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Soil Classification

Soil Classification

Soil is basically classified as Course grain soil and Fine grain soil, where Course grain consist of Gravel (G) and Sand (S) where as in Fine grain we have Silt (M) and Clay (C).
Course Grain:
Here we will be classifying it further with help of Coefficient of Uniformity (Cu) and Coefficient of Curvature (Cc),
Sieve size distribution,
Percentage finer (%f) i.e. percentage of total soil which passes 75 micron sieve.

So First will be Gravel
If 50% or more is retained on 4.75 mm sieve this is termed as "G".

When %f < 5% and Cu>4 and 1<Cc<3 than soil is Well graded as said as Well graded Gravel "GW".

When %f < 5% but Cu and Cc doesn't meet the criteria than it is Poorly Graded and said as Poorly Graded Gravel "GP".

When %f > 12% and Ip lies above A line , soil is Clayey and said as Clayey Gravel "GC".

When %f > 12% and Ip lies below A line, soil is Silty and said as Silty Gravel "GM".

If 5% < %f <12% the it is combination of Silty and Clayey soil and written as "GW-GM" "GW-GC" "GP-GM" "GP-GC".


So now Sand
If 50% or more passes 4.75mm sieve and gets retained on 75 micron sieve it comes under Sand "S".

When %f < 5% and Cu>6 and 1<Cc<3 than soil is Well graded as said as Well graded Sand "SW".

When %f < 5% but Cu and Cc doesn't meet the criteria than it is Poorly Graded and said as Poorly Graded Sand "SP".

When %f > 12% and Ip lies above A line , soil is Clayey and said as Clayey Sand "SC".

When %f > 12% and Ip lies below A line, soil is Silty and said as Silty Sand "SM".

When 5% < %f <12% the it is combination of Silty and Clayey soil and written as "SW-SM" "SW-SC" "SP-SM" "SP-SC".

civil engineering soil classification and its details


Now in Fine Grain soil we have threes types which are
Highly compressible (H)
Intermediate compressible (I)
Low compressible (L)

When Ip value lies above A line and
water content is more than 50% it is Highly Compressible Clay "CH".

water content is between 35% and 50% it is Intermediate Compressible Clay "CI".

water content less than 35% is Low Compressible Clay "CL".


When Ip value lies below A line and
water content is more than 50% it is Highly Compressible Silt "MH"

water content is between 35% and 50% it is Intermediate Compressible Silt "MI".

water content less than 35% is Low Compressible Silt "ML".

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