Question

As per the Indian Standard soil classification system, a sample of silty clay with liquid limit of 40% and plasticity index of 28% is classified as

1. CH
2. CI
3. CL
4. CL - ML

Answer 1

Correct Answer - Option 2 : CI

Concept: 

It is adopted by IS code. It was given by A-line Casagrande. It uses particle size distribution for Coarse soils and plasticity for fine Soils. 

​Equation of A-line I_P  = 0.73 (W_L- 20)

Soil classification as per Indian standards:

Coarse-grained soils (More than half of the material is larger than 75 μ sieve)
Gravels				Sands
More than half of the coarse fraction is larger than 4.75 mm sieve				More than half of the coarse fraction is smaller than 4.75 mm sieve
Clean gravel		Gravel with appreciable fines		Clean sands		Sand with appreciable fines
GW	GP	GM	GC	SW	SP	SM	SE
Fine-grained soil (More than half of the material is smaller than 75 μ sieve)
Silts and clay
Low compressibility (wL < 35)			Medium compressibility (50 < wL > 35)			Higher compressibility (wL > 50)
ML	CL	OL	MI	CI	OI	MH	CH	OH
High organic peat soil → (Pt)

 

∴ According to IS Classification, there are 18 groups of soils:

8 groups of coarse-grained, 9 group of fine-grained, and 1 of peat

Calculations:

Given,  

LL =  40%  and   PI  =  28%    

Now it is used,

As per , PI = 0.73 ( LL-20) = 0.73(40 - 20)  = 14.6

I_P of soil  > I_P of  A –line

It will lie above A-line and also 35 < W_L < 50

So it is CI.

Equation of U- line IP  =  0.9 (WL - 8)  

Note:

 For IPs between 4,  and 7, dual Symbols are used.