The shuttering for the polygonal structure (14) is made of several shuttering wall sections (20) with rigid external supports (24) to form a circular outer profile. This is secured by external ties (22) to form a rigid self supporting structure ready for casting. Suitable ties include ropes with tension control, steel pipe rings etc.The rigid supports for the wall sections can be made from plywood ribbing and secured to the outer profile. Simple tension control clamps secure the fitting before casting.
