We have here each step fixed at each
end and free only on the top, the bottom, and one face. Let us assume
that there is a back seam, that is, that the step is not fixed at the
back. In a quarry, this seam, unless a natural one, should be made by
a channeling machine. In order to throw this step put of place it must
be cut off at both ends, and for this purpose the V-shaped holes are
put in at right angles to the face. It is well, however, to put the
first two holes next the back seam in a position where the grooves
will converge at the back so as to form a sort of key, which serves a
useful purpose in removing the block after the blast. In quarries
where there are no horizontal beds a channeling machine should be used
to free the block on all sides and to a suitable depth, and then the
ledge may be "lofted" by holes placed horizontally.
Where "pressure" exists in quarries, the new system has certain
limitations. After determining the line of "pressure" it is only
practicable to use the system directly on the line of thrust, or at
right angles to it.
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