soil compaction.
• Evaluating the bearing capacity of soil.
• Estimating the stability of earth structures.
• Determining the suitability of soil for construction projects.
• Calculating stresses and settlements in geotechnical engineering.

Accurate determination of field unit weight is essential for quality control during the construction of roads, embankments, foundations, and earth dams.

Principle of the Core Cutter Method

The Core Cutter Method is a standard field method used to determine the in-situ unit weight of cohesive soils.

In this method, a cylindrical steel core cutter of known dimensions is driven into the ground. The cutter is carefully removed along with the soil contained within it. The mass of the soil sample and the known volume of the cutter are then used to determine the unit weight of the soil.

The volume of the core cutter is given by:

$$V = \frac{\pi D^2 H}{4}$$

where,

• $D$ = internal diameter of the core cutter,
• $H$ = height of the core cutter.

The Core Cutter Method is simple, reliable, and widely used for cohesive soils that can retain their shape during sampling.

Other Methods of Determining Field Unit Weight

Apart from the Core Cutter Method, the field unit weight of soil can also be determined by:

1. Sand replacement method.
2. Rubber balloon method.
3. Water displacement method.
4. Gamma ray method.

The choice of method depends on the type of soil and field conditions.

Engineering Applications

Determination of the unit weight of soil is an important part of geotechnical investigations and construction quality control. It is commonly used in:

• Field compaction control.
• Foundation engineering.
• Highway and pavement construction.
• Earth dam and embankment construction.
• Estimation of earth pressures and settlements.
• Evaluation of soil improvement works.

Since the unit weight of soil directly influences its strength and deformation characteristics, its accurate determination is essential for the safe and economical design of geotechnical structures.