Pump room — catenary
vaults without formwork

Two catenary vaults built without formwork — each brick laid to a string line defining the catenary geometry, self-supporting from the first course.

A catenary is the curve a hanging chain takes under its own weight. Inverted, it is the shape in which a masonry arch carries that weight in pure compression, with no bending in the section. Build to that curve and the vault does not need a rib, a tie, or a centering frame to stand while it is being built: each course leans on the one below it and the arch is stable from the first brick. The string line is the whole apparatus.

The consequence is economic as much as structural. Formwork for a vault is expensive, and in the Klein Karoo it is also scarce — the timber has to be brought in. Building to a hanging line removes it entirely, which is what made the method viable here with unskilled labour, and what made the two vaults the proof of concept for the ten-vault Dam Wall Building that followed.

The sequence is visible in the photographs: a line struck across the span, the courses rising to meet it, and the vault closing on itself without a single piece of timber beneath it.