1.
Soil Mechanics
Topic: Phase Analysis of Soil
Presented by,
Suvadeep De (13001320032)

2.
Introduction to Soil Mechanics
The term ‘soil’ according to engineering point of view is defined as the material, by
means of which and upon which engineers build their structures.
And Soil Mechanics is a branch of Civil Engineering involving the studies about the
properties and behaviour of soils in the action of various forces and its applications as
an engineering material.

3.
Formation of Soil
Soil is formed due to the weathering of rocks by physical and chemical processes.
Exposed parts of rocks and decayed by that processes and coarse aggregates are
made. That aggregates then are transferred to another places by physical carriers and
mixed with various organic and inorganic things and then it turned into the thick level,
that’s soil.

4.
What’s Phase Diagram?
The diagrammatic representation of different phases separated in soil mass is called as
phase diagram.
If the soil is partially saturated (air, water, and solid included), that diagram is called
Three Phase Diagram.
And if the soil is completely saturated (water and solids) or completely dry (air and
solids, eg.- oven dried soil), that’s called Two Phase Diagram.

5.
Three Phases of Soil and its Diagram
Soil mass is generally referred to as three phase system because it consists of solid particles,
liquid and gas.
Where, Wa= Weight of air part
Ww= Weight of water part
Ws= Weight of solid part
Va= Volume of air part
Vw= Volume of water part
Vs= Volume of solid part

6.
Two Phase Diagrams of Soil
Completely Saturated Soil Completely Dry Soil

7.
Volumetric Ratios
(1) Void ratio, e= Volume of Voids/volume of solids = vv/vs
(2) Porosity, n%= (Volume of voids/Total volume of soil)x100% = (Vv/Vt)x100%
(3) Degree of Saturation, S% (0–100%)= (Volume of water/Volume of voids)x 100%
=(Vw/Vv)x100%
(4) Water Content, (W%)= (Weight of water/Weight of solid part)x100%
= (Ww/Ws)x100%

8.
Soil Unit Weights
Unit weight or weight density of soil refers to its weight per cubic meter volume Unit=
KN/m3
1. Dry unit weight, γd= weight of soil solids/total volume of soil= Ws/Vt
2. Total/Bulk/Wet/moist unit weight, γ = Total weight of soil/Total volume of
soil
= Wt/Vt
= (Ws+Ww)/Vt
Or, γ = (Ws/Vt) + (Ww/Vt)
= γd+(Ww/Vt)

9.
3. Saturated Unit Weight
γsat = (Weight of solid + weight of water)/Volume of soil= (Ws+Ww)/Vt
= Total weight of saturated soil/Vt
4. Submerged/ Buoyant Unit Weight
When the soil is submerged in the water, water pushes the soil particles upwards, that force is
called Buoyant force.
γ’= γsat- γw
Or, γ’=
(Ws)sub
𝑉𝑡
where, (Ws)sub= Submerged soil solids weight
=Ws (in air)–weight of solid displaced by water

10.
Relations Between the Properties
1. Specific Gravity, Gs=
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑜𝑖𝑙 𝑠𝑜𝑙𝑖𝑑𝑠 𝑜𝑓 𝑎 𝑣𝑜𝑙𝑢𝑚𝑒
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑎𝑡 4℃ 𝑜𝑓 𝑠𝑎𝑚𝑒 𝑣𝑜𝑙𝑢𝑚𝑒
=
𝑈𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑
𝑈𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑎𝑡 4℃
=
γ𝑠
γ𝑤
=
𝑊𝑠
𝑉𝑠
.γ𝑤
as,( γs=
𝑊𝑠
𝑉𝑠
)
2. Relation between e and n: e=
𝑛
𝑛−1
; n=
𝑒
1+𝑒
3. Relation between e ,S, W, and Gs:
S.e=W.Gs as W=
𝑉𝑤
𝐺𝑠
𝑉𝑠
4. Relation between γ, e, S and Gs:
γ=
𝑆𝑒+𝐺𝑠
1+𝑒
γw

11.
Example
The moist unit weight of a soil is 19.2 kN/m2. Given that G, 2.69 and w = 9.8%,
determine a. Dry unit weight b. Void ratio c, Porosity d, Degree of saturation.
Ans. γ= 19.2 KN/m3 Gs= 2.69 W= 9.8%=
9.8
100
γw= 9.81 KN/m3 S.e=
9.8
100
x2.68 ≅ 0.264
Gs =
γ𝑠
γw
= 2.69x9.81= γs = 26.4 KN/m3
γ= (
𝑠𝑒+𝐺𝑠
1+𝑒
) γw => 19.2= (
0.264+2.69
1+𝑒
) x 9.81
=>1+e= 1.51 => e= 0.51 ∴ S =
0.264
𝑒
=
0.264
0.51
= 0.52
∴ n =
𝑒
1+𝑒
=
0.51
1.51
=0.34.

12.
Conclusion of Soil Phase Analysis
Soil analysis has proved to be very useful in both agriculture and horticulture. In the
forest, however, soil analysis has seldom proved to be of consistent value. In part this is
because the perennial roots of trees, together with their mycorrhizae, seem able to
access forms of nutrient elements not accessible to short-lived arable plants so the
chemical soil extractants developed for agriculture may not be appropriate. Perhaps
more significant, however, is that over time tree roots can exploit all the rooting
volume available to them.