Flyrock Incident and Exclusion Zone
You are the shotfirer at a surface coal mine. The blast is a production shot with 120 holes, each 15 m deep, 311 mm diameter, loaded with 250 kg of ANFO per hole. The burden is 6 m, spacing 7 m, and stemming 3 m. The blast is initiated with a non-electric system. The nearest public road is 500 m away, and the site's exclusion zone is set at 300 m. The blast is fired, and after the smoke clears, you observe that some flyrock has landed on the road, approximately 450 m from the blast. There are no vehicles or people on the road at the time, but the road is a public highway with occasional traffic. The mine's flyrock limit is 200 m. You have a crew of 10 and a supervisor. The police have been notified of the blast as per procedure.
1. What immediate actions should you take, and what are the potential legal and safety consequences?
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Immediately after observing flyrock on the public road, the shotfirer must ensure safety: stop all operations, check for injuries, and secure the area. The road should be checked for any debris that could cause accidents, and the police should be informed immediately to manage traffic. The shotfirer must report the incident to the site supervisor and the mine manager. The flyrock exceeded the site limit of 200 m and reached a public road, which is a serious incident. Legal consequences may include investigation by the regulatory authority (e.g., HSE in the UK or MSHA in the US), potential fines, or prosecution for breach of safety regulations. The shotfirer must preserve evidence: document the scene with photos, note the location of flyrock, and record the blast parameters. The shotfirer should also check the blast design: the stemming length of 3 m may be insufficient for the burden, causing flyrock. The shotfirer must review the blast plan and consider factors like geological conditions, stemming material, and delay timing. The incident must be reported under RIDDOR or equivalent. The shotfirer should also consider the need for a risk assessment review and additional controls, such as increasing the exclusion zone, using heavier stemming, or reducing the charge per hole.
2. How would you redesign the blast to prevent future flyrock, and what specific changes would you make?
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To prevent flyrock, the shotfirer should analyze the causes. Common causes include insufficient stemming, poor burden, and inappropriate delay timing. With a stemming of 3 m in a 311 mm hole, the stemming ratio (stemming length / hole diameter) is about 9.6, which is low; typically, stemming should be at least 20-30 times the hole diameter, i.e., 6-9 m for 311 mm. So increase stemming to at least 6 m. Also, the burden of 6 m may be too small for the charge; check the powder factor. The charge per hole is 250 kg, burden 6 m, spacing 7 m, bench height 15 m. The volume per hole is 6715 = 630 m³, so powder factor = 250/630 = 0.4 kg/m³, which is reasonable for coal. However, the stemming is short. Increase stemming to 6 m, which reduces the explosive column to 9 m. The charge weight per hole becomes 9/15 * 250 = 150 kg. This reduces the powder factor to 0.24 kg/m³, which may be too low for good fragmentation. So adjust burden and spacing: increase burden to 7 m and spacing to 8 m, volume = 7815 = 840 m³, powder factor = 150/840 = 0.18 kg/m³, still low. Alternatively, keep charge but use a stemming plug or crushed stone stemming to improve confinement. Also, check the delay sequence: ensure that the holes detonate in the correct order to provide relief. Use a staggered pattern or increase delays between rows. Consider using electronic detonators for precise timing. Additionally, conduct a flyrock risk assessment and increase the exclusion zone to at least 500 m, with road closure during blasting. The shotfirer should also monitor the blast with a video camera to analyze flyrock. Finally, ensure that the blast area is clear and that communication with the road authority is improved.