Page 187 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Density and Unit Weight
182 Geotechnical Engineering

Figure 9.2
The largest man-
made structures
are not earth
dams or the
Pyramids, but
highways built on
engineered and
compacted soil.
U.S. Interstate
Highway system.
Adapted from the
U.S. Federal
Highway
Administration.

voids that it will turn to mush after rain, and a lightweight roller will walk out
more readily than a heavy one.

Density and moisture content measurements are routinely conducted in all but the
smallest compaction projects in order to ensure adherence to specifications.
Modern instruments allow this to be done indirectly and quickly using non-
destructive gamma-ray transmission and neutron tests that are discussed in
Chapter 13. This chapter discusses the procedures used to calculate density
relationships in a soil.

9.2 SOLID þ LIQUID AND GAS

9.2.1 Three Phases

A block diagram such as shown in Fig. 9.3 is useful for dealing with
many definitions related to soils. The total volume is 1.0, and portions of the
volume made up by solids, water, and air are designated with subscripts s, w,
and a.

9.2.2 Density and Unit Weight

Density is a mass property per unit volume that is the same regardless of the
influence of gravity, whereas unit weight is the tug that is measured per unit
volume in response to gravity. Nuclear instruments measure density that is
converted to unit weight based on gravitational pull at the Earth’s surface through

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