sm spec grav

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The University of Toledo

Soil Mechanics Laboratory

1Specific Gravity of Soil Solids

IntroductionThe specific gravity of a material is defined as the ratio of the mass of a unit volume of a

material to the mass density of gas-free distilled water at a stated temperature. Specific gravity

of soil solids is written as

Gs = s / w (1)

where s and w are the mass density, mass per unit volume, of the soil solids and water,respectively. A material with a specific gravity greater than water is denser than water so it will

not float in water. Specific gravity is used in computations involving phase relationships thatare expressed in terms of unit weight, where unit weight is defined as the weight of material per

unit volume. The specific gravity of soil solids falls within the following ranges of values.

Soil Type Range of GsSand 2.63 2.67

Silty Sand 2.67 2.70

Silts 2.65 2.70Silty Clay 2.67 2.80

Clay 2.70 2.80

Organic Soil 1+ to 2.60

Apparatus

1. 250ml, 500 ml. or 1000ml Volumetric Flask2. Vacuum pump, aspirator or Bunsen burner

3. Thermometer4. Oven5. Precision balance capable of measuring samples to 0.1 g

Procedure

A. Preparation

1) Measure out approximately 1000 ml. of either distilled, deaired or tap water and place inthe laboratory to minimize temperature fluctuations during the test (at least 24 hoursbefore test).

2) Determine the mass of a clean, dry volumetric flask (pycnometer), Mf.

3) Add approximately 50 g. of oven dry soil to the flask and obtain the mass of the flaskand the dry soil, Mfs. Compute the mass of the soil, Ms.

4) Carefully add enough distilled, deaired or tap water to cover the soil and soak. [ASTMrequires soaking for at least 12 hours].

1ASTM D 854 - 92

Specific Gravity - 1


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B. Laboratory

1) Remove the entrapped air from the soil by either applying a partial vacuum of at least660 mm Hg (12.8 psig) pressure for at least 10 min. [ASTM requires 30 min.] or bygently boiling the specimen for 10 min. Gently agitate the pycnometer periodically to

assist in removal of the air. It is necessary to bring the specimen back to room

temperature if boiling is used. (Note: Failure to carry out this step of the procedurecan result in significant error.)

2) Tilt the flask slightly and slowly add the temperature-stabilized water to the flask, toavoid entrapping air in the specimen, until the bottom of the meniscus is just level with

the 500 ml. fill line etched in the stem of the flask. Determine the mass of the flask,water and soil, Mfws.

3) Measure the temperature of the soil and water.

4) Empty the flask making sure that all of the soil is removed and that the outside of theflask is clean and dry.

5) Fill the flask with the temperature-stabilized water to the 500 ml. level line as before,dry the outside of the flask and obtain the mass of the flask and water, Mfw.

Calculations

Since Equation 1 requires the mass density of the soil solids, it is necessary to determine the

volume of a known mass of soil. The volume can be obtained using Archimedes principle that

states that a body submerged in water will displace a volume of water equal to the volume of thesubmerged mass. The volume of soil solids is determined by first determining the mass of

displaced water, Mdw, and then dividing by the mass density of water. The mass of displaced

water is computed using the following procedure. Consider the mass of the flask and 500 ml. of

water and soil. The mass must be equal to:

Mfws = Mfw - Mdw + Ms (2)

Rearranging Equation 2 gives

Mdw = Mfw - Mfws + Ms (3)

The equation for specific gravity is then obtained as follows.

Vs = Vdw = Mdw / w (4)

s = Ms / Vs = (w Ms) / (Mfw - Mfws + Ms) (5)

Gs = Ms / (Mfw - Mfws + Ms) (6)

According to usual practice, the specific gravity at 20oC is given. It is computed by multiplying

by a correction factor that accounts for differences in water density with temperature.



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Results

Equation 6 is used and results are show in the table below.

Specific Gravity of Soil Solids Group ___________ Date ___________

Soil Description: Temperature Density Correction

Test Number 1 2 (C) (g/ml) Factor

Volumetric Flask No. 16.0 0.99897 1.0007

Mass of Flask, Mf (g) 17.0 0.99880 1.0006

Mass of Flask + Soil, Mfs (g) 18.0 0.99862 1.0004

Mass of Soil, Ms (g) 19.0 0.99843 1.0002

With water level with the fill line: 20.0 0.99823 1.0000

Mass of Flask + Water + Soil (g) 21.0 0.99802 0.9998

Temperature, T (oC) 22.0 0.99780 0.9996

Mass of Flask + Water, Mfw (g) 23.0 0.99757 0.9993

(Mfw - Mfws + Ms) (g) 24.0 0.99732 0.9991

Gs (at T) 25.0 0.99707 0.9988

Correction Factor 26.0 0.99681 0.9986

Gs (at 20 C) 27.0 0.99654 0.9983

28.0 0.99626 0.9980

Conclusions

Is the computed specific gravity representative of the type of soil tested in the laboratory?According to ASTM specifications (ASTM D 854 - 92), the average of two tests should be

used to determine the specific gravity. The difference between two tests should be 0.06

or less or the ratio of the two values should be less than 1.02. What is an acceptablerange of values for a second test to obtain an acceptable value for the average of two

tests?

How would it affect the test results if entrapped air were not removed from the soil?



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500ml Volumetric Flask

Vacuum Pump and Tubing

Thermometer

Picture 1 Specific Gravity Apparatus


sm spec grav

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