Blast Pattern Design Calculator

Burden and spacing are both calculated as a multiple of hole diameter (converted to feet), scaled by a rock-hardness factor -- a wider hole holds more explosive per foot of depth, so it can effectively fracture and displace a proportionally larger volume of rock around it. Doubling hole diameter roughly doubles both burden and spacing in this calculator's formula, which is why drill-bit selection is one of the first decisions in any blast design.

Less. This calculator uses a tighter burden factor for Hard Rock (22) than for Medium (25) or Soft Rock (28), meaning holes are spaced closer together in hard rock at the same diameter. Harder rock resists fracturing more, so a smaller burden concentrates more explosive energy per unit of rock to achieve adequate breakage, while softer rock fractures more easily and can be broken with a wider, more economical pattern.

Subdrill is the extra depth drilled below the visible bench floor -- calculated here as 30% of the burden -- to ensure the explosive charge breaks rock cleanly down to the design grade. Without subdrill, the blast tends to leave an uneven "toe" of unbroken rock at the base of the bench, since the explosive's fracturing effect is weakest right at the free face beneath it.

Bench height increases both the hole's charge length (more explosive column) and the rock volume broken per hole (burden × spacing × bench height), so a taller bench raises both total explosive load and tons broken per hole together. Because those two quantities move together rather than independently, powder factor -- the pounds-of-explosive-per-ton efficiency ratio -- responds much less to bench height than either the explosive load or the tonnage does on its own.