Page 193 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Density and Unit Weight
188 Geotechnical Engineering

Degree of saturation is defined as the volume of water divided by the volume of
voids, in percent:

V w
S ¼ 100 ð9:11Þ
V v
A formula that may be used to determine saturated unit weight from the void
ratio, specific gravity, and wet unit weight is as follows:

sat ¼
w G þ eð Þ= 1 þ eÞ ð9:12Þ
ð

9.5 VALUES FOR SPECIFIC GRAVITY

The specific gravity of the solid particles of a soil, represented by G s in Fig. 9.3, is
the ratio of the average density of the solids to that of water. Specific gravities of
different minerals vary widely. That of borax is only about 1.7, whereas that of the
iron oxide mineral hematite is 5.3, and some less common minerals have much
higher values. The usual preponderance of quartz, with a specific gravity of 2.65,
and feldspars, with values of 2.55 to 2.75, narrows the range in most soils to
between 2.6 and 2.7.

Some values of specific gravities are shown in Table 9.1. The specific gravity of
volcanic ash, which is mostly glass, is low because glass has no regular crystal
structure with requisite packing of atoms. That of the Oxisol is high because of its
iron content.

9.6 FACTORS AFFECTING SOIL UNIT WEIGHT

The theoretical upper limit for unit weight can be calculated assuming a zero void
ratio. With quartz as the soil mineral the upper limit is:
cgs system: Maximum
d ¼ 2:65 Mg=m 3

English units: Maximum
d ¼ 2:65 x 62:4 ¼ 165:4lb=ft 3
SI: Maximum
d ¼ 2:65 x 9:81 ¼ 26:0kN=m 3

Table 9.1 Volcanic ash, Kansas 2.32
Specific gravities of Kaolinite 2.61
solids in selected Alluvial smectite clay 2.65
soils. Platte River sand 2.65
Loess from central U.S. 2.70
Micaceous silt, Alaska 2.76
Oxisol (latosol), Hawaii 3.00

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