The Pycnometer is used for determination of specific gravity of soil particles of both fine grained and coarse grained soils. The determinination of specific gravity of soil will help in the calculation of void ratio, degree of saturation and other different soil properties.
Specific Gravity of Soil By Pycnometer Method
Equipment for Specific Gravity Test of Soil
The major measuring equipment in this test is Pycnometer. This is a glass jar of 1 litre capacity that is fitted at its top by a conical cap made of brass. It has a screw type cover as shown in figure-1.
There is a small hole at its apex of 6mm diameter. The leakage is prevented by having a washer between the cap and the jar. While closing the jar, it is screwed till the mark so that the volume of the pycnometer will remain constant throughout the calculation.
Mentioned below are the equipment and apparatus required to conduct the test for specific gravity of soil by pycnometer method.
- Pycnometer of about 1 litre capacity
- Weighing balance, with an accuracy of 1g.
- Glass rod
- Vacuum pump
Procedure for Specific Gravity of Soil by Pycnometer Method
1. Clean and dry the Pycnometer. Tightly screw its cap. Take its mass (M1) to the nearest of 0.1 g.
2. Mark the cap and Pycnometer with a vertical line parallel to the axis of the Pycnometer to ensure that the cap is screwed to the same mark each time.
3. Unscrew the cap and place about 200 g of oven dried soil in the Pycnometer. Screw the cap. Determine the mass (M2).
4. Unscrew the cap and add sufficient amount of de-aired water to the Pycnometer so as to cover the soil. Screw on the cap.
5. Shake well the contents. Connect the Pycnometer to a vacuum pump to remove the entrapped air, for about 20 minutes for fine-grained soils and about 10 minutes for coarse-grained soils.
6. Disconnect the vacuum pump. Fill the Pycnometer with water, about three-fourths full. Reapply the vacuum for about 5min till air bubbles stop appearing on the surface of the water.
7. Fill the Pycnometer with water completely upto the mark. Dry it from outside. Take its mass (M3).
8. Record the temperature of contents.
9. Empty the Pycnometer. Clean it and wipe it dry.
10. Fill the Pycnometer with water only. Screw on the cap upto the mark. Wipe it dry. Take its mass (M4).
Observations and Calculations for Specific Gravity of Soil
The specific gravity of soil is determined using the relation:
Where M1=mass of empty Pycnometer,
M2= mass of the Pycnometer with dry soil
M3= mass of the Pycnometer and soil and water,
M4 = mass of Pycnometer filled with water only.
G= Specific gravity of soils.
Table.1: Observations and Calculations for Specific Gravity of Soil
|Sl. No.||Observations an Calculations||Determination No.|
|3||Mass of empty Pycnometer (M1)|
|4||Mass of Pycnometer and dry soil (M2)|
|5||Mass of Pycnometer, soil and water (M3)|
|6||Mass of Pycnometer and water (M4)|
|7||M2 – M1|
|8||M3 – M4|
|9||Calculate G using formula|
Results of Pycnometer Test
Specific gravity of soil at ___0C = _____.
Unless specified, the specific gravity values of the soil will be reported at 27-degree Celsius.
The specific gravity of soil particles will come within the range of 2.65 and 2.85. If the soils consist of porous and organic materials, a specific gravity value of less than 2 will be obtained. A specific gravity value greater than 3 will be shown by soils that have heavy substances.
- The soil sample to be tested for specific gravity must be completely free from lumps. If present, they have to be broken down into the original form.
- Two main reasons for error in the calculation are weighing inaccuracies and the presence of entrapped air. Entrapped air have to be eliminated completely before testing the sample. The weighing balance has to be checked before conducting the test.
- The soil sample taken for testing have to be completely oven dried.
Read More on Geotechnical Lab Tests on Soils