Building Design and Construction Handbook - Merritt - Ventech!

SOIL MECHANICS AND FOUNDATIONS 6.27
calculate the different types of unit weights. Note in Table 6.6 that w � water
content and G � specific gravity of soil solids. The void ratio (e) and degree of
saturation (S) are discussed in the next article.
6.3.4 Phase Relationships
Phase relationships are the basic soil relationships used in geotechnical engineering.
They are also known as weight-volume relationships. Different types of phase relationships
are discussed below:
Void Ratio (e) and Porosity (n). The void ratio (e) is defined as the volume of
voids (V v) divided by the volume of solids (V s). The porosity (n) is defined as
volume of voids (V v) divided by the total volume (V). As indicated in Fig. 6.7, the
volume of voids is defined as the sum of the volume of air and volume of water
in the soil.
The void ratio (e) and porosity (n) are related as follows:
n e
e � and n � (6.5)
1 � n 1 � e
The void ratio and porosity indicate the relative amount of void space in a soil.
The lower the void ratio and porosity, the denser the soil (and vice versa). The
natural soil having the lowest void ratio is probably till. For example, a typical
value of dry density for till is 2.34 Mg/m 3 (146 pcf), which corresponds to a void
ratio of 0.14. A typical till consists of a well-graded soil ranging in particle sizes
from clay to gravel and boulders. The high density and low void ratio are due to
the extremely high stress exerted by glaciers. For compacted soil, the soil type with
typically the lowest void ratio is a well-graded decomposed granite (DG). A typical
value of maximum dry density (Modified Proctor) for a well-graded DG is 2.20
Mg/m 3 (137 pcf), which corresponds to a void ratio of 0.21. In general, the factors
needed for a very low void ratio for compacted and naturally deposited soil are as
follows:
1. A well-graded grain-size distribution
2. A high ratio of D 100/D 0 (ratio of the largest and smallest grain sizes)
3. Clay particles (having low activity) to fill in the smallest void spaces
4. A process, such as compaction or the weight of glaciers, to compress the soil
particles into dense arrangements
At the other extreme are clays, such as sodium montmorillonite, which at low
confining pressures can have a void ratio of more than 25. Highly organic soil,
such as peat, can have even higher void ratios.
Degree of Saturation (S). The degree of saturation (S) is defined as:
100 V w
S(%) � (6.6)
V v
The degree of saturation indicates the degree to which the soil voids are filled