How to Determine Specific Gravity of Soils ?

There are two basic methods of determining specific gravity of soils: 1. Pycnometer method 2. Density bottle method.

1. Pycnometer Method:

Pycnometer method is used to determine the specific gravity of coarse-grained soils.

Apparatus:

In this method, a pycnometer of 900-mL capacity is used for determination of specific gravity. However, IS – 2720 (Part 3/sec 2) – 1980 recommends the use of a gas jar of 1-L capacity, in place of the pycnometer, for determination of specific gravity. The gas jar is fitted with a rubber bung and is found to make a very effective pycnometer. A pyc­nometer, where used, should be shaken in a mechanical shaking apparatus and its mouth be sealed in such a way that its volume is constant.

Other apparatus consists of a ground glass plate or a plastic slip cover for closing the gas jar and a mechanical shak­ing apparatus, capable of rotating the gas jar, end to end, at about 50 rpm. The soil sample used is about 200 g in the case of fine-grained soil and 400 g in the case of medium- and coarse-grained soils and should be oven-dried. The method is not suitable if > 10% of soil is retained on 40-mm IS sieve, and such stones should be broken down to less than this size.

Procedure:

Figure 4.17 illustrates the principle of determination of specific gravity by pycnometer method.

The procedure is described in the following steps:

i. The weight of the clean and dry gas jar is taken to the nearest 0.2 g (W₁).

ii. The soil sample is placed in the gas jar. The gas jar with slip cover and with the contents is weighed (W₂) to the nearest 0.2 g.

iii. About 500 mL of water is added to the soil in the gas jar. The rubber stopper is inserted on to the gas jar. For medium- and coarse-grained soils, the gas jar with the contents is set aside for at least 4 h. At the end of this period or immediately after the addition of water in the case of fine-grained soils, the gas jar is shaken by hand until the soil particles are in suspension and then placed in the shaking apparatus and shaken for a period of 20-30 min.

iv. The rubber stopper is then removed and any soil adhering to the stopper is washed carefully into the jar. Water is then added to the gas jar to within 2 mm of the top surface. The soil is allowed to settle for a few minutes and the gas jar is then filled with water to the brim. The slip cover is then placed on the top of the jar taking care not to trap any air under the cover.

v. The gas jar and the cover are wiped with a cloth and the whole assembly is weighed (W₃) to the nearest 0.2 g.

vi. The gas jar is emptied, washed thoroughly, and filled completely with water. The glass plate is placed in position taking care not to trap any air under the plate or slip cover. The gas jar and the plate or cover are then wiped with a cloth on the outside and the whole assembly is then weighed (W₄).

Derivation of Expression for Specific Gravity:

G = γ_S/γ_w = W_d/V_sγ_w

where W_d is the weight of dry soil and V_s is the volume of soil solids. Referring to Fig. 4.17,

W_d = W₂ – W₁

V_s = (W₄ – W₁) – (W₃ – W₂)/ γ_w

Therefore,

2. Density Bottle Method:

The density bottle method is used for the determination of specific gravity of fine-grained soils. It may be also used for medium- and coarse-grained soils provided that the soil is pulverized and passed through a 4.75-mm IS sieve. The density bottle is a pycnometer of 50-mL capacity with a stopper.

Procedure:

Figure 4.19 illustrates the principle of determination of specific gravity by density bottle method.

The procedure is described in the following steps:

1. The weight of the clean and dry density bottle with stopper is taken to the nearest 0.001 g (W₁).

2. About 5-10 g of the soil sample (passed through a 2-mm IS sieve) is dried at 105°C – 110°C and cooled in a desic­cator. Where soils are suspected to contain gypsum or organic matter, the oven-drying of the soil is restricted to a maximum of 60°C – 80°C but for longer periods. The soil sample from the desiccator is placed into the density bottle. The bottle along with the soil and stopper is weighed to the nearest 0.001 g (W₂).

3. Sufficient air-free distilled water is added to the density bottle such that the soil in the bottle is just covered. The bottle with soil and water without stopper is placed in a vacuum desiccator, which is then evacuated gradually up to a pressure of 20 mm Hg (2.72 kN/m²). The air in the density bottle is removed in the process, which takes about 1 h.

4. The vacuum is then released and the contents in the density bottle are stirred with a glass needle or spatula. The soil adhering to the glass needle is washed carefully into the density bottle. The density bottle is kept in the desiccator once again and vacuum is applied.

This procedure is repeated until no more entrapped air evolves from the soil. Alternately, the entrapped air may be removed by heating the density bottle along with the soil and water in a water bath, accompanied by occasional stirring.

5. The bottle is then removed from the desiccator (or water bath) and air-free distilled water is added to the bottle in increments, accompanied by occasional stirring, until the bottle is full. The stopper is placed and the outside surface is wiped with a cloth. The density bottle with stopper is weighed to the nearest 0.001 g (W₃).

6. The density bottle is emptied, washed with water thoroughly, and filled with air-free distilled water completely. The stopper is placed and the outside surface is wiped with a cloth. The bottle with stopper is then weighed to the nearest 0.001 g (W₄). The specific gravity of soil solids is determined from Eq. (4.69).

Experience has shown that the major source of error in specific gravity test is the incomplete removal of entrapped air. The procedure should be scrupulously followed and all care should be taken to remove the entrapped air.

The test is repeated and the average of two test results is reported as the specific gravity. Kerosene or white spirit may be used in place of water for soils containing soluble salts and also because they are better wetting agents.

When kerosene or carbon tetra chloride (CCl₄) is used for determination of specification, the equation for calcula­tion of specific gravity is modified as follows –

G = G_k(W₂ – W₁)/[(W₄ – W₁) – (W₃ – W₂)] …(4.70)

Here, G_k is the specific gravity of kerosene or CCl₄.

Specific gravity is reported at a standard temperature of 27°C (G₂₇) by applying the following temperature correction –

G₂₇ = G_T(γ_wT/γ_w27) …(4.71)

where G_T is the specific gravity at room temperature (T°C), γ_wT is the density of water at room temperature (T°C), and γ₂₇ is the density of water at 27°C.