To obtain a weight of vaulting in each segment, we have to find the area between circles k1 and k2 and multiply it by the segment width (1e) and the density of granite (960 kg/e3).
The area is defined by:
where x1 and x2 are borders of the particular segment.
All we have to do is plug in the numbers and then multiply the result by one ell and 960kg/e3.