Page 167 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
162 Geotechnical Engineering

represents the weight divided by the weight of an equal volume of water, which by
definition is the specific gravity:

W
G ¼ ð7:6Þ
W W b

where G is the specific gravity and W and W b are the weight and buoyant weight
respectively.

A slightly different procedure is used for soils and is a bit more tricky. A flask
is filled with water and weighed; call this A. Then W, a weighed amount of soil,
is put into the flask and displaces some of the water, giving a new total
weight, C. As shown in Fig. 7.8, the weight of the water displaced is (A þ W C).
Hence,

W
G ¼ ð7:7Þ
A þ W C

Experimental precision is unhappy with subtracting a weight from the
denominator, so measurements are exacting. Recently boiled or evacuated
distilled water ensures that there is no air that might come out of solution to
make bubbles, and clay soils are not previously air-dried. Less critical is a
temperature-dependent correction for the specific gravity of water, which at 208C
is 0.99823. (Specific gravities are reported to three significant figures.) Details are
in ASTM Designation D-854. It will be noted that weights and not masses are
measured, even though the data are usually recorded in grams.

Example 7.8
A flask filled to a reference mark with water weighs 690.0 g on a laboratory scale. When
90.0 g of soil are added, the filled flask weighs 751.0 g. The water temperature is 208C.
(a) What is G? (b) What effect will the temperature correction have? (c) What if as a result
of measurement error the soil weight is 1 g too high, an error of 1.1%?

Fig. 7.8 Using a
pycnometer to
measure specific
gravity.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.