Aren't you both right? i.e. they knew exactly what it would look like and were well aware of the soil mechanics that would be involved. It wasn't the whim of a long haired disorganised artist but a planned project that started from a blueprint (or slateprint...) and involved their equivalents of surveyors, quantity surveyors, civil engineers etc etc.

As for what if anything it's meant to represent, that's unlikely to be figured out although if you Google Silbury and Womb you get 87,000 hits - which is testament only to how the internet can spread utter cobblers worldwide IMHO.