Specific Gravity Determination

The knowledge of the Specific Gravity is essential in relation to other tests, especially for calculating porosity and voids and for computation of particle size analysis from a sedimentation procedure (Hydrometer analysis). It is also important when compaction and consolidation properties are considered.

Required equipment

•        2 nos. 50 mL density bottles (pyknometers) with stoppers

•        Constant temperature water bath

•        Vacuum desiccator

•        Vacuum pump and suitable rubber tubing

•        Drying oven capable of maintaining temperature of 105 °C – 110 °C

•        Distilled water in a wash bottle

•        Test sieve 2 mm

•        Balance readable to 0.01 g

•        Mortar and piston.

Sample preparation

An oven-dried soil sample of about 100 g shall be prepared. Large particles shall be ground to pass a 2 mm sieve. Two specimens, each between 5 g and 10 g shall be obtained by riffling and stored in airtight containers.

Test Procedure

1. A density bottle with a stopper shall be dried with a cloth and weighed to the nearest 0.01 g (m₁).
2. The first soil specimen shall be transferred to the density bottle direct from the sealed container. The weight of the bottle with soil sample and stopper shall be recorded to the nearest 0.01 g (m₂).
3. Air-free distilled water shall be added so that the soil in the bottle is just covered.
4. Place the bottle containing soil and water, without the stopper, in the vacuum desiccator. Evacuate the desiccator gradually. The bottle shall be allowed to remain in the desiccator for at least one hour until no further loss of air is apparent.
5. Release the vacuum and remove the lid of the desiccator. Vibrate the bottle till no more air is released. Replace the lid and evacuate again as specified above.
6. Remove the bottle from the desiccator and add air-free water until the bottle is full. Insert the stopper and immerse the bottle up to the neck in the water bath for at least 1 hour.
7. If there is an apparent decrease in the volume of water, remove the stopper, add water to fill the bottle, and replace the stopper.
8. Return the bottle to the bath and again allow the contents to attain the constant temperature.
9. The stoppered bottle shall then be taken out of the bath, wiped dry carefully, and weighed to the nearest 0.01 g (m₃).
10. Clean out the bottle, fill it completely with air-free water, insert the stopper and immerse in a constant temperature water bath for 1 hour. If required, top up the bottle as described above.
11. Take the stoppered bottle out of the bath, carefully wipe it dry and weigh it to the nearest 0.01 g (m₄).

Calculation

1. Calculate the Specific Gravity, ρ_s from the equation:

where

m₁ is the mass of the density bottle (in g)

m₂ is the mass of bottle and dry soil (in g)

m₃ is the mass of bottle, soil, and water (in g)

m₄ is the mass of bottle full of water only (in g).

2. Calculate the average of the two results.

References

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