Blast vibration compliance near a village
A limestone quarry blasts 12 m benches with 115 mm holes at 3.0 m burden and 3.5 m spacing. Each hole contains 35 kg of explosive. The nearest village is 300 m from the blast. The permit limits peak particle velocity (PPV) to 5 mm/s at the village. Site calibration from previous blasts gives PPV = 850 (D/√Q)^-1.6, where D is distance in metres and Q is charge per delay in kilograms. The current design uses eight holes per delay with 42 ms delays. The quarry needs 6,000 t/d of broken rock and cannot afford long stoppages. Residents have complained of vibration and one cracked window. The blasting engineer must decide whether to change delay timing, charge per hole, or pattern geometry without losing production.
1. Calculate the current PPV and the maximum charge per delay allowed by the 5 mm/s limit.
Read the worked answer
Current charge per delay is Q = 8 x 35 = 280 kg. Then PPV = 850 x (300 / √280)^-1.6. √280 = 16.73, so 300 / 16.73 = 17.93, and 17.93^1.6 = 101.4. PPV = 850 / 101.4 = 8.4 mm/s, well above the 5 mm/s limit.
For the limit, 5 = 850 x (300 / √Q)^-1.6. Thus (300 / √Q)^1.6 = 170, so 300 / √Q = 170^(1/1.6) = 24.8, giving √Q = 12.1 and Q = 147 kg per delay. The current 280 kg per delay must be roughly halved.
2. Recommend a blast design that meets the limit without reducing daily production. Consider timing, charge per hole and pattern.
Read the worked answer
Use four holes per delay: 4 x 35 = 140 kg, below the 147 kg limit. Predicted PPV is 850 x (300 / √140)^-1.6 = 4.8 mm/s. Keep the same total number of holes per blast and the same powder factor, but divide the blast into more delays using electronic detonators. A 25-35 ms inter-hole delay with 4 holes per delay should maintain fragmentation and avoid misfires. Do not simply reduce the number of holes per blast, because that cuts production. If more capacity is needed, consider decking or a smaller-diameter charge, but verify that the charge per delay remains below 147 kg. Monitor PPV at the village and recalibrate the site constants after the first trial.
3. What compliance monitoring and communication measures should accompany the change?
Read the worked answer
Place a seismograph at the nearest village and record PPV, airblast and frequency for every blast. Keep a blast log with charge per delay, delay sequence and pattern. Notify residents of blast times and maintain a complaint-response procedure. Check for flyrock and cracked plaster after each blast, and compare measured PPV with the prediction. If measured PPV exceeds 5 mm/s, stop and reduce charge per delay before continuing.