Properties of Soil: Fundamental Elements in Soil Mechanics part 1

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.

Here

• Solid
  • Inorganic Minerals
  • Organic Matters
• Liquid
  • Water
• Gases
  • Air

Where,
V = Total volume of soil
V_v = Volume of voids in soil
V_s = Volume of solid
V_w = Volume of water in soil
V_a = Volume of air in soil

From the above phase diagrams,
V = V_v + V_s = (V_a + V_w) + V_s
W = W_v + W_s = (W_a + W_w) +W_s = W_w +W_s

important questions

Q: Why we neglected W_a in the above equation?

Sol: We know, that ρ_air = 1.2 kg/m³, r_water = 1000 kg/m³, ρ_solid = 2600 kg/m³ (i.e ρ_solid > ρ_water > ρ_air) And W_air <<<W_water or W_solid.

Hence, W_air can be neglected

Q: Why we neglected V_a and write V = (V_a + V_w) + V_s ≠ V_v + V_s ?

Sol: No we can't neglect V_a as value of V_a is not very less in compare to V_W and V_s Hence,

V = V_v + V_s = (V_a + V_w) + V_s

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

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 = (w_w/w_s) * 100 = (M_w/M_s) * 100 ]

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

• Relation W_s, W_Total, w: w = (W_w/W_s)
  • W_s = (W/1 + W)

• Water content can also reported in terms of total weight of soil (W)
  • {W' = (wt. of water / wt. of soil) * 100 = (W_w / W) * 100}

NOTE 1

• Range of w' : 0 ≤ w' ≤ 100%
  • w' ≠ 100%
  • ∵ if w' = 100% W_w = W
  • It is not possible in soil mass

NOTE 2

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) = (V_v / V_s) ]

NOTE 3

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.

NOTE 4

Relationship between V_v , V_T and e; e = ( V_v / V_S )

(V_S = V / 1 + e )

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

Denseness ∝ 1 / e

For same total volume, if V_v2 > V_v1 then e₂ > e₁

therefore, (Denseness)₂ < (Denseness)₁

NOTE 5

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.

	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	↑	↓

NOTE 6

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

V_SA + V_SB = V_SC → Volume of solid in soil C

W_SA + W_SB = W_SC → Weight of solid in soil C

NOTE 7

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

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 = (V_v / V) * 100 ]

NOTE 1

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

NOTE 2

Relationship between e and n

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

NOTE 3

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

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IMPORTANT NOTE

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.

IMPORTANT QUESTION

Q: If the porosity of a soil sample is 20%, then the void ratio is__?
(GATE-1997)

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

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