In order to make the arched bridge wider that we had only partially completed last year in California, this year we are building forms to support a nine inch arch to be added to both sides. This is in order to spread the parapets a bit more so that there is enough walking area over the bridge between them.
Sean, Patrick and I use junky stones laid carefully in an arc and then add a layer of cement (shaped with a trammel) to form the appropriate Roman arch shape.
It doesn't have to look pretty. It's just a temporary support for the terrazzo voussioirs we will be adding afterwards.
Talk about false work !