The Bridge Arch Retaining Wall
The basic 'arch shape' is very strong. A stone bridge can take a lot of weight because the arch is a fixed concave barrier to the downward pressure of gravity . Also, the stones in an arch are set on end, radiating slightly in such a way their strength and structure is maximized by their length relative to the direction of the forces coming down from above.
So then, why are there so few retaining wall applications utilizing this stronger 'concave' shape in the design. More importantly, why, in a wall that is supposed to hold back the horizontal force exerted by the ground behind it, are the stones laid flat? Shouldn't they be upright, standing on their ends the way they would if they were in an elongated bridge laid on its side?