At http://bit.ly/BookArch. The question is posed, will this book arch stand. Well, clearly it is standing. Here is a quick spreadsheet thrust model showing that it would stand at almost indefinite height at this span to thickness ratio. If the span were much greater, it would fail.
The outline obviously doesn't follow the photograph exactly. I have pasted the Photo as fill to the Plot Area in an excel graph. The model divides the arch into 20 segments and plots the thrust at each intersection and then at 150mm steps down the piers. Arches like this fail by spreading at the shoulders. THere is no way this one is absolutely symmetrical so itwill tip one way rather than failing symmetrically. The limiting case is the minimum horizontal thrust which means that the thrust goes close to the extrados at the crown and the intrados at the haunches and then slowly drifts out towards the back of the piers. If the arch and/or the pier is thinner, failure occurs at a lower height.
The minimum thickness at which a semicircular arch on rigid supports will stand is about span/18.