Fire in the hole! That warning has been sounded for years to warn of an impending controlled explosion. For a manager to address safety around rock blasting and explosives, it is first necessary to understand the basics of the technology, says Peter G. Furst, director of contracting at Liberty Mutual Group.
Most rock-blasting explosives are composite explosives. They contain fuels, oxidizers, and other self-explosive ingredients. Ammonium nitrate-fuel oil (ANFO) is the classic example. All composite explosives contain some mixture of carbon, oxygen, and nitrogen.
Various properties of explosives define how they will perform under field conditions, says Furst. Such properties include fume class, density, water resistance, temperature effects, detonation velocity, detonation pressure, borehole pressure, sensitivity, and strength.
The density of a material is defined as its weight per unit volume for blasting. Explosive densities are expressed in grams per cubic centimeter (g/cc). Blasters can relate to these metric units because water density is 1 g/cc. Therefore, explosives with densities higher than 1 g/cc will sink in clear water. That is important for blasting in wet areas, where the blasting material needs to sink to the bottom of the borehole.
For two reasons-safety and performance-the relative sensitivity of various explosive compositions is measured. From a safety perspective, it is important to know how sensitive an explosive is to impact, friction and heat. From a performance standpoint, such measures as gap sensitivity, critical diameter, and minimum primer sensitivity define the “functional sensitivity” of explosives.
When compared with newer water-based explosives, nitroglycerin (NG) explosives are far more sensitive to detonation by impact or friction. Of the four classes of explosives, NG has the greatest sensitivity, followed by high-explosive water gels, ANFO, and then emulsion explosives. NG can be set off by shock, whereas ANFO emulsion blends cannot.
The measure of an explosive’s sensitivity determines the minimum size of the primer or detonator required. This ignition sensitivity varies widely for various explosive types. A blast can be initiated with high-sensitivity explosives by using a detonator.
Low-sensitivity explosives, such as ANFO emulsion blends, require a booster (usually a very small amount of high explosives) that is initiated by a detonator to cause an explosion. Obviously, low-sensitivity explosives are safer to use because they cannot be set off accidentally.
When explosives detonate, they produce shock and heave energy. The total theoretical energy of explosives can be expressed in calories per unit weight or calories per unit volume.
Contractors often use ANFO-mix blasting agents for construction blasting. When used properly, ANFO can produce good blasting results for a relatively low cost. Number 2 diesel fuel is commonly used as the fuel. ANFO, however, has poor water resistance and will not fire when wet. Obviously it’s important to check the water resistance of an explosive used in wet conditions.
Electric or Non-Electric
Initiating explosives are designed to safely activate larger explosive charges at a controlled time and in a predetermined sequence. There are two types of initiating systems-electric and non-electric, depending on their signal transmission method. Electric systems use wire to transmit a current from a power source to the detonators.
Non-electric (or non-el) systems use plastic tubes. These are detonating cords with narrow trains of high explosives, or slow-burning pyrotechnic compounds, to transmit initiation signals. Furst says you can use timing systems, but delay is usually produced by means of pyrotechnic delay elements inside detonators. Delay detonators are available with millisecond or long delays, with approximately half-second timing intervals.
Detonators are compact devices that safely and efficiently initiate and control the performance of larger explosive charges. They contain relatively sensitive high explosives initiated by a signal or energy from an external source. Delay detonators incorporate components that introduce a controlled time delay to sequence explosions in blast holes for optimal results.
Furst says the most common initiation systems are the non-electric detonators: thin plastic tubes that resemble wire. These convey the signal by a shock front generated by the reaction of powdered aluminum and other chemicals coating the inside surface of the tubes. Field hookup is very easy, and various delay combinations allow for an infinite variety of blast sequencing.
Non-electric detonators provide a high level of safety against accidental initiation by static electricity, stray electric currents, and radio-frequency energy. They also cannot be initiated by flame, friction, or impact normally encountered in construction blasting.
However, a shock tube can be accidentally initiated by stretching the tube until it breaks. If the tube is slit or cut, then a misfire will result, so the tubes must be handled with care.
Blasting Design
For large projects, Furst recommends performing some small-scale test blasts to provide the most complete information about existing rock types, physical properties, structure, and blasting characteristics. Blast fragmentation size will influence equipment selection, and vice versa if you already have drill equipment. Excavation schedules and drill-bench dimensions will influence blast hole size, explosive selection, and labor requirements. The proximity of blasting to structures can profoundly affect blast planning.
Blasting near structures or urban areas adds a special set of concerns. The need for pre-blast structure surveys, vibration and air-blast monitoring, stringent blast-effect control measures, and effective blast area security and warning methods must be evaluated for close-in blasting.
Over the years, Furst says, bench-blast design rules of thumb have been established to help blast designers prepare initial blast designs and make preliminary cost estimates. These rules of thumb are not intended to predict actual blast performance. Bench-blast design includes calculations for overburden, hole spacing, minimum bench height, load factor, charge weight, and powder factor, among other factors.
Blast designers can select from a virtually unlimited variety of drill patterns and hole orientations. The drill patterns and spacing are bound by practical limits on hole size. Typically, hole sizes range from 1.5 inches in diameter up to 9 inches. Hole orientation is influenced by project topography, excavation boundary geometry, equipment limitations, bench height, bench access, and many other factors.
Another factor in blast design is the delayed (millisecond) timing of the initiation of the explosives in relation to one another.
Proper millisecond timing will enhance fragmentation and improve excavation productivity.
Major advantages of millisecond blasting include reduction of ground vibration and air blast, improved fragmentation, reduction of overbreak and flyrock, improved productivity, and lower costs.
Manufacturers offer many easy-to-use non-electric systems that allow blasters almost infinite timing flexibility. A combination of surface and in-hole delays are widely used in construction blasting operations.
About Air Blast
When explosive charges detonate in rock, most of the energy is used in breaking and displacing the rock mass. However, some of the energy is released in the form of ground vibration and air overpressure or air blast. Air-blast pulses are usually in frequencies below the threshold of human hearing, but this energy can be felt.
