Specific Gravity of Soil Solids:
“The Soil Specific Gravity is defined as the ratio of the weight of a given volume of the material to the weight of an equal volume of distilled water.“
It is an important parameter in soil mechanics for calculation of the weight-volume relationship.
Thus specific gravity, G” is defined as
Gs = Unit weight (or density) of soil sample only / Unit weight (or density) or water
The general values for specific gravity for different soils are given in Table -1. This method of determining the specific gravity of soil is given here is applicable for soils composed of particles smaller than 4.75 mm (No.4 U.S. sieve) in size.
Table -1. General Ranges of Gs for Various Soils:
The Specific gravity of soil generally ranges from 2.60 to 2.90. Organic matter and porous particles may have specific gravity values below 2.0 and Soil which has heavy substance or particles may have values above 3.0.
|Soil Type||Range of GS|
|Sand||2.63 – 2.67|
|Silts||2.65 – 2.7|
|Clay and Silty Clay||2.67 – 2.9|
|Organic Soil||Less than 2|
1. Specific gravity bottle (500 ml)
2. Thermometer graduated in O.soC division scale
3. Balance sensitive up to 0.01 g
4. Distilled water
5. A vacuum pump or aspirator)
6. Evaporating dishes
8. Plastic squeeze bottle
9. Drying oven
The equipment for this experiment is shown in Fig. 3-1.
Procedure to determine Specific Gravity of Soil :
1. Clean the specific gravity bottle well and dry it.
2. Carefully fill the bottles with de-aired, distilled water up to the 500 ml mark
3. Determine the weight of the bottle and the water-filled to the 500 ml mark (W1).
4. Insert the thermometer into the bottle with the water and determine the temperature of the water T = T1 ºC.
5. Fill approx 100gm of dry soil into an evaporating dish.
6. In the case of cohesive soil, add water (de-aired and distilled) to the soil and mix it into the form of a smooth paste. Keep it soaked for about 1 hour at least in the evaporating dish.
7. Now, Fill this soil (if granular) or the soil paste (if cohesive) into the volumetric specific gravity bottle.
8. Add distilled water to the gravity bottle containing the soil (or the soil paste) to make it about two-thirds full.
9. Remove excess air from the soil-water mixture. This can be done by:
- a. Gently boiling the specific gravity bottle containing the soil-water mixture for about 15 to 20 minutes. Accompany the boiling with continuous agitation of the flask. (Don’t use high flame otherwise, the soil may boil over.)
- b. The second option is to vacuum by a vacuum pump or aspirator until all of the entrapped air is out.
This is the most important step for testing. If this entrapped air not removed completely it can cause result error.
10. Bring the temperature of the soil-water mixture in the Bottle down to room temperature, i.e T1 ºC sees Step 4. (This temperature of the water is at room temperature.)
11. Now fill the distilled water to the bottle until the bottom of the meniscus touches the 500 ml mark. Also, clean the top and side of the bottle and remove any excess water.
12. Find out the combined weight of the bottle + Soil + Water as (W2).
13. Just as a precaution, check the temperature of the soil and water in the flask to see if it’s T1± 1 ºC or not.
14. Empty Soil and water in an evaporating dish. Clean the bottle such that no soil particle left in the side.
15. Transfer evaporating dish into the oven to dry to a constant weight.
16. Find out the mass of the dry soil in the evaporating dish (Ws).
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1. Calculate the specific gravity
G = Mass of soil / Mass of equal volume of water
Mass of Soil = Ws
Mass of the same volume of water, Ww = (W1 + Ws) – W2
Gs (T1 º C) = Ws / Ww
The specific gravity of water at various temperature:
|Temperature (T1º C)||A||Temperature (T1º C)||A|
For more accurate result it is recommended to conduct tests 3 times on the same soil sample. Take an average of 3 values these values should not vary by more than 2 to 3%.
A sample calculation for Specific Gravity of Soil is shown in the table given below:
Detail of soil: Light brown sandy Silt Sample No. 23
The volume of Bottle: 500 ml. The temperature of the test: 23 ºC.
Location : Water Density (23ºC) = 0.9993 (Table-3) .
Tested by: Date:
|Description||Sample – 1||Sample – 1||Sample – 1|
|Mass of Bottle + Water Filled, (W1) (g)||666.0||674.0||652.0|
|Mass of Bottle +soil + Water, (W2) (g)||722.0||738.3||709.93|
|Mass of dry Soil, Ws (g)||99.00||103.0||72.0|
| Mass of equal volume of water as the soil sample |
Ww (g) = (W1 + Ws) – W2
|Specific Gravity of Soil (G) = Ws / Ww||2.68||2.66||2.70|
From above table, we can say that spcific gravity of soil sample is 2.68.
(Note: Specific gravity of any element is unit less parameter)
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