Blasting
Good Practice Page 3.
Good Design for Ground Vibration
Table 6 summarises the important aspects of keeping control of level of ground vibrations.
Table 6. Options available for reducing ground vibrations.
| Allow sufficient margins of 'safety' when designing from blasting trials of charge size versus ground vibration at given distances. |
| Use smaller MICs; many individual charges fired at small time intervals only increase the vibrations by 50% of the first charge, the shock however lasts longer. |
| Use decked charges and in-hole delays which permit further reductions in MICs without increasing the number of holes to be drilled, which would be the case if bench heights were to be reduced instead. |
| Carefully design the detonation sequence and inter-hole/inter-row delays. When using non-electronic detonators, delays should not be too small otherwise the explosions will effectively coalesce and the benefits of small MICs lost. This problem is largely avoided when using electronic detonators. |
| Consider changing the frequency spectrum of the shock by varying blast designs in terms of the inter-hole delays, using electronic detonators to ensure accurate detonation. |
| Ensure that a comprehensive monitoring programme is in place, supported by a well maintained database, to enable detailed analysis and improved blast design. |
These measures may or may not add to cost. One report is of tighter control having a "major impact [increase] on mining costs"; another of "considerable costs". Other reports say that such measures actually improve blasting, fragmentation and safety, and consequently the economic efficiency of the workings. One operator reported a reduction in explosive related costs, after introducing decked charges to meet ground vibration limits. Overall it is difficult to disentangle any cost changes because of the considerable interaction between vibration control and effective fragmentation.
Although the cost of electronic detonators is considerably higher than other types, the advantages can be considerable, particularly in sensitive locations. The "hybrid" system of initiation allows a non-electric detonation system to be used as a backup in tandem with the electronic system, thus fulfilling the UK regulatory requirements to have two detonators on every delay, while minimising the costs.
In the general, non-mineral related literature, an oft quoted method of reducing vibration propagation through the ground is to use a trench as a barrier. Usually this is not practicable on a scale that is effective. One reference does however suggest that, in the context of mineral workings it is an under-used approach, pointing out that the first box-cut is in effect a massive trench.
Good Practice for Overpressure
Overpressure can be minimised at source (Table 7), but the debate continues as to whether it is practicable to use planning controls to monitor and enforce limits in view of the fluctuations caused by varying weather conditions.
Table 7. Minimising the overpressure levels at source
| Avoid using surface detonating cord and, if it has to be used, cover it adequately. |
| Reduce the degree of surface heave by minimising the total charge and using a low charge weight per delay. |
| Use an appropriate sequence of detonation and consider the orientation of the working face in relation to sensitive areas; if the direction of blast initiation is away from or at right angles to, rather than towards a sensitive location, then reductions of 10-15dB and 6dB respectively may be possible |
| Avoid gas venting through local rock weaknesses (also a cause of flyrock) by accurate drilling and placement, and regular face surveys, ensuring that the trace velocity between holes is significantly less than the speed of sound, i.e. the delay between holes is more than 5 ms/m; this will avoid air-blasts from individual holes reinforcing each other. |
| Avoid resonance with floors, which can increase the acoustic response (shaking and rattling) of nearby buildings, by using delays of less than 25-40ms. |
If possible, avoid blasting in adverse weather conditions which include:
|
Clearly it would be advantageous to be able to predict the effects of the weather on overpressure in a quantified way. The Meteorological Office has a computer programme which can predict the propagation of overpressure but sufficient local weather information to use it is unlikely to be available.
Good Practice for Flyrock
Flyrock is produced when there is too much explosive energy for the amount of burden, when stemming is inadequate, or when the explosive energy is too rapidly vented through a zone of weakness. The operator should adapt blasting methods to suit the conditions when blasting in conditions that favour the production of flyrock, e.g. heavily faulted and fragmented rock.
Table 8. Factors controlling flyrock.
| Avoid misfires by all means available. |
| Choose the bench height, burden and stemming such that the blasted rock movement is primarily horizontal and outward and not upward. |
| Select the orientation of the face. |
| Inspect the blast site for natural joints, bedding planes, mud seams, voids and any other weaknesses; inspect the working face for overhang/backbreak or raggedness before the blast hole pattern is laid out. Face profiling by scanning or photographic methods will add valuable data. |
| Ensure that blast holes are not located close to points of weakness; this problem is reduced by lower bench heights. |
| Design the blast to have sufficient confinement (burden, stemming, delays and sequence of firing) using the face profile information, and avoid detonator leads being cut. This problem can be averted by using non-electric and electronic detonators. |
| Ensure that, in multiple row shots, the delay between rows is long enough to allow the rock from an earlier row to move out so that the next row will have adequate relief. |
| Use toe-priming rather than collar priming/detonating. |
| Ensure that drilling is accurate. |
| Check the explosive column rise during loading to avoid overloading due to cavities around the hole and to avoid loading too near the collar of the hole. |
| Checking that all holes are loaded. |
| Avoid secondary blasting. |
Good Practice for Fumes
Explosives should only be used under the conditions specified by the original manufacturers. Site-mixed ANFO is known to be a particular problem and attention should be given to ensure the correct Ammonium Nitrate prills are used, that the correct amount of diesel is added and that the product is thoroughly mixed251.
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