Shear strength

Sometimes they fail catastrophically.

And that brings us to shear strength.

I always assumed a foundation failure meant the dirt beneath it was just, you know, crushed under the weight.

It's a super common misconception.

A soil mass yields when one portion of the soil slides past another portion along an internal surface.

We call this shear failure.

So the soil's ability to resist that internal sliding is its shear strength, and the text relies heavily on Coulomb's theory here.

It states that shear strength comes from two primary sources, internal friction and cohesion.