Page 195 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Density and Unit Weight
190 Geotechnical Engineering

The average density from the cubic and rhombic packing arrangements is
3 3 3

d ¼ 2:65 0:75 ¼ 1:7Mg=m , ð105 lb=ft Þ,16:6kN=m

which is within the usual range of densities obtained after compaction.

Example 9.7
Calculate the saturated and submerged unit weights for the cubic packing arrangement.
Answer: Subtracting the volume of solids from 1.0 gives V v ¼ 0.4764. The weight of water
3
3
3
to fill all voids is 0.476 Mg/m , or 0.476 62.4 ¼ 29.7 lb/ft , or 0.476 9.807 ¼ 4.67 kN/m .
3
3
This is added to the dry density to give
sat ¼ 29.7 þ 86.6 ¼ 116.3 lb/ft (8.3 kN/m ). Then
3

sub ¼ 116:3 62:4 ¼ 53:9lb=ft or 18:2 9:81 ¼ 8:5kN=m 3
9.7 MEASURING SOIL MOISTURE CONTENTS

Most but not all water in moist soil is ‘‘free water’’ that is liquid and occupies the
open void spaces. Other water is adsorbed on mineral surfaces, particularly the
clay mineral surfaces because of the fineness of clay and its surface activity, and
in expansive clays part of the water is loosely held between the layers in coordina-
tion with exchangeable cations. In addition, OH ions that are part of the clay
mineral crystal structure are released as water if the clay is heated to a high
temperature.

These several categories of water, therefore, do not all boil off at 1008C, so a
standardized drying temperature is used to determine moisture content. That
temperature is 110 58C (ASTM Designation D-2216). To determine moisture
contents, samples of soils are weighed, dried overnight in an oven held at 1058C,
removed, and allowed to cool in a desiccator, which is a sealed vessel containing
calcium chloride, CaCl 3 , as a desiccant. After cooling, the samples are removed
and re-weighed. Controlled cooling is required or the soil clay can re-adsorb water
from the atmosphere.

Nuclear instruments used to measure moisture contents in the field actually
measure hydrogen atoms regardless of whether they are in water or are part of the
clay mineral structure or the content of organic matter, and require calibrations to
oven determinations for each soil type. The use of nuclear instruments and other
means for measuring soil densities in the field is discussed in Chapter 13 on soil
compaction.

Example 9.8
A soil sample plus an aluminum dish weighs 195 g before oven-drying and 136 g afterwards.
The dish weighs 15 g. (Most small laboratory balances weigh in grams.) What is the
moisture content?

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.