In this paper, the collapse of a symmetric masonry arch due to a generic supports settlement is analyzed. The arch is modeled as a system of rigid-plastic blocks with frictional sliding. Since frictional sliding is not described by an associated flow rule, generally the collapse state is not uniquely determined. However, in a previous work [4] it has been shown that in a collapse state due to a horizontal settlement the horizontal thrust attains its minimum statically admissible value and then it is uniquely determined. Further, in a collapse due to a generic (both horizontal and vertical) settlement, the collapse thrust due to an only horizontal settlement provides a lower bound to the actual collapse thrust. Here, extensive numerical analyses for various circular arches and settlements have been carried out in order to illustrate the applicability of the two previous results.