Nature and Phase Properties of Unsaturated Soil - Unsaturated Soil Mechanics in Engineering Practice - page 67

where: 2.4.2 Void Ratio

Void ratio e is defined as the ratio of the volume of voids,
V v , to the volume of soil solids, V s :

n w = water porosity, %, and

n a =

air porosity, %.

V v
V s

e = (2.53)

The volume associated with the contractile skin can be
assumed to be negligible or part of the water phase.
The water porosity n w , expressed in decimal form, is
synonymous with the volumetric water content θ , used in
unsaturated soil mechanics The volumetric water content
notation is used throughout this topic, primarily because of
its association with the SWCC. Much of the early research
related to the SWCC has been conducted within the soil
physics and agriculture disciplines where the amount of
water in the soil has been defined in terms of volumetric
water content. Consequently, it has become necessary
for geotechnical engineers to become familiar with both
the mass and volumetric definitions of water content.
Typical values of porosity for some soils are given in
Table 2.16.

The relationship between overall soil porosity and void
ratio can be obtained by equating the volume of voids, V v
in Eq. 2.50 and Eq. 2.53:

e

n = (2.54)

1 + e

Typical values for void ratio are shown in Table 2.16.

2.4.3 Degree of Saturation
The percentage of the void space which contains water is
expressed as the degree of saturation S (%):

V w ( 100 )
V v

S = (2.55)

Table 2.16 Typical Values of Porosity, Void Ratio, and Dry Density

Density, ρ (kg/m 3 )

Void Ratio, e Porosity, n (%)

Soil Type maximum minimum maximum minimum maximum minimum

Granular Materials: Uniform Materials
(a) Equal spheres (theoretical values)

0.92 0.35 47.6 26.0 — —

(b) Standard Ottawa sand 0.80 0.50 44.0 33.0 1762 1474

(c) Clean, uniform sand (fine or medium) 1.0 0.40 50.0 29.0 1890 1330

(d) Uniform, inorganic silt 1.1 0.40 52.0 29.0 1890 1281

Granular Materials: Well-Graded Materials
(a) Silt sand

0.90 0.30 47.0 23.0 2034 1394

(b) Clean, fine to coarse sand 0.95 0.20 49.0 17.0 2210 1362

(c) Micaceous sand 1.20 0.40 55.0 29.0 1922 1217

(d) Silt sand and gravel 0.85 0.14 46.0 12.0 2239 1426

Mixed Soils
(a) Sandy or silty clay 1.8 0.25 64.0 20.0 2162 961

(b) Skip-graded silt clay with stones or
rock fragments

1.0 0.20 50.0 17.0 2243 1346

(c) Well-graded gravel, sand, silt, and
clay mixture

0.70 0.13 41.0 11.0 2371 1602

Clay Soils
(a) Clay (30-50% clay sizes) 2.4 0.50 71.0 33.0 1794 801

(b) Colloidal clay ( − 0 . 002 mm ≥ 50%) 12.0 0.60 92.0 37.0 1698 308

Organic Soils
(a) Organic silt

3.0 0.55 75.0 35.0 1762 641

(b) Organic clay (30-50% clay sizes) 4.4 0.70 81.0 41.0 1602 481

Source: Modified from Hough, 1969.
Note: Tabulation is based on G s =

2 . 65 for granular soils, G s = 2 . 70 for clays, and G s = 2 . 60 for organic soils