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## Varun

First, let us take a look at this

grain size distribution curve.In this curve,

x axis –Particle size(dia) in mm in log scaley axis– Percentage finer.From this curve, we can find the

D60,D30andD10of a particular soil.D60 –

60 % of the soil particles are finer than this size.D30 –

30% of the particles are finer than this size.D10 –

10% of the particles are finer than this size.Above definitions are just self explanatory. For example, if you have 100 particles of diameter ranging from 1 mm to 100 mm, D60 is 61 mm (below which 60% of particles are there).

Why do we find this?To find

CuandCc.Cu –

Uniformity coefficient.Cu = D60/D10.

Cc –

Coefficient of curvature.Cc = (D30)^2 /(D60)(D10).

What is the use of this Cu and Cc?Cuis alwaysgreater than 1(equal to 1 is possible only by theoretical). If Cu is closer to 1 ( ie.D60 and D10 sizes are close to each other, which means there are more no. of particles arein the same size range), the soil is considered asuniformly graded.If

Cuisaway from 1, the soil iswell graded(ie.it has a vareity of size range distributed well). Forgravel, if Cu>4, it is well graded. Forsand, if Cu>6, it is well graded.Ccis alsogreater than 1( equal to 1 is possible only by theoretical). For a well gradedsoil, Cc ranges between 1 to 3.So,

Cu and Cc gives us idea about particle size distribution of a soil. These values are used in the soil classification.Bonus points :

Why log scale is used instead of normal scale?If we use normal scale, it requires veryyyy length sheet to plot the soil size. Soil particles sizes ranges from few micrometers to centimeters range( 10^(-6) to 10^(-2) m ). So, it is impossible to cover this range with normal scale. Hence, log scale is used. And also getting a good curve is possible only with log scale.uniformly gradedsoil, the grain distribution curve isalmost vertical. For awell gradedsoil, the curve has agood slope. For agap graded soil, the curve is almosthorizontalin between any two points.