Page 192 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Density and Unit Weight
Soil Density and Unit Weight 187

‘‘Sudden drawdown’’ is when a submerged soil is left high and wet, as during
receding flood levels. This converts a submerged unit weight to a saturated unit
weight, which approximately doubles the effective weight of the soil. Collapse of
river banks therefore occurs during waning stages of a flood and not during the
flood. The same situation can occur in a lake if the level is rapidly lowered, which
is an important consideration in reservoir management as a sudden drawdown
can create sudden landslides.

9.4.3 Summary of Use of the Block Diagram

A general procedure is as follows:
The wet unit weight and moisture content of a soil are measured.
Dry unit weight is calculated using eq. (9.7).
The two unit weights are written on a block diagram.
A mineral specific gravity is assumed or measured and its unit weight calculated.
The volume of solids is calculated from the dry unit weight and mineral unit
weight.
The volume of solids is subtracted from 1.0 to obtain a volume of voids.
The void ratio is calculated from eq. (9.1).
To predict the saturated unit weight, assume that all voids are filled with water
and calculate its weight from the volume and unit weight of water, then add
this weight to the dry unit weight to obtain a saturated unit weight.
The submerged unit weight is calculated from eq. (9.8).

A block diagram can be used equally effectively with the cgs or SI systems of
3 3
measurement with the unit weight of water equal to 1.0 g/cm or 1 Mg/m ,or
3
9.807 kN/m , respectively. Specific gravity values are dimensionless and the same
in any system.

9.4.4 Other Measures Related to Density and
Moisture Content

Porosity is defined as the volume of pores divided by the volume of voids, in
percent:

V v
n ¼ 100 ð9:9Þ
V
Porosity is related to the void ratio by the following formula:

e
n ¼ 100 ð9:10Þ
1 þ e

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