The angle of internal friction of soil is something that can be overlooked, but one that can be easily explained is rather simple. We encounter it when we see a side of a soil pile slough or soil start to mobilize. The cause of both those instances comes right down to a simple principle all of us understand: friction.
This is the ability of any soil to resist shear forces. Soil is balanced when not disturbed. Once a trench is cut, the soil is no longer balanced and forces like water pressure – not only washes out the “glue”, but adds weight to the soil, making it harder for the soil to resist and lead to failure – the measurement of that failure is the basis of lateral earth pressure.
There is a moment in all soils when this angle gets to a point that the soil no longer has the grip (friction) to keep everything in place. It is when the shear resistance is overcome and soils begin to fail. Gravity also plays into the shear forces as well.