No definite rule can be laid down as to the amount of powder
to be used, but it should be as small as possible. Very little powder
is required in most rocks. Hard and fine grained stone requires less
powder than soft stone. Mr. Knox tells of a case which came under his
observation, where a block of granite "more than 400 tons weight,
split clear in two with 13 oz. of FF powder." He compares this with a
block of sandstone of less than 100 tons weight "barely started with
21/2 lb. of the same grade of powder, and requiring a second shot to
remove it."
It is obvious that enough powder must be inserted in the hole to
produce a force sufficient to move the entire mass of rock on its bed.
In some kinds of stone, notably sandstone, the material is so soft
that it will break when acted upon by the force necessary to shear the
block. In cases of this kind a number of holes should be drilled and
fired simultaneously by the electric battery. In such work it is usual
to put in the holes only 4 or 5 ft.
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