Page 186 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Density and Unit Weight
Soil Density and Unit Weight 181

Figure 9.1
Collapse of a wall
of a municipal fire
station was
triggered by
removing the
restraining effect of
weight of the soil
from alongside the
foundation. Photo
is courtesy of
Prof. J. M. Hoover.

9.1.2 Variations in Weight Depending on Moisture Content

In some ways a soil resembles a sponge—light in weight when dry and heavy when
wet. As the dry weight is constant, it is the reference for density and unit weight
calculations. For example, if a sample of soil weighs 100 g wet and 75 g dry, the
percent moisture is based on 75 g, not on the total weight of 100 g. The moisture
content, instead of being 25 percent, is 100 (100 – 75)/75 ¼ 33 percent.

This convention can yield some rather curious results: For example, if a soil
sample weighs 100 g wet and 50 g dry, its moisture content is 100 (100 – 50)/
50 ¼ 100 percent. Some natural soils contain more water than solids, in which
case the moisture content is over 100 percent! For this one needs a college
education.

The relation between moisture content and degree of saturation can be critical,
because a load applied to and acting to compress a saturated soil will be partly
carried by the pore water, which has zero strength. The soil that is firm enough to
support a heavy truck when dry can turn into a mudhole when the soil becomes
saturated with water.

9.1.3 Influence of Density on Strength
Generally the more dense the soil, the stronger it is, although there are important
exceptions. Compaction specifications define the kind of soil, its moisture content,
and a target density. Other criteria such as ‘‘walk-out’’ of a sheepsfoot roller are
not reliable because a roller can ‘‘walk-out’’ if the soil is dry but contains sufficient

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.