Page 166 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
Particle Size and Gradation 161

Example 7.7
What is the textural classification for the soil in Section 7.5.3?

Answer: The soil contains 31% sand and 49% silt. These figures are adjusted for the 4%
gravel content: 31/0.96 ¼ 32.6% sand and 49/0.96 ¼ 51.0% silt. The texturally is ‘‘silty clay
loam.’’

7.8 SPECIFIC GRAVITY OF SOIL PARTICLES

7.8.1 Definition and Use
Specific gravity is defined as the density of a material divided by the density of
water at 48C, which is water at its densest. According to eq. (7.3) the specific
gravity is required in order to interpret settlement analyses. Some representative
specific gravities for different minerals are shown in Table 7.4. Most sands have
a specific gravity of 2.65–2.68; most clays, 2.68–2.72.

7.8.2 Measurement

A common method for measuring the specific gravity of a large object is to weigh
it in air and then submerge it in water. The difference equals the weight of the
water displaced, a discovery made by Archimedes in his search for a way to
determine the purity of gold. The weight divided by the weight lost therefore

Gold 19.3 Terribly expensive Table 7.4
Silver 10.5 Pocket change Specific gravities of
Galena (PbS) 7.5 Cubes that look like silver but aren’t some selected solids
Pyrite (FeS 2 ) 5.0 Cubes that look like gold but aren’t
Hematite (Fe 2 O 3 ) 4.9–5.3 Red iron oxide in soils
Limonite (Fe 2 O 3 nH 2 O) 3.4–4.3 Yellow or brown iron oxides in soils
Iron silicate minerals 2.85–3.6 Dark minerals in basalt, granite
Calcite (CaCO 3 ) 2.72 Most abundant mineral in limestone
Micas 2.7–3.1 Flakey
Quartz(SiO 2 ) 2.65 Most abundant mineral in soils
Feldspar (Na and Ca silicates) 2.55–2.65 Most abundant mineral in rocks
Kaolinite 2.61 Clay mineral
Smectites 2.2–2.7 Expansive clay minerals
Glass 2.2–2.5 Lead glass ¼ 3
Halite (NaCl) 2.1–2.3 Rock salt
Liquid water (H 2 O) 1.00 At its densest, 48C
Ice (H 2 O) 0.918 Floats on liquid water

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