# What do D60, D30 and D10 mean in soil? What do Cu & Cc refer in Soil mechanics?

First, let us take a look at this **grain size distribution curve.**

In this curve,

*x axis – *Particle size(dia) in mm in log scale

*y axis* – Percentage finer.

From this curve, we can find the **D60**, **D30** and **D10** of 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 **Cu** and **Cc**.

Cu – *Uniformity coefficient.*

Cu = D60/D10.

Cc – *Coefficient of curvature*.

Cc = (D30)^2 /(D60)(D10).

**What is the use of this Cu and Cc?**

**Cu** is always **greater 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 are* *in the same size range*), the soil is considered as **uniformly graded**.

If **Cu** is **away from 1**, the soil is **well graded**(ie. *it has a vareity of size range distributed well*). For *gravel*, if Cu>4, it is well graded. For *sand*, if Cu>6, it is well graded.

**Cc** is also **greater than 1 **( equal to 1 is possible only by theoretical). For a well graded *soil*, 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.- For a
*uniformly graded*soil, the grain distribution curve is*almost vertical*. For a*well graded*soil, the curve has a*good slope*. For a*gap graded soil*, the curve is almost*horizontal*in between any two points.