10:14:00
OBJECTIVE:
To
familiarize the students with general method of obtaining the specific gravity
of a mass of any type of material composed of small particles (specifically
soil).
THEORY
A
value of specific gravity is necessary to compute the void ratio of a soil, it
is used in the hydrometer analysis, and it is useful to predict the unit weight
of a soil. Occasionally, the specific gravity may be useful in soil mineral
classification; e.g., iron minerals have larger value of specific gravity than
silicas.
The
specific gravity of any substance is defined as the unit weight of the material
divided by the unit weight of distilled water at 4 oC. Thu, specific
gravity of soil can be found as;
As long as equal volume of water
and soil are involved, the above stated form can be simplified as;
Strictly speaking above mentioned
equation is only valid if we do not consider any density change with
temperature. However, a slight increase in precision to account for temperature
effects on the density of water can be obtained by rewriting above stated
equation as;
Where, α is the ratio of the unit
weight of water at temperature T of the test and at 4oC. The value
of Gs obtained at temperature T (which will be too large if T > 4oC)
is appropriately reduced.
APPARATUS
1.
Pycnometer
2. Weighing balance (Least count of
0.01 grams)
3. Thermometer
4. Hot plate or Bunsen burner
5. Funnel
6. Drying oven
7. Paper towel
PROCEDURE:
1. Weigh the dry pycnometer to nearest
0.01 gram and record it as W1.
2. Take about 100 grams of oven dried
soil and put it into the flask. Weigh the flask and dry soil to the nearest
0.01 gram. Record this weight as W2.
3. Add water in the pycnometer until
it is about two-third full. In order to remove the entrapped air from soil and
water, heat the mixture for at least 2 h after the soil-water mixture comes to
a full boil. Use only enough heat to keep the slurry boiling. Agitate the
slurry as necessary to prevent any soil from sticking to or drying onto the
glass above the slurry surface.
4. Allow the mixture to cool, and then
fill the flask with distilled water to above the calibration mark.
5. Place the stopper in the bottle
while removing the excess water. Be sure the entire exterior of the flask is
dry. Weigh the flask to the nearest 0.01 gram and record this weight as W3.
6. Empty the flask, wash it thoroughly
and fill it completely with water. Dry the exterior of the flask. Weigh the
flask and record it as W4.
7. Repeat the procedure three times.
8. Record the temperature of soil water
mixture.
PRECAUTIONS
1.
Make
sure no air is entrapped within the soil water mixture.
2.
Weights
should be obtained from a properly balanced weighing scale.
OBSERVATIONS
AND CALCULATIONS
|
Test No.
|
1
|
2
|
3
|
|
Volume of flask
|
|
|
|
|
W1
(grams)
|
|
|
|
|
W2
(grams)
|
|
|
|
|
W3
(grams)
|
|
|
|
|
W4
(grams)
|
|
|
|
| a
|
|
|
|
|
Gs
|
|
|
|
Typical
values of correction factor, α;
|
T (oC)
|
Correction Factor, α
|
|
4
|
1.0000
|
|
15
|
0.9999
|
|
20
|
0.9982
|
|
25
|
0.9971
|
|
30
|
0.9957
|
|
35
|
0.9941
|
REFERENCE
ASTM
D854 – 02
Standard
Test Methods for Specific Gravity of Soil Solids by Water Pycnometer
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