The next step in our bridge 'reno' was to construct a goalpost-shaped trammel which we centered on the wooden platform we had made between the stone bridge abutments.
Then using it as our height gauge, we rotated the trammel and built up a barrel-shaped pile of left-over rubble stones, being careful not to extend stones anywhere above the bar . Talk about moving goal posts! We covered these stones with landscape fabric.
We mixed concrete and applied a four inch layer with trowels over the barrel of stones. The surface was shaped to the exact contour of the bottom of the bar of the trammel . We left this over night to set. This was now to become the centering for the new arch.