Blast Hole Stemming Length and Flyrock Screen

Stemming is the plug that keeps explosive gases in the hole, and when it is short everything above the charge leaves at speed. The gases take the path of least resistance straight up, and the collar region is also where the burden is least confined, so it is doubly the place where things go wrong. Two rules govern and the LARGER wins: 0.7 to 1.0 times the burden, and at least 20 hole diameters. An 8 ft burden with 3.5 in holes wants 5.6 ft by the first rule and 5.83 ft by the second, so 5.83 ft governs. NOW THE FAILURE CASE. A hole loaded with 4 ft of stemming on that same 8 ft burden has a ratio of 0.50, well under the low end of the range. That hole will vent, and the material above the charge is going somewhere. Where a nearby structure, road, or occupied area is in play the practice is to move toward 1.0 times the burden -- 8.0 ft here -- accept slightly worse fragmentation at the collar, and deal with the oversize at the crusher instead of on the neighbour's roof. THE MATERIAL MATTERS NEARLY AS MUCH AS THE LENGTH. Angular crushed stone sized around a tenth to a twentieth of the hole diameter locks and holds; on 3.5 in holes that is 0.18 to 0.35 in, a clean quarter-inch angular chip. Drill cuttings, which are free and right there, fluidize and blow out, and using them is a documented contributor to flyrock incidents. One more field reality: run this against the SHORTEST stemming in the shot, not the average, because one short hole is enough. A screen from published ratio ranges. It does not predict flyrock distance or throw, which depends on rock structure, voids and mud seams, face burden variation, initiation timing, and confinement -- and face burden variation, not the design burden, is the usual real cause; a face that has sloughed leaves a hole with far less burden than the drill pattern says, and measuring the actual face profile before loading is what catches that. It does not design blast area security, determine the blast danger zone or evacuation radius, or address mats and covers. Blasting is a licensed activity and flyrock is a fatality mechanism: the blaster in charge, the state and federal explosives regulations, MSHA or OSHA jurisdiction, and the site's blast plan govern.

stemming by the burden rule = ratio x burden (0.7 to 1.0); by the diameter rule = 20 x hole diameter / 12; the LARGER governs; achieved ratio = proposed stemming / burden; stemming stone = hole diameter divided by 20 to 10.

The published stemming rules by name -- 0.7 to 1.0 times the burden and at least 20 hole diameters, whichever is larger -- with angular crushed stone sized a tenth to a twentieth of the hole diameter named as the material, rather than drill cuttings. 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: burden_ft, hole_diameter_in, proposed_stemming_ft, burden_ratio_low, burden_ratio_high, diameter_multiple, by_burden_ft, by_diameter_ft, governing_ft, achieved_ratio, stone_min_in, stone_max_in

• The larger of the two rules governs the burden rule and the diameter rule disagree, and the longer one winsblasting practice
• Drill cuttings are not stemming they fluidize and blow out; angular crushed stone locks and holdsblasting practice
• Check the SHORTEST hole one short hole is enough, so an average tells you nothingblasting practice