In clay, what is the angle of internal friction?

The angle of internal friction in clay is typically considered to be zero degrees when it is fully saturated and undrained. Clay is cohesive, which means that it relies heavily on its particle interactions and cohesion rather than friction between particles for stability. In saturated conditions, the resistance to shear failure in clay is mainly due to its cohesive strength rather than frictional resistance.

This is in contrast to granular materials, like sand, where friction plays a significant role in shear strength, and therefore, typical angles of internal friction would be observed at much higher values. The cohesive nature of clay and its unique geotechnical properties leads to the conclusion that when evaluating undrained conditions, it is appropriate to consider an angle of internal friction of zero degrees.