reason because of the added pressure on the slope. The slope area has so much pressure from overweight of materials and erosion that may have occured....Am I on the right track?
I think it?s best to think in terms of plastic flow. If you take a pile of sand and allow it to trickle through your fingers it forms a natural angle of 43.5 degrees. The same sort of thinking can be applied to earth. Of course if you go to the other extreme and talk about solid rock then you can have a vertical wall. Interestingly the Bent Pyramid in Egypt had it?s steep initial angle altered because the Egyptian engineers became aware of this problem when one of their other pyramids collapsed.
