# Angle of Internal Friction

Think of this post as a direct continuation of the previous two posts of this series, shear strength of soil and lateral earth pressure. Now let’s present another very important characteristic of soil, the angle of internal friction, which we briefly mentioned already before in those sections.

Let’s consider a block resting on a surface, where there is its weight and the table has friction.

In the figure above, when we try to push this block, we must apply a certain horizontal force, shown here as T. If we apply less than that, we can not push it. Again, let’s think about the ratio of horizontal to vertical as we just showed.

This angle Ø, is called, “Angle of Internal Friction”.

And if we applied the concept for soil loading it looks like this:

So, this is the angle where the soil fails, when stresses are applied in all directions. Here σ2 would be perpendicular to the page you are looking at and equal to σ3, thus, it is usually not shown. This is also how we test the strength of soils in the lab, in triaxial test, which we will talk about shortly.

Rearranging the equation above, we reach to the famous equation in geotechnical engineering, which relates shear stress to normal stress as:

And if we put it in graph form, it will look like: