Classification of Soils - GATE Civil Soil Mechanics Free MCQ Test

Fine grained soil are classified on the basis of plastic chart. Clay is found to exist above A-line whereas silt & organic soil found to exist below A-line.

Soil type:
Liquid limit = W_L = 15 %, Plasticity index = I_P = 3 
I_P according to the plasticity chart = 0.73(15 - 20) = - 3.65
And we know I_P cannot be negative, and when it comes out negative, we report it as 0
So, according to plasticity chart, I_P = 0
Firstly,
The actual plasticity limit is 3, so the soil will lie in the region above A-Line
Secondly, 
By plotting the plasticity index (3) and liquid limit (15 %), in the graph, we get the region of the given soil in the graph above
So, the soil is in the region marked as ML in the graph

Concept:
Coefficient of Uniformity / Uniformity Coefficient:
It is defined as the ratio of D₆₀ and D₁₀ sieve sizes in the sieve analysis of granular material.
Higher is the value of C_u larger is the range of the particle size.

For uniformly graded soil, C_u = 1
For well graded sand, C_u > 6
For well graded gravel C_u > 4
Coefficient of Curvature / Curvature Coefficient:

For well graded soil, 1 < C_c < 3
For gap graded soil, 1 < C_c or C_c > 3
where
D₆₀ = size at 60% finer by weight
D₃₀ = size at 30% finer by weight
D₁₀ = size at 10% finer by weight = Effective size

Plasticity chart:

• It is a graph between plasticity index (IP) and liquid limit (WL) in percentage which is used for the classification of fine-grained soils as per the Indian Standard Soil Classification System(ISSCS).
• If more than 50% percent of soil passes through a 75-micron sieve, then it is classified as fine-grained soil.
  
• The 'A' line in this chart is expressed as IP = 0.73 (WL - 20), , where IP = plasticity index , WL = liquid limit
• Depending on the point in the chart, fine soils are divided into clays (C), silts (M), or organic soils (O).
• The organic content is expressed as a percentage of the mass of organic matter in a given mass of soil to the mass of the dry soil solids.
• The 'U' line in this chart is expressed as IP = 0.9(WL - 8).

Clays generally plot above the plasticity chart's A-Line, whereas silts generally plot below it.

If the point gets plotted below A-line, then we need to find out whether the soil is organic or inorganic. If it's inorganic below A-line then it is silt (M)

Concept:

• Plasticity index of soil, I_p = W_L - W_p
• Plasticity index of A-line, I_p = 0.73 (W_L - 20)

Calculation:
Given,
liquid limit (W_L) = 60%
plastic limit (W_p) = 20%

I_p of soil = W_L - W_p = 60 - 20 = 40%
I_p of A-line = 0.73 (W_L - 20) = 0.73 (60 - 20) = 29.2%
I_p of soil > I_p of A-line, So it will lie above A-line
and also W_L > 50%, so soil is CH

Concept:
Soil is also classified based on the mode of deposition or mode of transportation.
Soil deposit formed due to transportation by wind is called Aeolian deposit e.g. Loess

As per IS 383, Table 4(Clause 4.3)

• There are 4 types of grading zones for fine aggregates(sand) namely Grading zone I, II, III, IV.
• All grading zones and their recommended value for different sieve sizes are given below:
  

Concept:

• Plasticity index of soil, I_P = W_L – W_P
• Plasticity index of A-line, I_P = 0.73 (W_L - 20)

Calculation:

Given,
Liquid limit (W_L) = 60%
Plastic limit (W_p) = 20%
I_P of soil = W_L – W_P = 60 - 20 = 40%
I_P of A-line = 0.73 (W_L - 20) = 0.73 (60 - 20) = 29.2%
I_P of soil > I_P of A-line, So it will lie above A-line and also W_L > 50%,
∴ Soil is CH.

Soil is also classified based on the mode of deposition or mode of transportation.
Soil deposit formed due to transportation by wind is called Aeolian deposit e.g. Loess

Soil:

• According to soil engineering the unconsolidated material, composed of solid particles, is produced by the disintegration of rocks.
• The void space between particles may contain air, water, or both.
• The soil particle may contain organic matter.
• On the basis of the geological origin of their constituent sediments, soil can be divided into two main groups those which owe their origin to the physical and chemical weathering of the parent rocks, and those which are chiefly of organic origin.

Physical weathering:

• The agencies responsible for physical weathering are the impact and grinding action of flowing water, ice, wind, and splitting actions of ice, plants, and animals.

Chemical weathering:

• Chemical weathering or decomposition of rocks is caused mainly by oxidation, hydration, carbonation, and leaching by organic acids and water.
• Clay and, to some extent, silts are formed by chemical weathering.

• Plasticity chart gives the relationship between plasticity index and liquid limit.
• Equation of A-line, I_p = 0.73 (WL - 20) , where I_p = plasticity index , W_L = liquid limit
• If the point gets plotted above A-line, classify soil as clay.
• Further,
  • If, 35 > W_L ; classify as C_L (low compressible clay)
  • If, 35 < W_L < 50; classify as CI (intermediate compressible clay)
  • If, 50 < W_L ; classify as CH (high compressible clay)
• If the point gets plotted below A-line, then we need to find out whether the soil is organic or inorganic. If it's inorganic below A-line then it is silt (M)
• Further,
  • If, 35 > W_L ; classify as ML (low compressible silt) or OL (low compressible organic)
  • If, 35 < W_L < 50; classify as MI (intermediate compressible silt) or OI (intermediate compressible organic)
  • If, 50 < W_L ; classify as MH (high compressible silt) or OH (high compressible organic)