Brunel's 1838 cast iron bridge does have a precedent.
Magdelene Bridge, Cambridge, 1823, is designed as an elliptical arch with flat springings and dovetailed joints at the apex, but like Brunel's bridge, has cast iron plates between the beams at soffit level and earth fill above.
In the early 1970s the DoT wished to replace it with a concrete bridge. I put the case together for Cambridge Preservation Society. It is the 10th oldest cast iron bridge in the world and was at that time comfortably carrying full highway loading.
Judging from the line of the ellipse, it had settled about 150mm at midspan, probably within the first year or two of its life.
The DoT preserved it but chose not to repair it as we proposed. It built a steel portal frame bridge within it. Thus it now stands as a shell.
Jim Platts (M) mjp@eng. cam. ac. uk, University of Cambridge Institute for Manufacturing, Mill Lane, Cambridge