Basic Definitions In Geotechnical Engineering

A soil mass consists of solid particles which form a porous structure. The voids in the soil mass may be filled with air, water or partly with water and partly with water. Soil is a three phase system in general.

Volumetric Relationship

Void Ratio

Void ratio is the volume of voids to the volume of solids. It is denoted by ‘e’

e=Vv/Vs

It is expressed as a decimal.

Porosity

It is defined as the ratio of volume of voids to the total volume. It is denoted by ‘n’

n=Vv/V

It is generally expressed as a percentage

1/n=V/Vv= (Vv+vs)/Vv

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

                             n=e/ (1+e)                              (a)

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

                               e=n/ (1-n)                               (b)

In equations (a) and (b), the porosity should be expressed as a ratio and not percentage.

Degree of saturation

The degree of saturation is the ratio of the volume of water to the volume of voids. It is denoted by ‘S’.

S=Vw/Vv

The degree of saturation generally expressed as a percentage. It is equal to zero when the soil is absolutely dry and 100% when the soil is fully saturated.

Percentage air voids

It is the ratio of volume of air to the total volume.

na= Va/V

It is also expressed as a percentage.

Air content

Air content is defined as the ratio of the volume of air to the volume of voids

ac= Va/Vv

Also,

na=n* ac

Water content

The water content (w) is defined as the ratio of the mass of water to the mass of soilids

w= Mw/Ms

It is also known as the moisture content (m). It is expressed as a percentage but used as a decimal in computation.

Volume-Mass Relationships

1. Bulk Mass Density

The bulk mass density (ρ) is defined as the total mass (M) per unit volume (V)

ρ=M/V

2. Dry Mass Density

The dry mass density (ρd) is defined as the mass of solids per unit total volume

ρd= Ms/V

3. Sasturated Mass density

The saturated mass density (ρsat) is the bulk density of the soil when it is fully saturated

ρsat= Msat/V

4. Submerged Mass density

When the soil exists below water, it is in a submerged condition. The submerged mass density (ρ’) of the soil is defined as the submerged mass per unit total volume.

ρ’= Msub/V

5. Mass Density Of Solid

The mass density of solids (ρs) is equal to the ratio of the mass of solids to the volume of solids

ρs=Ms/Vs

Volume Weight Relationship

Bulk Unit Weight (γ)= W/V

DRY Unit Weight (γd) = Ws/V

Saturated Unit Weight (γsat) = Wsat/V

Submerged Unit Weight (γsub or γ’) = Wsub/V

Unit WeightT Of Soil Solids (γs) = Ws/Vs

Specific Gravity of Solids

The specific gravity of soil particles (G) is defined as the ratio of the mass of a given volume of solids to the mass of an equal volume of water at 4° C.

G = ρs/ρw

The mass density of water ρw at 4°C is 1gm/ml, 1000 kg/m3 or 1 Mg/m3