PHASE RELATIONSHIPS OF SOIL

Basic soil properties and parameters can be subdivided into physical, index, and

engineering categories. One of the youngest disciplines of civil engineering, soil

mechanics involving the study of soil, its behavior and application as an engineering

material. In soil there are three kinds of common composition, namely the solid, liquid

and gas.

Terzaghi (1948) once said, “Soil Mechanics is the application of laws of mechanics and

hydraulics to engineering problems dealing with sediments and other unconsolidated

accumulations of solid particles produced by the mechanical and chemical disintegration

of rocks regardless of whether or not they contain an admixture of organic constituent.”

The Three Phases of Soil

Three Typical Phase Diagrams of Soil

Referring to figure above, soil is generally three phases material which contains solids

(soil particles), water (in liquid state) and air (in gas state). Within the soil, it contains

solid particles and voids, and whereby the voids contains water and/or air. Phase diagram

that contains all three state called partially saturated (unsaturated soil), while diagram

with solids and water named fully saturated (wet soil condition) and lastly dry soil with

soil solids with lots of voids.

In general, the soil physical properties include mass density, particle sizes, specific

gravity, and water content. Within this physical properties, these are the important terms

used in soil mechanics (as below):

Water content of a soil sample represents the weight of free water contained in the

sample expressed as a percentage of its dry weight.

Degree of saturation of the soil sample is the ratio, often expressed as percentage,

of the volume of free water contained in the sample to its total volume of voids and

has an important influence on soil behavior.

Void ratio to the volume occupied by the soil particles defines the void ratio, in

other words the volume of voids in a mixture divided by the volume of solids.

Porosity which is a measure of the relative amount of openings, voids (air or

gases), is the ratio of void volume to the total volume of soil and it represents the

storage capacity of the geologic material.

Specific gravity of a substance is a comparison of its density to that of water.

The Phases Relationship in Soil

Phase Relation of Soil: Weight and Volume

For defining purposes of the soil physical and index properties, it is more convenient to

represent the soil components (in physical forms) by projecting it into a block or phase

diagram (as figure above). In soil mechanics, the unit weight of soil varies in which

depending on the amount of water contained in the soil. This is often known as the

relationship of weights (W) and volumes (V) in soil.

Formula of Soil Physical Properties

Notes and Legends:

1. Specific gravity, Gs = 2.67 +/- 0.05 for inorganic soils.

2. Unit weight of water = 62.4 lb/ft3 or 1001 kg/m3 for freshwater and 64.0 lb/ft3 or

1026.7 kg/m3 for seawater.

Wtotal, Wt = Total weight of soil mass

Wsolids, Ws = Dry weight of soil mass

Wwater, Ww = Weight of water in soil mass

Vtotal, Vt = Total volume of soil

Vsolids, Vs = Volume of solids in soil mass

Vvoids, Vs = Volume of voids in soil mass

Vwater, Vw = Volume of water in soil mass

The Phases Relationship in Terms of Formulation

Phase Relation of Soil: Weight and Volume Formulas

The phase relations in soil materials, between weight and volume can be further derived

and then produces the above formulas.

Further Formulation of The Phases Relationship in Soil

Formula of Moist Unit Weight

Formula of Dry Unit Weight

Formula of Saturated Unit Weight

The series of soil weight-volume relationship formulated in three categories; moisture,

dry and saturated. Hmm, life could be easier after this…for soil mechanics…

n conclusions, these are the basic fundamentals in terms of relationship in soil as

engineering materials which acquired through soil mechanics subject.