Blast Near a Protected Structure
You are the shotfirer at a quarry where a new blast is planned. The nearest protected structure, a 19th-century stone cottage, is 120 m from the blast. The site's vibration limit is 6 mm/s peak particle velocity (PPV) at that structure. The site's blasting consultant has provided a scaled distance graph indicating that, for a 50 kg maximum instantaneous charge (MIC), the predicted PPV at 120 m is 5.5 mm/s. However, the cottage's owner has reported recent cracking and is concerned. The blast design uses 32 mm diameter holes, 3 m spacing, 3 m burden, and a bench height of 10 m. The explosive is a bulk emulsion with a density of 1.2 g/cm³. The site has a seismic monitoring station at the cottage, but it is currently out of service. The shot is scheduled for tomorrow.
1. What are the key regulatory and safety considerations you must address before proceeding with the blast?
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The shotfirer must ensure compliance with the site's vibration limit (6 mm/s PPV) and consider the owner's concerns. Under relevant legislation (e.g., Quarries Regulations 1999 in the UK), the shotfirer has a duty to ensure that the blast does not cause injury or damage. The predicted PPV of 5.5 mm/s is close to the limit, leaving little margin for error. The out-of-service monitoring station means there is no real-time verification, so the shotfirer should arrange for alternative monitoring or repair the station. Additionally, the recent cracking report must be investigated; a pre-blast condition survey by a qualified structural engineer should be conducted to document existing cracks and establish a baseline. The shotfirer should also review the blast design to reduce ground vibration, such as reducing the MIC by using smaller charge weights per delay or changing the delay sequence. Communication with the owner is essential to address concerns and provide information. The shotfirer must also check that all relevant permissions and insurance are in place. Finally, the shotfirer should consider using a lower-risk method, such as reducing the blast size or using non-explosive methods, if feasible.
2. How would you adjust the blast design to reduce vibration while maintaining productivity, and what specific calculations would you perform?
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To reduce vibration, the shotfirer can reduce the MIC by increasing the number of delays or using a different initiation sequence. The scaled distance (SD) is given by SD = D / √MIC, where D is distance in meters and MIC in kg. Currently, SD = 120 / √50 = 16.97 m/kg^0.5. To achieve a PPV below 5.5 mm/s, the shotfirer could aim for a lower MIC. For example, if the site-specific PPV predictor is PPV = k * SD^(-b), using the given data, we can estimate k and b. With one data point, we cannot determine both, but we can assume a typical k and b for the site. Alternatively, use the consultant's graph to find the MIC that gives a PPV of, say, 4 mm/s at 120 m. From the graph, interpolate: if 50 kg gives 5.5 mm/s, then to get 4 mm/s, MIC = 50 * (4/5.5)^(2/b) approximately, but without b, we can use the square root scaling: MIC2 = MIC1 * (PPV2/PPV1)^2 = 50 * (4/5.5)^2 ≈ 26.4 kg. So reduce MIC to about 26 kg. This can be achieved by dividing the charge into more delays. For example, if each hole contains 26 kg, then only one hole per delay. With 32 mm holes, the charge per hole is limited by the hole diameter. The volume of explosive per meter of hole is π*(0.032/2)^2 = 0.000804 m². With density 1.2 g/cm³ = 1200 kg/m³, mass per meter = 0.000804 * 1200 = 0.965 kg/m. For a 10 m bench, hole depth maybe 11 m, so total charge per hole ~10.6 kg. So each hole has about 10.6 kg. To keep MIC ≤ 26 kg, at most two holes per delay (2*10.6=21.2 kg) or three holes (31.8 kg) which exceeds 26 kg. So use two holes per delay. This increases the number of delays, requiring more detonators. The shotfirer should also consider using electronic detonators for precise timing to improve fragmentation and reduce vibration. Additionally, adjust the burden and spacing to ensure good fragmentation with the reduced charge. The shotfirer should recalculate the predicted PPV with the new MIC and verify with monitoring. Also, ensure that the stemming and confinement are adequate to prevent flyrock.