My bad... Horntoad. I just got excited when I saw it. Thanks for sharing those photos. I had wondered if the tree at the base was a Laurus nobilis (bay) or some other herbal/edible tree.
Yes, a more shallow spiral would not have the outward pressure on the upper portion of the construction. I've seen some spirals that use rock for the spiral, wide enought that one could actually walk up on the rock. The rocks can also be placed inward as it is built up to support the weight of the soil.
Interesting thread. Thanks.
