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, involves the study of soil, its behaviour and its application as an engineering material. In soil, there are three kinds of standard composition: 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

soil is generally a three-phase 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 contain water and/or air. The phase diagram that contains all three states is called partially saturated (unsaturated soil), while the diagram with solids and water is called fully saturated (wet soil condition) and lastly, dry soil with soil solids with lots of voids.

The soil's physical properties generally include mass density, particle sizes, specific gravity, and water content. Within these physical properties, these are the essential terms used in soil mechanics (as below):

• The 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 a percentage, of the volume of free water, contained in the sample to its total volume of voids and has an important influence on soil behaviour.
• 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 and 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 produced above formulas.

Further Formulation of The Phases Relationship in Soil

The series of soil weight-volume relationships is formulated in three categories: moisture, dry and saturated. Hmm, life could be easier after this…for soil mechanics…

In conclusion, these are the basic fundamentals in terms of the relationship between the soil and engineering materials acquired through soil mechanics subject.