As soon as the cut was out, a second round of holes was started for the first squaring up, as shown in the drawings, where they are numbered 1, 1, 1, 1, &c. In these and in the subsequent rounds, numbered 2, 2, 2, 2, &c., and 3, 3, 3, 3, &c., the resistance to be overcome is, of course, not so great as in the cut. In the first and the second squaring-up rounds, from 50 lb. to 60 lb. of dynamite was used, and, in the third round, this quantity was increased to 80 lb. or 90 lb., the resistance becoming greater as the roof arch falls at the sides. In this third round, there were generally one or two additional roof holes; these are not shown in the drawing, as their position varied, according to the lay of the rock. The top holes in the first round are also intended to bring down any roof not shaken by the cut, and these are therefore angled sharply towards the centre, and bored from 12 feet to 14 feet deep. In the plan, Plate XII., the number 3 indicates the cut holes, and 4, 5, and 6, the squaring-up rounds. The holes of the first squaring round were always drilled about a foot deeper than the cut holes; when blasted, these generally brought out an additional foot of shaken rock at the apex of the cut. The following table shows approximately the number and the depth of the holes required, and the quantity of dynamite used for a linear advance of 10 feet.
---------------------+------+---------+------+------+------ | | |Total | | |No. of| Depth |Depth | | |Holes.|of Holes.| of |No. 1.|No. 2. | | |Holes.| | +------+---------+------+------+------ | | ft. in. | ft. | lb. | lb. Cut | 12 | 10 6 | 126 | 25 | 50 1st square up | 8 | 12 0 | 96 | .. | 55 2nd „ | 8 | 12 0 | 96 | .. | 55 3rd „ | 6 | 12 0 | 72 | .. | 85 Additional roof holes| 2 | { 10 0} | 18 | .. | .. | | { 8 0} | | | +------+---------+------+------+------ | 36 | .. | 408 | 25 | 245 ---------------------+------+---------+------+------+------
The cut holes being 10 feet 6 inches deep, the blast usually brought out about 9 feet full, which, as explained above, was increased to 10 feet in the subsequent rounds. The cross section being about 175 square feet, in an advance of 10 linear feet, there are about 65 cubic yards of rock to be broken; this gives on an average 0·4 lb. of No. 1 and 4 lb. of No. 2 dynamite, and a little over 6 feet of drilling per cubic yard.