Pattern rules of thumb, and when they fail

A hole can only hold so much explosive per metre, and that amount rises with the square of the diameter. The distance that charge can break to a free face rises with it. Across a wide range of production blasting, burden works out at somewhere between 25 and 35 hole diameters: a 102 mm hole wants a burden of roughly 2.5 to 3.6 m, a 165 mm hole roughly 4 to 6 m. The low end is for dense or hard rock and lower-energy products; the high end is for soft, well-jointed ground and higher-energy products.

Spacing is usually 1.1 to 1.4 times the burden for a staggered pattern. Below about 1, holes interfere and the middle of the burden is starved; well above 1.4, the gap between holes is under-broken.

Stemming of about 0.7 to 1.0 burden keeps the top of the charge confined long enough to do work instead of venting. Too little gives airblast and flyrock; too much leaves a hard collar.

Subdrill of about 0.2 to 0.3 burden carries breakage below the design floor so the floor can be dug level. A free-breaking floor or a convenient bedding plane needs little or none.

Low benches. When the bench is less than about twice the burden, the geometry no longer gives the charge room to break forward and the pattern has to be tightened.

Blocky or open ground. Large open joints let gases vent early and let blocks pass unbroken. The ratio that worked in massive ground gives oversize here.

Water and product. A wet hole that needs a different product, or a product loaded at a different density, changes the energy per metre and so the burden it can carry.

The result wanted. Fine fragmentation for a crusher, a clean final wall, or a controlled throw each pull the pattern in its own direction. A ratio is a place to start a design, not a reason to stop designing.

Take the ratio as the first number, drill the pattern, measure what came out (fragmentation, dig rate, floor, wall, vibration), then move the ratio and measure again. Within a few blasts in the same ground the site has its own rule of thumb, which is the only one that matters.