While taking a shortcut through George Square we noticed, among all the digging and the smart new pipes ready to be installed, a small hollow log, then another which tapered to one end, then a pile of longer, more substantial examples.
At once we took these to be wooden pipes, similar to those of C17 Edinburgh's first fresh piped water supply, between their Comiston origin and the Castlehill reservoir. 
It appears that wooden pipes were still in use in the period that George Square was constructed, some hundred years later.