The University of Toledo
Soil Mechanics Laboratory
1
Specific Gravity of Soil Solids
Introduction
The 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 = ρs / ρw
(1)
where ρs and ρw 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 that
are 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
Sand
Silty Sand
Silts
Silty Clay
Clay
Organic Soil
Range of Gs
2.63 – 2.67
2.67 – 2.70
2.65 – 2.70
2.67 – 2.80
2.70 – 2.80
1+ to 2.60
Apparatus
1. 250ml, 500 ml. or 1000ml Volumetric Flask
2. Vacuum pump, aspirator or Bunsen burner
3. Thermometer
4. Oven
5. 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 in
the laboratory to minimize temperature fluctuations during the test (at least 24 hours
before 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 flask
and 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. [ASTM
requires soaking for at least 12 hours].
1
ASTM D 854 - 92
Specific Gravity - 1
B. Laboratory
1) Remove the entrapped air from the soil by either applying a partial vacuum of at least
660 mm Hg (12.8 psig) pressure for at least 10 min. [ASTM requires 30 min.] or by
gently 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 procedure
can result in significant error.)
2) Tilt the flask slightly and slowly add the temperature-stabilized water to the flask, to
avoid 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 the
flask 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 the
submerged 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 / ρw
(4)
ρs = Ms / Vs = (ρw 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.
Specific Gravity - 2
Results
Equation 6 is used and results are show in the table below.
Specific Gravity of Soil Solids
Soil Description:
Test Number
Volumetric Flask No.
Mass of Flask, Mf
(g)
Mass of Flask + Soil, Mfs
(g)
Mass of Soil, Ms
(g)
Group ___________
Temperature
1
2
(C)
16.0
17.0
18.0
19.0
Date ___________
Density Correction
(g/ml)
Factor
0.99897
1.0007
0.99880
1.0006
0.99862
1.0004
0.99843
1.0002
With water level with the fill line:
20.0
0.99823
1.0000
Mass of Flask + Water + Soil (g)
Temperature, T
(oC)
Mass of Flask + Water, Mfw (g)
(Mfw - Mfws + Ms)
(g)
21.0
22.0
23.0
24.0
0.99802
0.99780
0.99757
0.99732
0.9998
0.9996
0.9993
0.9991
Gs (at T)
Correction Factor
Gs (at 20 C)
25.0
26.0
27.0
28.0
0.99707
0.99681
0.99654
0.99626
0.9988
0.9986
0.9983
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 acceptable
range 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?
Specific Gravity - 3
500ml Volumetric Flask
Vacuum Pump and Tubing
Thermometer
Picture 1 – Specific Gravity Apparatus
Specific Gravity - 4