Mohammad Syarifudin: 3 Phases in Soils

O: 4/X-1/GTK/10

Volume-weight properties

The volume-weight properties of a soil define its state. Measures of the amount of void space, amount of water and the weight of a unit volume of soil are required in engineering analysis and design.

Soil comprises three constituent phases:

Solid: rock fragments, mineral grains or flakes, organic matter.
Liquid: water, with some dissolved compounds (e.g. salts).
Gas: air or water vapour.

In natural soils the three phases are intermixed. To aid analysis it is convenient to consider a soil model in which the three phases are seen as separate, but still in their correct proportions.

Volumes of solid, water and air: the soil model

The soil model is given dimensional values for the solid, water and air components: Total volume,
V = V_s + V_w + V_a

Since the amounts of both water and air are variable, the volume of solids present is taken as the reference quantity. Thus, the following relational volumetric quantities may be defined:

Note also that:

n = e / (1 + e)
e = n / (1 - n)
v = 1 / (1 - n)

Degree of Saturation

The volume of water in soil can only vary between zero (i.e. a dry soil) and the volume of void, this can be expressed as a ratio:

For a perfectly dry soil:
S_r = 0
For a saturated soil:
S_r = 1

Note: In clay soils as the amount water increases the volume and therefore the volume of voids will also increase, and so the degree of saturation may remain at S_r = 1 while the actual volume of water is increasing.

Air - Void Content

The air-voids volume, V_a , is that part of the void space not occupied by water.

Air-voids content, A_v
Gama _v = (air-voids volume) / (total volume)
= Va / V
= e.(1 - S_r) / (1+e)
= n.(1 - S_r)

For a perfectly dry soil:
A_v = n
For a saturated soil:
A_v = 0

Masses of solid and water: water content

The mass of air many ignored. The mass of solid particles is usually particle density or grain spesific gravity.

Grain Spesific Gravity

Particle Density

_{The ratio of mass of water present to mass of} solid particle is called the water content or sometime the moisture content

Densities and Unit weights

Density is a measure of the quantity of mass in a unit volume of material.
Unit weight is a measure of the weight of a unit volume of material.

There are two basic measures of density or unit weight applied to soils: Dry density is a measure of the amount of solid particles per unit volume. Bulk density is a measure of the amount of solid + water per unit volume.