Page 189 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Density and Unit Weight
184 Geotechnical Engineering

Example 9.2
Calculate wet and dry unit weights using SI.
Answer: The wet and dry weights are 13.52 N and 11.43 N respectively. The sample volume
3
3
is 695(10) 6 m . The respective unit weights are 19.5 and 16.4 kN/m .
9.2.4 The Void Ratio, e
A convenient measure of a soil dry density that is not affected by specific gravity
of the soil particles or by the moisture content is the void ratio, e, which is defined
as the volume of voids divided by the volume of solids. The volume of voids is the
combined volume of water and air. Then by definition,
V v
e ¼ ð9:1Þ
V s
where e is the void ratio, V v the volume of voids, and V s the volume of voids.
This is one of only a few formulas in geotechnical engineering that should be
committed to memory.

9.2.5 Formula for Moisture Content, w
As previously noted, moisture content is defined as the weight (or mass) percent
water referenced to the dry weight (or mass) of the soil. By definition,
W w
w ¼ 100 ð9:2Þ
W s
where w is the moisture content in percent, W w is the weight of water, and W s is
the weight of the solids.

Example 9.3
Determine the moisture content in Example 9.1.

Answer: First subtract the dry weight from the wet weight to obtain the weight of water:
3.04 – 2.57 ¼ 0.47 lb. Then w ¼ 100 (0.47/2.57) ¼ 18.3%. A similar calculation can be
made using grams weight or Newtons.

9.3 DRY UNIT WEIGHT

One of the most frequently used formulas in geotechnical engineering converts a
measured wet unit weight to a dry unit weight. With reference to Fig. 9.3, the
definition for wet unit weight is
W W s þ W w

w ¼ ¼ ð9:3Þ
V V

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