Blast Burden (Konya's Formula)

The burden is the thickness of rock a blast hole has to push forward — the perpendicular distance from the hole to the nearest free face. It is the most important single dimension in a bench blast, and Konya's formula in FHWA-HI-92-001 is the industry's standard first estimate: $B = 3.15 D_e (SG_e/SG_r)^{1/3}$, with the burden in feet and the charge diameter in inches.

Note that this is a UNIT-BOUND coefficient. The 3.15 carries the foot-per-inch conversion inside it. With both lengths in the same unit — both metres, both feet, both inches — the coefficient becomes $3.15 \times 12 = 37.8$, exactly, since an inch is a twelfth of a foot. This page computes in the 37.8 form so that the metric and imperial answers agree, and it is worth knowing which form you are looking at whenever you meet the formula elsewhere.

The physical argument behind it is simple energy accounting. The explosive in the hole has energy proportional to its density and to the cross-sectional area of the charge; the rock in front has mass proportional to its density and to the volume the hole is responsible for. Balancing them gives a burden that scales linearly with charge diameter — the dominant term, and the one that matters — and with the cube root of the density ratio, which is a gentle correction. That cube root is worth understanding: switching from ANFO at 0.82 to an emulsion at 1.25, a 52 % increase in density, buys only about 15 % more burden. What a denser product really buys is water resistance and energy per metre of hole, not pattern.

BURDEN AND SPACING ARE NOT INTERCHANGEABLE, AND TRANSPOSING THEM IS THE CLASSIC BLASTING ERROR. The burden is measured perpendicular to the free face; the spacing is measured along the row, between neighbouring holes. In a properly designed bench pattern the spacing is always the LARGER of the two. Lay a pattern out with them swapped and every hole has an excessive burden in front of it and a crowded spacing beside it, and the shot behaves the way over-confined shots always behave: poor fragmentation, severe backbreak into the wall behind, cratering upward at the collar instead of movement forward, and the airblast and flyrock that come with energy venting out of the top of the hole instead of doing work. If the two dimensions are close together the error hides; if they are far apart it is spectacular.

What the formula cannot see is at least as important as what it contains. There is no term for JOINTING, and on badly broken ground the jointing governs the result completely — open joints parallel to the face vent the gases before they can do work, and joints dipping out of the face turn the shot into a slide. There is no term for rock strength beyond density, none for bench height, and none for initiation timing. Konya's own guidance is that the formula gives a starting point which is then corrected for geology and for the number of rows behind the face. And there is one measurement the formula cannot make for you: the ACTUAL burden in front of the actual holes. A face left irregular by the previous shot can have a real burden half or double the design value from one hole to the next, and that variation is where flyrock comes from. Walk the face and measure it before charging.