Selection of Explosives
The explosives used in blasting need to function safely and reliably under the environmental conditions of the proposed use, but ultimately the explosives selected need to meet the objectives and goals of the overall Master Blasting Plan.
The first and foremost goal is to break rock. However, some other objectives that may need to be considered are: (1) a specific fragment size, (2) future handling of rock, (3) minimum damage to remaining rock, (4) rock displacement, (5) diggability, (6) vibration, (7) air blast, and (8) cost. It is important to understand that each site is different and may involve multiple stakeholders.
The Master Blasting Plan must be tailored to meet the site-specific goals and objectives as defined by these stakeholders and reflected in the plans and specifications.
In the explosive selection process, the objectives and goals of the Master Blasting Plan can eliminate certain types of explosives from consideration on a particular project. After the objectives and goals have been considered, one must consider the limitations of blast site factors. Limitations of main concern are: (1) blast site location, (2) geology, (3) project specifications, (4) available explosives, and (5) available drill types.
a. Blast Site. The blast site can limit the selection of the explosives for a Master Blasting Plan. Concerns such as allowable blast size, proximity to sensitive locations, water conditions, and topography of site will all have an impact on the selection of the explosives. If the blast site is located near urban areas or sensitive structures where excessive vibrations are not tolerated the blast size (amount of explosives) will have to be reduced. As discussed earlier water greatly impacts the selection of explosives. The topography of a site may limit equipment mobility and could limit the selection of explosives by, for example, not allowing an explosives pump truck on site.
b. Geology. Geology greatly impacts the selection of explosives for a Master Blasting Plan. One of the first things to consider is the hardness of the rock and its resistance to blasting. ANFO will most likely work well on limestone, but it may have difficulty achieving proper fragmentation on a harder gneiss. Other geological considerations that may limit explosive selection are bedding planes, faults, joints, voids, and caverns.
c. Project Specifications. The project specifications will impact the selection of explosives for a Master Blasting Plan. Project specifications are usually developed by the primary stakeholder and can be very prescriptive. Prescriptive specifications can greatly limit the selection of explosives by specifying a particular type of explosive of minimum density of explosive. Performance-based specification such as fragmentation size and blast size will also have an impact on selection.
d. Available Explosives. The availability of explosives will impact the selection of explosives for a Master Blasting Plan. If a particular explosive is readily available, making it cheaper, it will most likely be used. Also, if a blasting contractor has an inventory of a particular type of explosive, it will probably be used. Certain explosives may not be available for use for the particular project. Always consult controlling regulations applicable at the site before final selection of explosive product. Special permits for some products may require additional time or cost and this must also be factored into the selection of an appropriate product.
e. Available Drill Types. The availability of drilling equipment will impact the selection of explosives for a Master Blasting Plan. If the available drill equipment for a particular site can only drill 3-in. blastholes, the selection of an explosive like some heavy ANFO would not be a good choice since some heavy ANFO has a critical diameter of over 3-in.