Soil Mechanics Lab: Specific Gravity by Pycnometer

Determination of Specific Gravity of Soil Using Pycnometer Method

Objective

The objective of this experiment is to determine the specific gravity of a given soil sample using the pycnometer method.

Apparatus

• Pycnometer (density bottle)
• Weighing balance (accuracy ±0.001 g)
• Sieve (No. 10, 4.75 mm opening)
• Distilled water
• Oven-dried soil sample
• Thermometer (for temperature control, if necessary)

Theory

Specific gravity (G) of soil solids is defined as the ratio of the weight of a given volume of soil solids to the weight of an equal volume of water at a specified temperature. It is a dimensionless quantity and is crucial in geotechnical engineering for calculations related to soil density, void ratio, and porosity.

The pycnometer method is based on Archimedes’ principle, where the volume of soil solids is determined by the volume of water displaced. The specific gravity is calculated using the formula:

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

Where:

• W₁ = Weight of empty pycnometer
• W₂ = Weight of pycnometer + dry soil
• W₃ = Weight of pycnometer + soil + water
• W₄ = Weight of pycnometer filled with water only

Procedure

1. Sample Preparation:
   • The soil sample was sieved through a No. 10 sieve (4.75 mm) to remove coarse particles.
   • The soil was oven-dried at 105–110°C for 24 hours to remove moisture.
2. Weighing the Pycnometer:
   • The empty, dry pycnometer was weighed (W₁).
   • Approximately 10 g of dry soil was placed inside the pycnometer and weighed again (W₂).
3. Adding Water and Removing Air Bubbles:
   • Distilled water was added to the pycnometer until it was nearly full.
   • The pycnometer was shaken gently to remove trapped air bubbles.
   • The weight of the pycnometer with soil and water was recorded (W₃).
4. Calibration with Water:
   • The pycnometer was emptied, cleaned, and filled only with distilled water.
   • The weight of the pycnometer with water was recorded (W₄).
5. Calculations:
   • The specific gravity was calculated for each trial using the given formula.
   • The average value was determined from three trials.

Observations and Calculations

Flask No.	W₁ (g)	W₂ (g)	W₃ (g)	W₄ (g)	Specific Gravity (G)
A	63.600	73.031	168.832	162.971	2.642
B	61.972	80.564	173.936	162.052	2.635
C	61.778	71.046	167.698	161.992	2.602

Average Specific Gravity: G_avg = (2.642 + 2.635 + 2.602) / 3 = 2.626

Result

The average specific gravity of the given soil sample was found to be 2.626.

Discussion

Consistency of Results: The values obtained from the three trials (2.642, 2.635, 2.602) show minor variations, indicating good experimental consistency.

Possible Sources of Error:

• Trapped Air Bubbles: Incomplete removal of air bubbles could lead to slight inaccuracies.
• Temperature Effects: If temperature was not controlled, water density variations might affect results.
• Weighing Errors: Minor instrumental errors in the balance could contribute to discrepancies.

Comparison with Standard Values: Typical specific gravity values for common soil types are:

• Quartz sands: 2.65–2.67
• Clayey soils: 2.68–2.80
• Organic soils: < 2.0

The obtained value (2.626) is close to that of quartz-based soils, suggesting the sample may contain a significant amount of silica minerals.

Conclusion

The specific gravity of the given soil sample was determined using the pycnometer method and found to be 2.626. This value falls within the expected range for mineral soils, indicating good experimental accuracy. The pycnometer method is reliable for determining specific gravity when performed carefully, ensuring minimal air entrapment and precise measurements.