Blast design review next to a village
A limestone quarry supplies a cement plant and blasts once a day at 14:00. Holes are 115 mm diameter on a 4.2 m burden and 5.0 m spacing, with a 12 m bench and 0.8 m subdrill. ANFO at 0.85 g/cm³ is loaded to 3.0 m below the collar; the remainder is stemmed with drill cuttings. The quarry boundary is 400 m from the nearest village. The current pattern fires two holes per delay with 25 ms delays. The blasting contractor reports inconsistent fragmentation, occasional flyrock beyond 80 m, and one misfire in the last 500 holes. The quarry holds a vibration limit of 5 mm/s at the village and a flyrock exclusion zone of 200 m. The site uses a site-specific vibration predictor of PPV = 1,140 (D/√Q)^−1.6, where D is in metres and Q is charge per delay in kilograms.
1. Calculate the explosive charge per hole, the powder factor, and the predicted peak particle velocity at the village for the current two-holes-per-delay pattern and for one hole per delay. Assess compliance.
Read the worked answer
Hole depth = 12 + 0.8 = 12.8 m. Charge length = 12.8 − 3.0 = 9.8 m. Hole area = π × 0.115² / 4 = 0.01039 m². Charge volume = 0.01039 × 9.8 = 0.1018 m³. Charge mass = 0.1018 × 850 = 86.5 kg per hole. Powder factor = 86.5 / (4.2 × 5.0 × 12) = 86.5 / 252 = 0.34 kg/m³, which is low for limestone and is consistent with poor fragmentation. For two holes per delay, Q = 173 kg, scaled distance = 400 / √173 = 30.4, and PPV = 1,140 × 30.4^−1.6 = 4.8 mm/s. This is below 5 mm/s but with only about 3% margin, which is not defensible given predictor scatter. For one hole per delay, Q = 86.5 kg, scaled distance = 400 / √86.5 = 43.0, and PPV = 1,140 × 43.0^−1.6 = 2.8 mm/s. The single-hole pattern gives a much better margin. Flyrock beyond 80 m is still inside the 200 m exclusion zone, but it shows the blast is not fully confined.
2. Recommend changes to improve fragmentation and reduce flyrock while keeping vibration within the limit. Support the recommendation with calculations.
Read the worked answer
Fire one hole per delay to reduce vibration to about 2.8 mm/s and retain margin for geological variation. Improve fragmentation by tightening the pattern rather than by increasing charge per hole, because a larger charge per hole would raise vibration. For example, use a 3.5 m burden and 4.0 m spacing: rock volume per hole = 3.5 × 4.0 × 12 = 168 m³, so powder factor = 86.5 / 168 = 0.52 kg/m³, which is more appropriate for limestone. With one hole per delay the vibration prediction remains about 2.8 mm/s because Q is unchanged. Stemming is currently 3.0 m, or 0.71 × burden; with a 3.5 m burden it becomes 0.86 × burden, which is better. Use crushed rock stemming rather than drill cuttings, avoid bridged or voided stemming columns, and check hole deviation because an reduced actual burden is a common cause of flyrock. Maintain the 200 m exclusion zone and clear it before each blast.
3. Outline the actions required if a misfire is suspected after a blast.
Read the worked answer
Stop all loading and hauling in the affected area, barricade and sign the approach, and keep personnel and equipment out. Do not dig or disturb the muck pile with an excavator bucket or hand tools until the misfire procedure has been authorised. Wait the specified time, then inspect from a safe position and look for unexploded explosive, cut-off holes or unusual fumes. Use a non-sparking method and follow the site explosive permit. If a misfired hole is found, it should be re-blasted or washed out only by a competent shotfirer under the site procedure, never by pulling the explosive. Record the event, preserve the delay pattern and loading records, and review the cause: hole deviation, cut-offs, water, poor priming, or delay failure. The misfire rate of one in 500 holes is a warning that initiation and loading practice need auditing before the next blast.