Specific Gravity Test
Concept
The Specific Gravity Test is a geotechnical procedure used to determine the specific gravity (Gs) of soil particles. Specific gravity is defined as the ratio of the unit weight of the soil solids to the unit weight of water. This parameter is crucial for various soil mechanics calculations, including the determination of void ratio, porosity, and degree of saturation. Specific gravity values also help identify and classify soil types. Standards governing the Specific Gravity Test include ASTM D854 for fine-grained soils and ASTM C127/C128 for coarse-grained soils.
Descriptions and Procedure
Apparatus Required
- Pycnometer or specific gravity flask
- Balance with an accuracy of 0.01 g
- Distilled water
- Vacuum pump (optional, for fine-grained soils)
- Thermometer
- Oven
- Desiccator
- Sample preparation tools (e.g., sieve, mortar and pestle for fine soils)
Procedure
1. Sample Preparation
- Coarse-Grained Soils: Obtain a representative sample of the soil, ensuring it passes through a 4.75 mm (No. 4) sieve.
- Fine-Grained Soils: Obtain a representative sample of the soil, ensuring it passes through a 2 mm (No. 10) sieve. If necessary, further pulverize the soil using a mortar and pestle to break up clumps.
2. Drying the Sample
- Dry the soil sample in an oven at 110 ± 5°C to a constant mass.
- Allow the sample to cool to room temperature in a desiccator to prevent moisture absorption.
3. Weighing the Dry Sample
- Weigh a dry, clean pycnometer or specific gravity flask and record its mass (W1).
- Add a known mass of the dry soil sample (typically 50 g for fine-grained soils and 500 g for coarse-grained soils) to the pycnometer and record the combined mass (W2).
4. Filling with Water
- Partially fill the pycnometer with distilled water, ensuring that the soil is completely submerged.
- For fine-grained soils, use a vacuum pump to remove air bubbles from the soil-water mixture. This step can be omitted for coarse-grained soils.
- Fill the pycnometer to the calibration mark with distilled water.
5. Weighing the Filled Pycnometer
- Weigh the pycnometer filled with soil and water, ensuring no air bubbles remain, and record the mass (W3).
6. Determining the Mass of Water
- Empty the pycnometer, clean it thoroughly, and fill it with distilled water to the calibration mark.
- Weigh the pycnometer filled with water only and record the mass (W4).
7. Temperature Measurement
- Measure the temperature of the water used in the test. If the temperature deviates significantly from the standard temperature (20°C), apply a temperature correction factor from the appropriate table.
Calculations
Specific Gravity (Gs)
Calculate the specific gravity of the soil particles using the following formula:
𝐺𝑠=(𝑊2−𝑊1)/(𝑊2−𝑊1)−(𝑊3−𝑊4)
Where:
- 𝑊1 = Mass of empty pycnometer
- 𝑊2 = Mass of pycnometer with dry soil
- 𝑊3 = Mass of pycnometer with soil and water
- 𝑊4 = Mass of pycnometer with water only
Temperature Correction
- If necessary, apply a correction factor to account for deviations in water temperature from the standard reference temperature. The correction factor can be found in standard tables, such as those provided in ASTM D854.
Data Interpretation
Typical Specific Gravity Values for Soils
- Sands: 2.65 - 2.67
- Silts: 2.65 - 2.70
- Clays: 2.70 - 2.80
- Organic soils: 2.00 - 2.50
These values can vary based on mineral composition and other factors. Specific gravity values outside these ranges can indicate unusual soil characteristics or potential testing errors.
Summary Table
The Specific Gravity Test is an essential method for determining the fundamental properties of soil particles, aiding in soil classification and various geotechnical engineering analyses.