Properties of Soil: Fundamental Elements in Soil Mechanics part 1

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Properties of Soil Fundamental Elements in Soil Mechanics

Properties of Soil: Fundamental Elements in Soil Mechanics

Properties of Soil: Fundamental Elements in Soil Mechanics,' where the intricate nature of soil is unveiled, revealing its essential elements and their pivotal role in understanding the three-phase system (Phase Diagram).

Soil mechanics delves into the fundamental characteristics that govern soil behaviour, composition, and interactions within its three-phase system: solid, liquid, and gas. In this study, we will navigate through the crucial properties defining each phase, shedding light on their influence on soil mechanics and how these elements collectively shape the intricate dynamics of the three-phase system within soil.

properties of soil (phase Diagram)

Soil is a three-phase system which consists of solid, liquid and gaseous matter, that do not occupy separate space but are blended with each other in definite proportion, which in turn governs the property of soil.


  • Solid
    • Inorganic Minerals
    • Organic Matters
  • Liquid
    • Water
  • Gases
    • Air
Three - Phase Diagram
Three - Phase Diagram

V = Total volume of soil
Vv = Volume of voids in soil
Vs = Volume of solid
Vw = Volume of water in soil
Va = Volume of air in soil

From the above phase diagrams,
V = Vv + Vs = (Va + Vw) + Vs
W = Wv + Ws = (Wa + Ww) +Ws = Ww +Ws

important questions

Q: Why we neglected Wa in the above equation?

Sol: We know, that ρair = 1.2 kg/m3, rwater = 1000 kg/m3, ρsolid = 2600 kg/m3 (i.e ρsolid > ρwater > ρair) And Wair <<<Wwater or Wsolid.

Hence, Wair can be neglected

Q: Why we neglected Va and write V = (Va + Vw) + Vs ≠ Vv + Vs ?

Sol: No we can't neglect Va as value of Va is not very less in compare to VW and Vs Hence,

V = Vv + Vs = (Va + Vw) + Vs

In some limiting cases, Soil can be represented as a two phase diagram as shown below.

Two - Phase Diagram
Two - Phase Diagram

Properties of Soil

I) Water Content (w)/ Moisture Content:

It is defined as the weight of water to the weight of solid, present in the given soil mass and expressed in percentage.

[ W = (wt. of water/wt. of solids) * 100 = (ww/ws) * 100 = (Mw/Ms) * 100 ]

Range of W: W ≥ 0% { i,e w can be greater than 100% also}

  • Relation Ws, WTotal, w: w = (Ww/Ws)
    • Ws = (W/1 + W)
  • Water content can also reported in terms of total weight of soil (W)
    • {W' = (wt. of water / wt. of soil) * 100 = (Ww / W) * 100}


  • Range of w' : 0  w'  100%
    • w' 100%
    • if w' = 100% Ww = W
    • It is not possible in soil mass


Relation in W and W' : W' = ( W / 1 + W) or W = ( W' / 1 + W')

Important Notes

Weight of solids is comparatively more stable in comparison to weight of soil, as it does not change with changes in the weight of water.
Hence engineering significance of w is more than w'.

ii) Void Ratio (e):

It is defined as the ratio of volume of voids to the volume of solids in given soil mass:

[ e = (Volume of Voids / Volume of solids) = (Vv / Vs) ]


Range of e: e > 0 ( i.e, it can be greater than than 1 also)

Important Notes

Volume of voids cannot be zero in a soil. For soil to be a system it should have at least two phases, i.e. either dry soil (solid + air) or saturated soil (soil + water). However, if there is only solid content (zero voids),then it is a rock.


Relationship between Vv , VT and e; e = ( Vv / VS )

(VS = V / 1 + e )

Void ratio can also be used to represent the degree of denseness of soil.

Denseness ∝ 1 / e

Denseness, Void Ratio
Denseness, Void Ratio

For same total volume, if Vv2 > Vv1 then e2 > e1

therefore, (Denseness)2 < (Denseness)1


Though volume of one void is more for coarse grained soil, total volume of voids in more for fine grained soil. Hence void ratio, water content of fine-grained soil is more than coarse-grained soil.

Comparison of volume of voids
Comparison of volume of voids
Fine Grain (i)Coarse Grain (ii)
Size of Void ( size of void ∝ size of solid ) 
Volume of one void 
Number of void (nvoid∝ nsolid ) 
Total Volume of voids 


Wherever two or more soil samples are mixed, quantity (weight and volume) of solids will not change in final soil (because of mass conservation).

Soil A + Soil B = Soil C

VSA + VSB = VSC → Volume of solid in soil C

WSA + WSB = WSC → Weight of solid in soil C


Quantity (volume + weight) of solids remains same, if soil is displaced from one place to another place.

Soil Mass Conservation
Soil Mass Conservation

Q: If a soil sample-A of weight 1 kg and water content 100% is mixed with another soil sample-B having same weight but water content is 50%, then what is the net water content for mixed soil sample C?

iii) porosity (n):

It is defined as the ratio of volume of voids to the volume of soil present in the given soil mass.

[ n = (Volume of voids / Total volume of soil) * 100 = (Vv / V) * 100 ]


Range of n: 0 < n < 100% ( n ≠ 100% if n = 100%, i.e Vv = V, it is not possible in soil mass


Relationship between e and n

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


Both e and n signifies the water storage capacity of soil as they represent volume of voids in it.

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Soil solid volume remains more or less constant, while the total soil volume is dynamic in nature because it is, composed of air, water, and, volume, which varies due to environmental conditions. Hence in engineering practices, void ratio is more significant than porosity.


Q: If the porosity of a soil sample is 20%, then the void ratio is__?

Sol: given n = 20% or 0.2, then e = ?

e = n / ( 1 - n ) = 0.2 / ( 1 - 0.2 ) = 0.2 / 0.8 = 0.25

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