This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Theoretically, this should be known before any calculations are made; but since the weight of filling and pavement are always large, and their unit-weight is but little less than that of the concrete, it is possible to estimate from experience on the required crown thickness, and to make the thickness at other points in the required ratio. If this should prove too thin (or too thick), all sections can be changed in the same ratio. If the outline of the intrados is determined (as in the case of an arch spanning railroad tracks), and the upper surface line (of earthwork or pavement) is also known, the change in the arch ring will mean only a change in weight due to the difference of unit-weight of concrete and earth filling. If the original assumption is even reasonably close, this difference will hardly exceed the uncertainties in the loading.