Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is specific gravity? Specific gravity (G) of soil tells you how many times heavier the soil particles are than the same volume of water. If G = 2.65, a cube of soil solids weighs 2.65 times as much as a cube of water of the same size.
Why is it needed? G is used to work out void ratio, porosity and degree of saturation, to draw the zero air voids line on the compaction curve, and in the hydrometer analysis.
This guide follows IS 2720 (Part 3/Sec 1): Determination of Specific Gravity of Fine-Grained Soils (density bottle method).
The Idea in One Picture
Typical Values
| Soil type | Typical G |
|---|---|
| Sand and gravel (mostly quartz) | about 2.65 |
| Silts and clays (most mineral soils) | 2.60 – 2.80 |
| Soils with iron minerals | above 2.80 |
| Organic soils (peat, decayed plants) | much lower, can be below 2.0 |
Apparatus Required
Density bottle (50 ml)
Glass bottle (pycnometer) with a stopper that has a fine hole.
Water bath (27°C)
Keeps the bottles at a constant 27 ± 0.2°C.
Balance (0.001 g)
Very accurate balance; reads to 0.001 g.
Oven
Dries the soil and the bottle at 105–110°C.
Vacuum pump
Removes trapped air bubbles from the soil and water.
2 mm sieve, riffle box, wash bottle
For sieving, splitting the sample and rinsing soil into the bottle.
Sample
Take about 50 g of oven-dried soil (split with a riffle box), passing the 2 mm sieve. Use 5 to 10 g of it for each density bottle test.
Procedure: Four Weighings
- m1: Clean the density bottle, dry it in the oven at 105–110°C, let it cool, and weigh it with its stopper.
- m2: Put the cooled, dry soil (5–10 g) into the bottle and weigh it with the stopper.
- Add water: Add enough air-free distilled water to just cover the soil. Stir gently, and rinse any soil on the stirrer back into the bottle.
- Remove the air: Keep the bottle under vacuum to remove trapped air, preferably for several hours or overnight, shaking it gently once or twice.
- m3: Fill the bottle with air-free water, put the stopper in, and keep it in the water bath at 27°C for about 1 hour. Take it out, wipe the outside dry and weigh it.
- m4: Empty and clean the bottle, fill it with air-free water only, keep it in the bath for about 1 hour, wipe it dry and weigh it.
- Repeat: Do at least two tests. If the results differ by more than 0.03, repeat the test.
Formula and Worked Example
| Description | Test 1 | Test 2 |
|---|---|---|
| Bottle + stopper, m1 (g) | 30.000 | 31.000 |
| Bottle + dry soil, m2 (g) | 40.000 | 41.000 |
| Bottle + soil + water, m3 (g) | 86.260 | 87.470 |
| Bottle + water, m4 (g) | 80.000 | 81.200 |
| Dry soil, m2 − m1 (g) | 10.000 | 10.000 |
| Water displaced, (m4 − m1) − (m3 − m2) (g) | 3.740 | 3.730 |
| Specific gravity G | 2.67 | 2.68 |
| Average G (at 27°C) | 2.68 (difference 0.01 < 0.03 ✓) | |
Try It: Specific Gravity Calculator
Precautions
🫧 Remove all air
Trapped air is the biggest source of error. Use air-free water and keep the bottle under vacuum long enough.
🌡️ Same temperature
Take m3 and m4 at the same temperature (27°C), as water’s weight changes with temperature.
🔥 Gentle drying
For soils that may lose chemically held water (e.g. gypsum), do not dry above 80°C.
🧽 Wipe dry, fill to the mark
Wipe the outside of the bottle dry before weighing, and fill it exactly to the top of the stopper hole each time.
Practical Tips for Site Engineers
1. Use G to Catch Wrong Field Density Results
For any soil, there is a maximum possible dry density at a given moisture content, when all the air is gone (the zero air voids density). A field dry density higher than this is impossible, so it shows a mistake in the field test.
Zero air voids density = 2.68 ÷ (1 + 0.12 × 2.68) = 2.68 ÷ 1.322 = 2.03 g/cc
If your sand replacement test shows a dry density of 2.10 g/cc, it is more than 2.03: the field test or the moisture value is wrong. Repeat it.
2. Know Where G Is Used
📈 Proctor curve
G is needed to draw the zero air voids line on the compaction curve.
⏳ Hydrometer test
The grain size of silt and clay in the hydrometer method uses G in its calculation.
🧮 Void ratio and saturation
e = G × ρw ÷ ρd − 1, and degree of saturation S = w × G ÷ e.
3. Practical Lab Habits
- Boil distilled water for a few minutes and let it cool in a closed container to get air-free water.
- If no vacuum pump is available, many labs gently boil the soil–water mix in the bottle for about 10 minutes to drive out air, then cool it back to 27°C before weighing.
- Find G once for each soil type (borrow area or layer) and use it in all related calculations, instead of assuming 2.65.
- Record the bottle number and use the same bottle and stopper for m1 to m4.
Key Takeaways
- G = weight of soil solids ÷ weight of an equal volume of water. Most soils: 2.60–2.80.
- Four weighings: m1 bottle, m2 + dry soil, m3 + soil + water, m4 + water.
- G = (m2 − m1) ÷ [(m4 − m1) − (m3 − m2)]; report to 0.01 at 27°C; repeat if two tests differ by more than 0.03.
- Remove all trapped air: it is the main source of error.
- On site: use G to check that field dry density does not exceed the zero air voids density.
References: IS 2720 (Part 3/Sec 1): Methods of Test for Soils – Determination of Specific Gravity, Fine-Grained Soils.