Bulk Density and Unit Weight for Undisturbed Samples

The Bulk Density of a soil is the ratio of the total mass to the total volume. The Unit Weight of the soil is the ratio of the total weight (a force) to the total volume.

This procedure describes a practical way of determining the Bulk Density and Unit Weight on undisturbed samples obtained by means of a U-100 sampling tube, a 54 mm sampling tube or from a block sample.

It may be convenient to include the determination of several other parameters when determining the Bulk Density of a sample, i.e. the Dry Density, the Void Ratio, the Degree of Saturation and the Unit Weight.

Required equipment

•        Balance readable to 1 g

•        Sample extruder

•        Apparatus and equipment for moisture content determination (optional)

Test Procedure – cylindrical tube sample

1. After registration cylindrical tube sample, the lids shall be unscrewed and the wax removed.
2. Weigh the cylinder with the sample inside. The mass shall be recorded to the nearest 1 g (m_T).
3. The length of the sample in the cylinder is determined by measuring the length of the cylinder (l1) and the depths from both ends of the cylinder (l2 and l₃). The average depths to the sample are recorded since the sample surface is never even.
4. Extrude the sample by following relevant procedures for further tests to be carried out on the sample.
5. When the sample has been extruded, weigh clean and dry the cylinder and record its mass (m_C).
6. (Optional) The moisture content shall be determined on three different specimens from the sample, in order to achieve the average condition of the sample.

Test Procedure – Block sample

1. Depending on the soil, the sample is cut to either a rectangular prism form or a cylindrical form, preferably by a cylindrical or rectangular tube that is pushed into the soil.
2. Trim the ends of the sample.
3. Weigh the tube with the sample inside. The mass shall be recorded to the nearest 1 g (m_T).
4. Record the internal dimensions of the tube or the dimensions of the sample if no tube is used.
5. Extrude the sample by following relevant procedures for further tests to be carried out on the sample.
6. When the sample has been extruded, weigh clean and dry the tube and record its mass (m_C).
7. (Optional) The moisture content shall be determined on three different specimens from the sample, in order to achieve the average condition of the sample.

Calculations

1.       The Bulk Density of the sample, ρ (in kg/m³), is calculated from the equation:

m_T is the mass of the cylinder/container + sample (in g)

m_C is the mass of the empty cylinder/container (in g)

V is the volume of the sample (in cm³)

2.       The Unit Weight of the sample, γ (in kN/m³), is derived from the equation:

Where

g is the acceleration due to gravity (= 9.81 m/s³)

The value of the Bulk Density shall be expressed to the nearest 1 kg/m³, and the Unit Weight to the nearest 0.01 kN/m³.

References

 The United Republic of Tanzania ministry of works, Laboratory Testing Manual 2000, Novum Grafisk AS, Skjetten Norway