Concept:

An Aluminium ion has 3 positive charges and one silicon ion has 4 positive charges.

When one silicon ion replaced by one aluminum ion, there would be deficiency of one unit (4 – 3 = 1) positive charge per substitution since silicon ion has more positive charge then aluminum ion.

The soils formed at a place may be transported to other places by agents of transportation, such as water, wind, ice and gravity.

1) Water transported Soils: Flowing water is one of the most important agents of transportation of soils. Swift running water carries a large quantity of soil either in suspension or by rolling along the bed.

The size of the soil particles carried by water depends upon the velocity.

All type of soils carried and deposited by water are known as alluvial deposits. Deposits made in lakes are called lacustrine deposits. Such deposits are laminated or varved in layers. Marine deposits are formed when the flowing water carries soils to ocean or sea.

2) Wind transported Soils: Soil particles are transported by winds. The particle size of the soil depends upon the velocity of wind. Soils deposited by wind are known as aeolian deposits.

Loess is a silt deposit made by wind. These deposits have low density and high compressibility. The bearing capacity of such soils is very low.

3) Glacier-Deposited Soils: Glaciers are large masses of ice formed by the compaction of snow. As the glaciers grow and move, they carry with them soils varying in size from fine grained to huge boulders.

Drift is a general term used for the deposits made by glaciers directly or indirectly. Deposits directly made by melting of glaciers are called till. The soil carried by thw meling water from the frint of a glacier is termed out-wash.

Clay-Matrix Structure:

• Clay-matrix structure is a composite structure formed by clay particles is very large as compared with bulky, coarse-grained particles. The clay forms a matrix in which bulky grains appear floating without touching one another.
• Soils with a clay-matrix structure have almost the same properties as clay. They are more stable, as disturbance has very little effect on the soil formation with a clay matrix structure.

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Concept:

The specific surface area is defined as the total surface area of any material per unit mass or volume.

Specific surface \(= \frac{{Surface\;Area}}{{Volume}}\)

Calculation:

Surface area = 10 cm², Volume = 10 cc – 6 cc = 4 cc

∴ Specific surface area = \(\frac{{10}}{4} = 2.5/cc\)

The clay particles carry a net negative charge on their surfaces. This is the result both of isomorphous substitution and of a break in continuity of the structure at its edges. In dry clay, the negative charge is balanced by exchangeable cations like Ca²⁺, Mg²⁺, Na⁺, and K⁺ surrounding the particles being held by electrostatic attraction. When water is added to clay, these cations and a few anions float around the clay particles. This configuration is referred to as a diffuse double layer.

The cation concentration decreases with the distance from the surface of the particle. The force of attraction between water and clay decreases with distance from the surface of the particles.

When two clay particles in suspension come close to each other, the tendency for interpenetration of the diffuse double layers results in repulsion between the particles. At the same time, an attractive force exists between the clay particles that is caused by van der Waals forces and is independent of the characteristics of water. Both repulsive and attractive forces increase with decreasing distance between the particles, but at different rates. When the spacing between the particles is very small, the force of attraction is greater than the force of repulsion.

Concept:

Classification of Coarse-grained soil is done on the basis of

(1) Grain Size

(2) Gradation characteristics 

(3) Percentage of fines present in soil by weight (fines means particle size < 75 μ)

Case – 1: When fines are less than 5%

Case-2 : When fines are. between 5-12%:-

• Dual symbols are used. The first part of the dual symbol indicates gradation (SW), where as second part indicates the nature of fines (SC).
• The soil having dual symbol SW — SC signifies well-graded sand with clay as fines which is plotted above A-line on Plasticity chart.

The Classification can be done as follows:

1. GW — GC → Well graded gravel containing clay as fines
2. GP — GC → Poorly graded gravel containing clay as fines
3. GW — GM → Well graded gravel containing Silt as fines
4. GP — GM → Poorly graded gravel containing Silt as fines
5. SW—SC → Well graded sand containing clay as fines
6. SP—SC → Poorly graded sand containing clay as fines
7. SW — SM → Well graded sand containing silt as fines
8. SP — SM → Poorly graded sand containing silt as fines.

Case – 3: When fines are greater than 12%:-

(1) Gravel:

(a) Fineness > 12% & I_p < 4% e.g GM → Silty Gravel

(b) Fineness > 12% & I_p > 7% e.g. GC → Clayey Gravel

(2) Sand:

(a) Fineness > 12% & I_p  < 4% e.g. SM → Silty Sand

(b) Fineness > 12%, & I_p > 7% e.g. SC → Clayey Sand

If I_p is between 4 – 7 then again Dual symbol are used.

Calculation:

% fines \(= \frac{2}{{10}} \times 100 = 20\%\)

∴ % fines > 12%

Gravel fraction (> 4.75 mm) = 3.0 kg

Sand fraction (75 μ – 4.75 mm) = 5 kg

∵ Sand fraction > Gravel fraction, Thus the soil can be classified as sand.

I_p = ω_L - ω_p = 40 – 20 = 20%

I_p > 7% and fineness > 12%

∴ Soil is clayey sand i.e SC.

Concept:

Soil classification as per Indian standards:

Computation:
% retained on 75 μ = 100 – 5 = 95% > 50 > Thus soil is classified as coarse grained soil.

Since the soil ranges between 4.75 mm to 75 μ, thus it a sandy soil.

Here fines are less than 5%.

For classification of finer soil D₆₀, D₃₀, and D₁₀ are required to be computed.

D₆₀ = 600 μ, D ₃₀ = 222 μ, and D₁₀ = 150 μ

Coefficient of uniformity \(\left( {{{\rm{C}}_{\rm{u}}}} \right) = \frac{{{{\rm{D}}_{60}}}}{{{{\rm{D}}_{10}}}} = \frac{{600{\rm{\mu }}}}{{150{\rm{\mu }}}} = 4< 6\) 

Coefficient of curvature \(\left( {{{\rm{C}}_{\rm{c}}}} \right) = \frac{{{\rm{D}}_{30}^2}}{{{{\rm{D}}_{60}} \times {{\rm{D}}_{10}}}} = \frac{{{{222}^2}}}{{600 \times 150}} = 0.547 < 1\)

For well graded soil: C_u > 6 & 1 < C_c < 3

For Soil A:

More than 50% of the soil fraction passed through 0.075 mm Is sieve is known fine Grained soil

I_P(A-line) = 0.73 (W_L - 20)

I_P = 0.73 (45 - 20) = 18.25

I_P for soil = 11

Thus soil lies below A-line of the Plasticity chart. it is known as silt.

W_L for soil lies between 35% to 50% W_L = 45% (Given)

Soil A is silt of Intermediate Compressibility Soil A = MI

For Soil B:

More than 50% of the soil is retained through 0.075 mm Is sieve. It is termed as coarse Grained soil.

Less than 50% of the soil is retained over 4.75 mm Is sieve, so it is sand

% fineness Given = 27% > 12% and I_P = 9% (Given)

i.e. I_P > 7%

So, it is termed as SC soil

Soil B = SC

Concept:

Coefficient of uniformity \(= \frac{{{D_{60}}}}{{{D_{10}}}}\)

Coefficient of curvature \(= \frac{{D_{30}^2}}{{{D_{60}}\; \times \;{D_{10}}}}\)

Calculation:

C_U = 4, C_c = 1.6

\({C_U} = \frac{{{D_{60}}}}{{{D_{10}}}} \Rightarrow 4 = \frac{{{D_{60}}}}{{{D_{10}}}} \Rightarrow {D_{60}} = 4\;{D_{10}}\)

\({C_c} = 1.6 \Rightarrow \frac{{D_{30}^2}}{{{D_{60}}\; \times \;{D_{10}}}} = 1.6 \Rightarrow \frac{{D_{30}^2}}{{4\;{D_{10}}\; \times \;{D_{10}}}} = 1.6\)

Concept:

Group Index (GI): It is used to give rating to the quality of soil within its group.
GI = 0.2a + 0.005ac + 0.01 bd

Where,

a = % passing 75 μ seive – 35 ; a ≯ 40

b = % passing 75 μ seive – 15 ; b ≯ 40

c = w_L - 40 ; c ≯ 20

d = I_p – 10 ; d ≯ 20

GI values ranges from 0 - 20.
GI value 0 → Soil is of Superior quality.

GI value 20 → Soil is of Poor quality.

Calculation:

% Passing 75 μ sieve = 80%, w_L = 70%, w_p = 30%

a = 80 – 35 = 45

But a ≯ 40 ⇒ a = 40

b = 80 – 15 = 65

But b ≯ 40 ⇒ b = 40

Also,

I_p = ω_L - ω_p = 70 – 30 = 40%

\(\therefore c = min\left\{ {\begin{array}{*{20}{c}} {{\omega _L} - 40}\\ {20} \end{array}} \right. = min\left\{ {\begin{array}{*{20}{c}} {70 - 40}\\ {20} \end{array}} \right. = 20\) 

\(d = min\left\{ {\begin{array}{*{20}{c}} {{I_p} - 10}\\ {20} \end{array}} \right.= min\left\{ {\begin{array}{*{20}{c}} {40 - 10}\\ {20} \end{array}} \right. = 20\)   

∴ G.I. = 0.2 a + 0.005 ac + 0.01 bd

= 0.2 × 40 + 0.005 × 40 × 20 + 0.01 × 40 × 20 = 20

The soil is of extremely poor quality as suggested by group index.