WHERE AND HOW TO PLACE CHARGES.
Masonry or Brick Arch.—Attack the haunches of the arch or the piers. In bridges of a single arch the haunches are the best points, two trenches being dug across the width of the roadway down to the back of the arch. When there is no time for this, the charge should be placed along the crown of the arch. With guncotton untamped use ¾T2 × B; if tamped with a depth of earth equal to the thickness of the arch, use ⅜T2 × B.
With gunpowder, if over crown of arch, use 3/2T2 × B; if behind haunch, ⅔T2 × B. In the former case use tamping if possible equal to the thickness of the arch, and in the latter case equal to twice the thickness. The quantity of powder obtained by these rules should be divided into two or more charges placed across the arch.
When the bridge consists of a series of arches, and the piers are short and thick, the haunches should be attacked by the rules given in table on next page.
If the piers be high and thin, it is better to place the charges against them, as the fall of one pier will involve the destruction of two arches. The charge should be slightly greater than for a wall of the same thickness.