Basic Definitions and Simple Tests on Soil Mechanics

Introduction

This page discuss some of the basic definitions and simple tests used throughout the subject.

• The phase diagram is a simple, diagrammatic representation of a real soil.
• The phase diagram is also known as block diagram.
• A soil mass consists of solid particles, water and air, which are segregated and placed separately, known as three-phase system.
• A three-phase system becomes a two-phase system when the soil is absolutely dry (solids + air) or when the soil is fully saturated (solids + water).
• In phase diagram, volumes are represented on the left side, whereas weights are represented on the right side.

Volumetric Relationships

In total, there are fi ve volumetric relationships. These are as follows:

1. Void ratio (e):

• It is defi ned as the ratio of volume of voids to volume of solids.
• Range: e > 0
• e = Vv/Vs
• For some soils, it may have a value even greater than unity.
• The void ratio of coarse grained soils is, generally, smaller than that of a fi ne-grained soil.

2. Porosity (n):

• It is defined as the ratio of volume of voids to the total volume.
• n Vv/V
• Range: 0 < n < 1
• Also called ‘percentage voids’.
• Both porosity and void ratio are the measure of the degree of denseness (or looseness) of soil. Relationship between n and e:
• e = n /(n-1)
• n = e /(e+1)

3. Degree of saturation (Sr ):

• It is defined as the ratio of volume of water to the volume of voids, in soil.
• It is expressed as a percentage.
• Sr = Vw/Vv
• Range: 0 ≤ S ≤ 100%
• For dry soil, V w = 0 ⇒ S = 0
• For saturated soil, V w = V v ⇒ S = 100%

4. Percentage air voids (na ):

• It is defined as the ratio of volume of air to total volume, of soil.
• na=Va/V
• Range: 0 ≤ n a ≤ n

5. Air content (ac ):

• It is defined as the ratio of volume of air to the volume of voids, in soil.
• ac=Va/Vv
• Range: 0 ≤ a c ≤ 100%
• For dry soil, V a = V v ⇒ a c = 100%
• For saturated soil, V a = 0 ⇒ a c = 0
• Relationships between a c , n a , n and s:
• ac + S =1
• na= n*ac