Explosives for blasting in civil excavations 05 Nov 2020

There are three primary applications for explosives in tunneling: for tunnel headings, cavern excavations and shaft sinking. Accordingly, blasting is applied horizontally, vertically and in all orientations. Aside from applying experience, principles and engineering to the application of explosives in blasting, safety is paramount. Misapplication, error or carelessness can have catastrophic results. Explosives and blasting knowledge today are highly advanced and the safety focus is all-prevailing.

Charged face initiated for a blast round

The application of explosives is subject to all jurisdictional and regulatory considerations. These can be applicable to the country, state, province or territory, and county or city in which the work is carried out. There are also occupational health regulations and governing rules of the owner under whose auspices the excavation is being advanced. Many contractors have their own, overarching rules concerning blasting and must, therefore, be aware specifically of the transportation, storage and application regulations to which adherence must be assured. There will almost certainly be licensing requirements specifically for the blaster-in-charge and those under that person’s command. Most contracts will define the regulations of which the contractor must be aware.

It is critically important to treat blasting holistically. Blast design is always a function of the rock being excavated, the orientation of the excavation, the explosives and initiation systems being used, and, perhaps most importantly in a vast majority of civil works, the location of the project that will govern environmental factors including ground vibration, airblast and dust generation.

Drilling is an interdependent factor with the explosives in the success of the blast. To obtain optimal results, the fragmentation of the broken rock and ground control implications are to be considered in the design and adjusted through measurement and observational engineering as the excavation progresses. For security and efficiency reasons, the logistics of getting the explosives to the worksite and to the working face must be considered.

Plastic and paper wrapped Dyno Nobel emulsion

Dynamite, the nitroglycerin-based explosive, was the dominant explosive used until the latter half of the 20^th Century. Ammonium nitrate fuel oil, or anfo, began to be seen in civil work in the 1970s with watergels and emulsions starting to appear in the 1980s. The advantages of these technologies range from less impact sensitivity, lower cost, and reduction of the risk of unwanted propagation during blasting, to beneficial velocities of detonation as emulsion technology has become fine-tuned. For those who have suffered from nitroglycerin headaches when handling or being in the vicinity of dynamite, the alternative technologies are a blessing.

These alternate technologies have some trade-offs. Lower bulk strengths require higher powder factors to achieve the same useful work at detonation and the number of required charge holes tend to increase, driving up drilling costs. Nor were all explosives created equally. Dead pressing, whereby explosives in a charge hole may be rendered insensitive due to the explosives in an adjacent charge hole detonating before them, became an issue to be reckoned with. The industry, through much experience and study and the application of technologies, including microballoon strength, secondary salts, emulsifiers, and ingredient mix variants, has dramatically improved the explosives resulting in the robust products of today.

Anfo, while cost attractive, has the detraction of being water soluble, resulting in the risk of high concentrations of nitrates in tunnel and groundwater outflow that can shut down jobs either temporarily or permanently. When applying packaged explosives to blasting in civil works, emulsions are the products most selected. For specific applications and challenges, dynamite still has a significant place in the industry. Where a specification is tight and revolves around limitation of mass per delay, dynamite, as the most energetic explosive available, may be the best or necessary choice. In challenging conditions, such as fragmented rock or water-bearing strata, its high energy, proven reliability and ruggedness, can dramatically reduce the risk of poor blast results. This combination of performance characteristics remains relevant today.

Titan 7000 sensitised emulsion

DynoMiner APS loading unit

In recent times, there has been an evolution from packaged to bulk explosives, especially for larger excavations. The advantages can be significant. Eliminating packaging reduces costs, there are reduced logistics costs, speedier loading rates, a reduction of manual aspects of the loading process, and more efficient filling of the available charge hole space. While anfo and watergels may also be applied in bulk, emulsions are now the preferred technology.

Solid density control, more stable emulsifiers and water-ring pumping technology whereby an annulus of water around the explosive to facilitate the flow of emulsion in the charge hose, have made re-pumpable emulsion possible, which is of primary importance from a logistics point of view. The ability to pump or blow the emulsion more than once is critical for most underground excavation projects. The application of bulk emulsions can reduce the loading cycle in a heading by sometimes up to 75% with hydraulic pumping units operating at capacities of up to 90kg/min.

When using bulk explosives, the reduction in packaging costs is partially offset by the need for investment in bulk loading equipment. However, on an all-in basis, given the right size and logistical and dimensional parameters, bulk products are less costly than the packaged explosive alternatives. Emulsions in bulk offer added flexibility. They may be:

• applied hydraulically or pneumatically;
• sensitized in different manners with solid or gas additives, or a combination of both;
• delivered to site as an oxidizer and sensitized at point of loading into an explosive, thereby eliminating some explosives handling restrictions or costs in storage or transportation,
• density-modified at point of loading, lending the option of flexibility to loading different charge holes at different strengths;
• used to fill the charge hole for blasting efficiency or, alternatively, string loaded thereby decoupling perimeter charge holes, for example, to reduce over-excavation.

Most bulk emulsion manufacturers specify the equipment and control systems which go with the various formulations. The companies either manufacture the loading equipment themselves or specify the use of a third-party manufacturer which produces specified equipment under license. For safety reasons, Dyno Nobel does not allow its bulk emulsions to be pumped through equipment other than those it recommends.

A tagger and blast initiator command set for a wired electronic initiation system

In choosing the right combination of bulk emulsion and loading equipment, a variety of factors must be considered. The choice of thicker formulations for hydraulic loading versus thinner formulations for pneumatic loading defines the viscosity specification of the explosive.

Hydraulic loading tends to be more robust, faster, and amenable to application technologies, including:

• using longer and smaller diameter hoses for loading, hose pushing and retraction;
• metering of loaded explosive;
• computer control of the loading process;
• gassing, or sensitizing the explosive by introducing trace chemicals at time of loading to create microscopic voids in the explosive matrix; and
• string loading.

With proper procurement and correct maintenance, hydraulic systems bring ease-of-use but require a higher level of training.

Pneumatic loading tends to demand less expensive equipment, simpler operation and may be more suited to smaller workspaces. The carrier required for heavier hydraulic loading equipment is avoided with skid-mounted units, and sling-mounted loading equipment for shaft work, where available.

Given these advantages, pneumatic equipment is often selected for testing or approval work. At the same time, shorter hose runs only are typical and metering options are limited. Pressure vessel pneumatic technology is favored primarily for shaft work but is suitable also for horizontal applications. It is uncomplicated, with limited moving parts and can facilitate a rapid loading rate, particularly on a crowded benching excavation. Using specifically formulated bulk emulsions, loading rates dramatically reduce the loading time of down and horizontal charge holes compared with packaged explosives. Uphole loading is not possible with emulsions, but this is rarely a limitation in civil applications.

Solid sensitization requires the incorporation of solid microballoons of glass or plastic into the explosive at point of manufacture. Sensitization using the gassing method requires greater care in equipment set-up, maintenance and training, but offers cost benefits when viewed holistically. Because the explosive is being modified at point of loading, minimum diameter and maximum density must be carefully monitored in order to ensure optimum blast results. The setup of the equipment and the skill of the operator is key.

String loading, or decoupled charging, involves reducing the energy dissipated by the detonation of explosives in a non-fully circumferentially loaded charge hole. Less explosive goes into the individual hole and its effect, beyond the unloaded perimeter of the charge hole, is reduced. This lends itself well to the loading of perimeter holes to control overbreak.

DigiShot Plus 4G tagger

DigiShot Plus 4G coil

Initiation systems

Many in the industry during the 1980s and before will remember loading holes with safety fuse and hooking up faces with igniter cord. While this was an important development at the time of its inception, safety fuse today belies its name. Thankfully, the civil excavation industry has now progressed through electric and nonelectric detonators to the state-of-the-art electronic detonators of today, which can be either wired-electric or shock tube nonelectric.

The accuracy available with today’s technology takes a lot of guesswork out of blasting, offering:

• Secure blast design: making available
  • A guaranteed single hole per delay and
  • Lower vibrations
• More reliable and controlled perimeter and cushion blasting; to
  • Reduced back break and damage of the rock beyond the excavation perimeter,
  • Limiting over-excavated material to be removed
  • Reducing overbreak
• Greater excavation scales per blast under tighter restrictions;
• An ability to verify the integrity of the initiation system up to the point of detonation; and
• Reduced inventory and procurement concerns.

This technology comes with a higher unit cost but considering the scope of a blasting operation, the benefits significantly outweigh the cost. The wired electronic technology typically involves a steeper learning curve but once mastered is applied as second nature. Wired electronic detonators can come with pre-set timing or can be programmed while loading or at any time prior to detonation.

The nonelectric electronic detonators, with timing set at the point of manufacture, provide high accuracy with ease of application. These detonators are applied identically to traditional pyrotechnic nonelectric detonators and a variety of configurations, either avoiding or requiring compatible detonating cord, are available.

When using the less sensitive explosives of today, particularly bulk explosives, the energy needed to set off an efficient detonation typically exceeds the detonator alone. This requires the use of boosters to ensure steady state velocity of explosive detonation. Boosters also minimize the likelihood of unwanted blast gasses being generated by sub-standard detonation and provide an additional level of dead press protection to electronic detonators. These boosters are typically manufactured of rigid high explosive and are affixed to the detonator prior to loading into the charge hole and being surrounded by explosive.

In the future, fully wireless initiation systems will be economically and technologically available for the diameters of charge holes typically applied in civil underground projects. These are in selected use today in the mining industry and further development is required before expected widespread application in civil work.

A set of Stinger boosters

Blast design

There are many aspects to consider in the design and application of explosives and blasting, including the design of the charge hole drilling patterns. These include:

• Fragmentation
• Perimeter control
• Ground vibration
• Airblast
• Size of excavation
• Cost

These cannot be assessed in isolation and must be evaluated holistically, particularly in urban environments, or in close proximity to other infrastructure where the environmental knock-on effects of ground vibration, airblast and noise can be paramount. Ground vibration and airblast complaints in residential areas can threaten shutdown of job sites or lawsuits. Residual blast gasses are at best a nuisance and at worst a risk to health and can also threaten job shutdowns and litigation. Avoiding damage to nearby structures from blasting is a prime driver in blast design considerations.

There is no substitute for training and experience when it comes to blasting design. Blasters, independent consultants and explosives manufacturer-based consultants rely heavily on these. There exists also a variety of technological tools that complement this expertise and knowledge, by providing measured parameters which can be integrated into blast planning, either at the outset or as part of the observational and applied engineering refinements as a project progresses. Some tools to consider for design implementation in the field and for troubleshooting issues, include:

• Seismographs and accelerometers
• Drillhole deviation measurement devices
• 3D excavation scanning
• Drillhole cameras
• Velocity of detonation recorders

The application of these technologies minimizes the need for guesswork, can provide valuable data with which to analyze challenges, and can be essential in avoiding complications from negative environmental aspects of the excavation.

Project interaction

Each civil excavation project comes with its own set of challenges and opportunities from an explosives and blasting perspective, with senior managers from the project owner, design, construction manager, contractor and Explosives supplier involved in the decision making. It is advisable to seek supplier consultation in the design, bidding, pre-planning and value engineering phases, as well as during the actual drilling and blasting operation itself. Suppliers, with their blasting expertise and product knowledge, can also be helpful in meetings up to and including the project owner. Blast evaluation, troubleshooting and fine tuning are all part of the observational engineering process.

Much knowledge and many tools exist to assist in the design of blast excavations and their implementation. Product technologies and delivery systems are highly advanced today and when specified correctly, are used safely and effectively in excavating shafts, tunnels and caverns. Blasting is carried out every day around the civil construction world, driving excavations forward while minimizing environmental effects and disruption to neighbors or surrounding infrastructure. Highly trained and experienced staff are available for consultation on what will work best for each project, considering the many different factors to be considered.