Blast Pattern Burden, Spacing, and Stiffness Ratio

The burden ratio carries the rock and the explosive together: a low ratio near 20 suits hard rock or a low-energy product, a high one near 35 suits soft rock and a high-energy product, and 25 is a common starting point for ANFO in medium rock. A 3.5 in hole at 25 gives a 7.29 ft burden, 8.39 ft of staggered spacing, 2.19 ft of subdrill and 5.10 ft of stemming -- call it 7 by 8 on the ground. THE STIFFNESS RATIO IS THE CHECK THAT STOPS A BAD PATTERN BEFORE IT IS DRILLED. On a 30 ft bench that burden gives 4.1, comfortably above 2, and the bench will break properly to the free face. Drop the bench to 12 ft and keep the same holes: stiffness falls to 1.6, and a charge that short relative to its burden behaves like a crater rather than a bench blast. It vents upward, and the result is airblast, flyrock, and poor fragmentation. THE FIX IS A SMALLER HOLE, NOT A WIDER PATTERN -- at 2.5 in the burden falls to 5.21 ft and stiffness comes back to 2.3, workable. Spreading the 3.5 in holes further apart makes it worse, which is exactly the instinct the ratio exists to correct. Starting-point geometry from published ratio ranges. These are a first design to be adjusted against the rock actually in the face: jointing, bedding, mud seams, voids, and a variable free face all move the correct burden more than the ratios do, and a burden right on one end of a bench can be wrong on the other. The ratios assume a vertical or near-vertical hole to a clean free face with adequate relief; angled holes, choked faces, presplit lines, trim rows, and secondary blasting follow different rules. It does not design the initiation sequence and timing, which controls relief, fragmentation, and vibration as much as the pattern does, and it does not evaluate flyrock, vibration, or airblast. Blasting is a licensed activity: the blaster in charge, the state and federal explosives regulations, MSHA or OSHA jurisdiction, and the site's blast plan govern.

burden = burden ratio x hole diameter / 12; spacing = spacing ratio x burden; subdrill and stemming are their own ratios of the burden; stiffness ratio = bench height / burden, and the largest hole a bench supports = bench height x 12 / (2 x burden ratio).

The published pattern ratios by name -- burden ratio typically 20 to 35, spacing 1.15 to 1.4 times the burden staggered, subdrill 0.2 to 0.5, stemming 0.7 to 1.0 -- with the stiffness check requiring bench height at least twice the burden. Starting-point geometry only. Blasting is a licensed activity: the blaster in charge, the state and federal explosives regulations, MSHA or OSHA jurisdiction, and the site's blast plan govern.

Ratio arithmetic on published starting ranges the user enters; no proprietary design data is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: hole_diameter_in, burden_ratio, bench_height_ft, spacing_ratio, subdrill_ratio, stemming_ratio, burden_ft, spacing_ft, subdrill_ft, stemming_ft, stiffness_ratio, max_diameter_in

• One ratio sets the whole pattern every dimension is a multiple of the diameter or the burdenblasting practice
• Stiffness below 2 is a crater, not a bench and the fix is a smaller hole, never a wider patternblasting practice
• The face governs, not the ratios jointing, mud seams, and a variable free face move the correct burden moreblasting practice