Mean Fragment Size (Kuz-Ram)
Estimates the mean fragment size X₅₀ of a blast with the Kuz-Ram model, from the rock factor, the volume broken per hole, the explosive mass per hole and the relative weight strength of the explosive. It is the screen size half the muckpile passes, the number that decides whether crushing will struggle. Enter the four blast design parameters.
Result
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The screen size half the muckpile passes through
Coarse fragmentation never shows up in the cost of the blast. It turns up three weeks later, in primary crusher power draw and in shovel queue time. A predicted X₅₀ closes the blast design: with it the mine engineer decides whether to tighten the pattern, switch explosive, or accept the muckpile as it comes. Lacking an estimate before firing, the argument degenerates into comparing photographs of the pile afterwards, once nothing can still change.
X₅₀ = A × (V₀/Qₑ)^0.8 × Qₑ^(1/6) × (115/RWS)^(19/30). A stands for Cunningham rock factor, in practice 7 for friable ground, 10 for hard jointed rock and 13 for hard massive rock. V₀ means the volume broken per hole in m³, that is burden × spacing × bench height. Qₑ covers explosive mass per hole in kg, and RWS the relative weight strength with ANFO set at 100. The values on screen, A = 10, V₀ = 168 m³, Qₑ = 92 kg and RWS = 115, return X₅₀ = 34.40 cm at a powder factor of 0.55 kg/m³.
The 19/30 exponent on (115/RWS) carries the Cunningham correction for explosives other than TNT, whose RWS reads 115 — which is why, at RWS = 115, that term equals exactly 1 and the equation reverts to the original 1973 Kuznetsov form. The model is well known for underestimating fines, since the crushed zone around the hole never enters the formulation; anyone needing the full curve moves to modified Kuz-Ram or the Swebrec function. Nearly all the uncertainty rides on the rock factor: A comes from summing rock mass descriptors, and shifting 10 to 13 lifts X₅₀ from 34.40 to 44.72 cm with nothing else in the design touched.
Frequently asked questions
Does an X₅₀ of 34.40 cm mean the largest block measures 34 cm?
How do I pick the rock factor A with no testing available?
Should the powder factor go into one of the fields?
Related Tools
Powder Factor
Compute the powder factor of a rock blast by dividing the explosive mass (kg) by the volume of rock broken (m³), in kg/m³. It is the central parameter of the blast design: too low produces boulders and poor fragmentation; too high wastes explosive and increases vibration and flyrock. Optimizing it reduces downstream crushing costs. Enter the explosive mass and the rock volume.
Blast Hole Count
Calculate the number of holes of a blast pattern, N = area ÷ (burden × spacing), dividing the bench area to blast by the pattern area of each hole (burden B × spacing S). The result is the number of holes needed to cover the area with the specified drilling pattern. In practice, round up. It is an essential quantity calculation in blast planning: it sets the drilling time, the amount of explosive and accessories, and the operation cost. Wider patterns (larger B and S) reduce the number of holes but may worsen fragmentation. Enter the area, the burden and the spacing.
Blast Burden
Calculate the burden of a blast pattern, B = k × d, multiplying a factor k (typically 25 to 40, depending on rock and explosive) by the hole diameter d. The result, in the unit of d, is the distance from the row of holes to the free rock face — one of the most critical geometric parameters of blasting. Too large a burden leaves the rock poorly fragmented (boulders) and creates toes; too small wastes explosive and causes flyrock and overpressure. Together with the hole spacing, the burden defines the drilling pattern and the resulting fragmentation. Enter the factor k and the hole diameter.
Advance per Blast (Pull)
Calculate the effective advance per blast (pull) in drill-and-blast tunnelling, advance = L·η, from the drilled hole length L (m) and the blast efficiency η (0-1). Not all drilled depth converts to advance: part is lost because the hole bottoms do not always break fully, leaving a 'socket'. Typical efficiency is 85-95% — depending on the blast pattern, rock type and execution. Advance per blast, times the cycles per day, sets the rock face productivity. Maximizing it reduces cycles and schedule, but very long holes lose drilling accuracy and efficiency. Enter the drilled length and the blast efficiency.
Blast Subdrilling
Calculate the subdrilling of a blast hole, S_p = 0.3 × B, multiplying the burden B by a typical factor of 0.3. The result, in the unit of B, is the length the hole must drill below the desired bench floor level. This extra depth ensures the blast fragments the rock down to the floor level, avoiding toes (ledges of unfragmented rock at the bench foot) that hinder equipment operation. Insufficient subdrilling leaves toes; excessive wastes drilling and explosive and damages the rock below the floor. Enter the burden.
Detonation Velocity (VOD)
Calculate the velocity of detonation (VOD) of an explosive, VOD = L ÷ t, dividing the distance traveled by the detonation wave L (m) by the time t (s) measured between two sensors. The result, in m/s, is the speed at which the detonation reaction propagates through the explosive column — one of the most important properties of an explosive, linked to its energy and fragmentation power. High-VOD explosives (4000-7000 m/s, like emulsions and dynamites) generate high detonation pressure and are effective in hard rock; low VOD (ANFO, ~3000-4500 m/s) suits softer rock. Enter the measured distance and time.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.