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

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

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.

Notes and Legends:

1. Specific gravity, G_s = 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.

• W_total, W_t = Total weight of soil mass
• W_solids, W_s = Dry weight of soil mass
• W_water, W_w = Weight of water in soil mass
• V_total, V_t = Total volume of soil
• V_solids, V_s = Volume of solids in soil mass
• V_voids, V_s = Volume of voids in soil mass
• V_water, V_w = Volume of water in soil mass

The Phases Relationship in Terms of Formulation

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

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…

In conclusions, these are the basic fundamentals in terms of relationship in soil as engineering materials which acquired through soil mechanics subject.

Read More Constructive Posts:

• Particle Distribution for Soil Test
• How to Test Moisture Content of Soil
• 7 Info You Should Know About Soil.
• Introductory to Laboratory Testing of Soil

Tagged as: Soil and Testing

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