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The Specific Gravity (Gs) is needed for various calculations purposes in Soil Mechanics,

e.g. void ratio, density, and unit weight.

For Sandy soil, i.e. soils mostly made of quartz, Gs can be accurately estimated to be

about 2.65, whereas for silty and clayey soils, it may vary from 2.6 to 2.9.

-  Specific gravity : 

Specific gravity is the ratio of the density (mass of a unit volume) of a substance to the

density (mass of the same unit volume) of a reference substance. Apparent specific

gravity is the ratio of the weight of a volume of the substance to the weight of an equal

volume of the reference substance. The reference substance is nearly always water for

liquids or air for gases. Temperature and pressure must be specified for both the

sample and the reference.

The main Idea of this test is to determine the specific gravity of a given soil specimen .

Calculating the specific gravity in the experiment will use the concept of :

If you know the amount of water that fulfill the same volume of the soil substance, then

the specific gravity equals the ratio between that number and the amount of soil

sample.

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-  A couple of flasks ( 6 & 4 ) of unknown volumes . see figure 1.

-  Water.

-  Digital scale with a precision 0.1 gm. See figure 2.

-  A spoon .

* Figure 2 * the scale we used * Figure 1* The flasks we used

-  A soil specimen of unknown mass.

-  The mass of the flask has been scaled and noted as M 1

-  The soil sample has been added to the flask and both has scaled and noted as M

2

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-  Water has been added to the soil in the flask until the water reaches a certain

level and the mass of the mixture has been scaled and noted as M 2 .

-  The mixture has been poured and a same volume of water has been added alone

in the flask , the mass has been calculated and noted as M 4. See Table 1-  The previous steps has been done to both flasks ( 6 & 4 ) for the same soil

substance

Flask alone

M 1

Flask + soil

M 2

Flask +soil +

water M 3

Water + flask 

M 4

96.5 140.2 390.6 363.4

93.5 172 407.6 358.6

First : flask ( 6 ) : 

-  To determine the mass of the soil specimen we use

M 5 = M 2 – M 1 = 140.2 – 96.5 = 43.7 gm.

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-  Determining the mass of the water in the mixture will be as followed

M 6 = M 3 – M 2 = 390.6 – 140.2 = 250.4 gm.

-  The mass of water of the same volume of soil specimen will be calculated as

followed :

M 7 = M 4 – (M 6 + M 1 ) = 363.4 – (250.4+ 96.5) = 16.5 gm

-  The specific gravity will be :

Gs = M 5 / M 7 = 43.7 / 16.5 = 2.65

Second : flask ( 4 ) : 

-  To determine the mass of the soil specimen we use

M 5 = M 2 – M 1 = 172 – 93.5 = 78.5 gm.

-  Determining the mass of the water in the mixture will be as followed

M 6 = M 3 – M 2 = 407.6 - 172 = 235.6 gm.

-  The mass of water of the same volume of the soil specimen will be calculated as

followed :

M 7 = M 4 – M 6 – M 1 = 358.6 – 235.6 – 93.5 = 29.5 gm.

-  The specific gravity will be :

Gs = M 5 / M 7 = 78.5 / 29.5 = 2.66

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By Calculating the Specific gravity of the same specimen we get :

-  from the first experiment by flask 6

-  from the second experiment by flask 4

We can say after lab experiment that the specific gravity of the specimen is the average

of these two numbers . that’s mean for the given sample

1.  This test covers the determination of soil sample that passes sieve 4 # , but we

didn’t check that in the lab .  

2.  Distilled water should be used in this test , we didn’t check the type of water in

the lab .

3.  Test temperature was about 34 C0

.

ASTM 854 : Standard test for specific Gravity of soil solids by water Pycnometer .

Importance of determining specific gravity : wikipedia .