below the surfaces used u2014 although one is not dealing properly speaking with u201c intermediate u201d surfaces additionally in the flowchart of fig3 the process is explained with a pillar by pillar calculation it is possible to change the order of the calculations proposed for example commencing with the search over the top surface for the points of transit of the pillars followed by searching for the points on the bottom surface and finally the interpolation of the pillars step 6 of the flowchart of fig3 also assumes that the pillar is a straight line in so far as the pillar is not a straight line it is possible to consider the projection of the points n 0 g h 1 and n 0 g h n on the surfaces s 0 1 and s 0 n in the same way as for the calculations relating to the top and to the bottom as described in step 8 and not the intersection proposed in step 6 of fig3 this consideration applies naturally in respect of fault surfaces which are not necessarily ruled surfaces in any event one obtains the points i 0 g h 1 and i 0 g h n of the straight line passing through these two points with the surfaces s 0 1 and s 0 n as in the case of the flowchart of fig3 it is possible to change the order of the steps of the flow chart of the other figures in particular as regards the sweeping of the surfaces and the irregular points it is not indispensable to implement the solution of fig9 in the absence.