Chapter 37: Specialty Blasting Applications

The three blasting applications discussed in this chapter are (1) concrete blasting, (2) secondary blasting, and (3) submarine blasting. Each is a distinct blasting application that poses unique risks and hazards. They also present different blasting challenges and limiting factors. When the blaster is involved in these applications they are encouraged to seek advice and assistance from blasters who routinely perform these applications. Sometimes explosives manufacturers can provide assistance or direction.

Commercial explosive uses for applications other than those discussed in Part III of this Handbook are very specialized and beyond the scope of this book. In these instances, the blaster is encouraged to seek advice from appropriate industry associations specializing in the desired work. Associations can direct the reader to companies that routinely perform these applications.

CONCRETE BLASTING

Blasting of reinforced concrete structures is the most economical demolition method. The use explosives in the demolition of concrete structures tends to separate the concrete from the rebar much more effectively than if the structure were demolished using an excavator equipped with a hydraulic demolition hammer. Therefore, explosives demolition is effective as aiding in the recycling of the reinforcing steel. Many of the challenges with use of explosives in demolition work can be solved with sound technical knowledge.

Drilling Challenges

Drilling of reinforced concrete offers unique challenges. The hardness of concrete can vary depending on its age, hardness of the aggregate used, and the presence of additives such as accelerators used in the mix. The presence of reinforcing steel adds to the challenge. Drilling in concrete is similar to drilling in conglomerate, where the hardness and abrasiveness depends on the make up of the pebbles that are fused into rock. Concrete can be brittle and is fairly weak and will shatter easily when blasted. Many structures specify use of 5/8 unspecified (5/8 pounds/inch² rock) concrete, however stronger concrete containing lightweight aggregate is often used on the upper floors of high-rise structures. Often the only way to tell this is to chip out a core of the concrete to expose the aggregate for visual comparison.

Concrete reinforcement will vary greatly depending on the type of structure. Often, heavier reinforcing bar on closer centers is used on lower floors while lighter reinforcing bar is used on upper floors. Structures such as nuclear plants, machine bases and bridge piers will contain heavy reinforcing. Drilling into reinforcing bar will have a major impact on drill productivity, bit life, and safety. Special safety precautions need to be taken to protect the driller from injury by hot steel shards from drilling into rebar.

Generally, drilling is accomplished using small diameter holes, with up to 75 millimeter (3.0 inch) boreholes used on massive structures such as bridge abutments, and 35 millimeter (1⅜ inch) boreholes used for drilling of shear walls and columns inside structures. Where blast vibration control is a factor, borehole size may be determined by the maximum charge weight per delay, and choice of drill type will be determined by access, borehole locations, presence of reinforcing steel and the drilling schedule. Where access is restricted, drilling using hand pluggers or jack leg drills may be necessary. Where more drill power is required to drill through rebar, a small rubber tired or tracked hydraulic drill may have to be used. Some structures may require use of drill bits specifically designed for drilling through rebar.

Notice in Figure 37.1 the absence of a center blow hole and continuous carbide on the bit designed for drilling through rebar.

Hazards And Risks

Containment of flying debris is one of the greatest challenges with blasting reinforced concrete. Many structures requiring demolition are in congested areas or may be contained within a mill site. Use of blasting mats, backfill, layers of chain link fencing and filter cloth, plywood, conveyor belting, and sand have all been used for control of flying debris. Test blasting should be used to evaluate the control method of choice.

In many cases, pre-weakening of the structure is required. This may involve the selective removal of columns and non-structural walls, and cutting of the floors or walls of the structure. A Structural Engineer should be consulted to prepare a pre-weakening plan for the structure. Care must be taken to ensure that the remaining structure is strong enough to support wind loading and mild earthquakes. A comprehensive demolition plan is drawn up and all columns and walls to be drilled and loaded are identified with a numbering sequence on both the structure and the drawings. An example would be 10-C4. for tenth floor, column C, borehole number 4. Charge sizes are then be calculated and tabulated in a loading plan, referencing each bore number.

Explosive Selection

The choice of explosives in demolition of reinforced concrete structures is important. An explosive with high brisance or shattering power is required, along with the ability to be cut into various lengths. Dynamite is generally the explosive used for the demolition of columns, whereas high core load detonating cord is generally used in blasting of shear walls.

Blast Design

Test blasting of concrete is similar in some respects to blasting in rock in that the tensile strength of concrete is approximately 10% of its compressive strength. Additional tensile strength is gained through the use of reinforcing bar. In designing blasts to demolish structures, it is necessary to know the size and distribution of the rebar within the structure. This can be determined through examination of the original construction plans coupled with the chipping of the concrete to expose the reinforcing steel to verify the "as built" condition of the structure.

In blasting high rise structures, the weight of the structure is used to bring it down. It is necessary to design the blasting to defeat the strength of the reinforcing steel in the structure. The explosives are used to not only shatter the concrete, but also to deflect the rebar in the column. Deflecting the rebar causes it to lose its strength, allowing the structure to collapse. Thus, drilling must take into account the pattern of the reinforcing steel. The explosives need to be in close proximity to the reinforcing steel to cause it to buckle. Deflection of the rebar can be seen in the following photo of a test blast. Notice the deflection in the rebar from the test blast on a shear wall shown in figure 37.2.

The powder factor used will depend on the situation and the strength of the concrete. Powder factor can vary from 0.30 kilogram/meter³ (0.51 pounds/yard³) for weaker structures to 0.90 kilogram/meter³ (1.52 pounds/yard³) for very heavily reinforced structures. Test blasting is generally carried out to determine what minimum quantity of explosives is required to achieve a full deflection of the reinforcing steel. Test blasting is conducted on columns or shear walls that are not critical for the stability of the structure. For control of flying debris, this step is critical for refinement of the control method being used.

Insensitive methods can be used to demolish concrete structures using very light charges. Explosives can be placed in boreholes containing water, the holes plugged and stemmed, and the hydrostatic energy used to break the concrete.

To avoid having cut-offs in demolishing a structure it is important to select a delay sequence that allows all boreholes to be energized before the first charges fire. Often, electric or detonating cord initiated shock tube systems are used in conjunction with long period delay detonators.

Delay sequencing is designed to direct the collapse of the structure. In the case of high rise buildings, wire rope cables are used to pull outer walls in towards the center.

Stemming of blast holes is accomplished using stemming bags, clay dummies, stemming plugs or polyurethane foam or a combination.

SECONDARY BLASTING

Secondary blasting is the process of reducing oversize material for processing and handling. Oversize material can be caused by many factors including blast geometry, predelineation, improper design, geology, explosives malfunction, and misfires. The breaking up or moving of large rocks or boulders in unconsolidated fill is also done with explosives.

Hazards And Risks

Secondary blasting can also be very hazardous. Dangers include flyrock, air blast, and personal injury related to climbing on or among large chunks. Care must be taken to protect the personnel performing the work and handling the equipment (drills) often used in the process.

The best way to handle secondary blasting as a result of mine or quarry blasting is to avoid its need through thorough and accurate blast design and execution. There is no substitute for doing the job correctly the first time, and the resulting minimal need for secondary breakage is a large economic saving. There are three basic methods for reducing oversize. These are snakeholing, external charge placement, and internal charge placement.

Snakeholing

This method is used for oversize in loose and unconsolidated fill, or smaller broken material. The snakehole is a hole or path dug to create a void underneath the oversize. In this method, the goal is to break or displace the oversize for easier handling.

It normally requires several pounds of explosives per yard³ or meter³. Precise amounts are difficult to determine and experience should be the guide, with progressively larger charges being tried rather than starting with a large amount.

A good starting point is 0.8 pounds to 1.2 pounds (NG dynamite or other high brisance (velocity) explosives) per foot of rock thickness, opposite or above the charge. Over blasting will likely only increase flyrock and air blast potential, and not create any more breakage.

Caution Confinement under the circumstances is very difficult to determine. Covering of the boulder is highly recommended to control flyrock and air blast.

External Charges

Using external charges to break oversize is the quickest and least expensive method for the task. It also creates the greatest hazard for flyrock and air blast, and requires larger amounts of explosives for the same result.

An accepted beginning powder factor is 1 to 2 kilograms/meter² (2 pounds/yard²). The explosives should be high velocity (brisance). The charge should be in tight contact with the surface and at the center of gravity of the boulder. Some method of attachment (wire, tape, rope) may be needed to keep the charge tight to the oversize.

An alternative method is mud capping. This requires placing the charge on the oversize and then putting mud or other containment on the charge. Mud may be the best containment as it has more coherent mass and thus loose material. Sand, sand bags, or other fine material can also be used.

The mud capping charge should be 1.2 kilograms/meter² to 1.8 kilogram/meter² (2 pounds/yard² to 3 pounds/yard²) to be effective. The charge should be covered with at least 102 millimeters to 152 millimeters (4 inches to 6 inches) of mud to provide confinement. Placing the charge in a depression or other area where good coupling occurs will give the best results.

Extreme caution must be taken to assure that the mud or sand does not contain any larger solid pieces within it. These will become projectiles at the time of detonation and will be very hazardous. The blaster should expect flyrock of some extent from mud capping.

Another method of using external charges is to use a cartridge explosive to form a cone with the base diameter to height ratio of 2:1. If the explosive is initiated from the apex or top of the cone it will create a concentration of the energy towards the base and the boulder to be broken. This may permit the use of up to a third less weight of explosive per yard² of oversize.

Loose emulsion or ANFO can also be shaped into a cone for the same effect. This would require a preformed container or shaped plastic bag or other container. If using non-detonative sensitive explosive, it must be primed with a high explosive booster, and conical cast primers have been marketed for this purpose. This conical method should also be expected to produce flyrock.

Internal Charges

Internal charges are the safest, most efficient, and cost effective method of dealing with oversize boulders. Drilling is required and significantly smaller amounts of explosives are required for the same or improved results.

The drilling of the oversize requires considerable set up and staging. If hand drilling is to be attempted, extreme caution must be taken to ensure safe footing for the drillers and safety for edfs provided. Large boulders, if free standing, can suddenly shift and cause serious injury to the driller attempting to drill the holes. The driller may also forget his or her location and make a mistake. Personal protective equipment, especially the safety harness, is necessary.

Drilling with mechanized equipment (boom drill or track drill) will require positioning of the boulders for safe and easy access by the equipment and driller. A safety harness is again critical if the driller is going to be climbing or standing on the oversize. The oversize must also be secured to prevent unsafe movement as the result of the higher down-pressure force of a mechanical drill.

Drilling diameters are usually quite small, less than 76 millimeters (3 inches) and of shallow depth. The driller must be sure to control the hole depth to approximately two-thirds of the oversize thickness at the drill hole location and to very close to 90° from the face being drilled.

Explosive charges used have been as low as 57 grams (2 ounces) yielding a powder factor of 0.08 kilogram/meter³ (0.13 pounds/yard³). The highest charge necessary should be limited to 0.30 kilogram/meter³ (0.5 pounds/yard³). A high brisance explosive should be used for its shattering effect and low relative gas pressure.

At the higher level of explosives charge, flyrock would be a certainty. At even higher levels of explosives charge, flyrock and air blast will increase, and there will be no significant increase in breakage.

The blaster will have to make a careful estimation of whether one borehole or several boreholes are required and mark the boulders for borehole location and depth. For a boulder requiring one borehole, that borehole should be drilled in the center of mass of the oversize. Large boulders that require more than one borehole must be more carefully evaluated for borehole location and depth.

Loaded boreholes should be tightly stemmed with fine material or clay. In all cases, flyrock should be expected. Multiple boreholes in one boulder are the most likely to cause flyrock. Adequate cover for the blaster is a necessity and it should protect against horizontal flyrock and skipping flyrock. Matting or covering should be considered if personnel are within a line standard feet.

A method that may reduce flyrock is an "air cushion" design. It requires the hole to be drilled between two and three-fourths of the thickness of the oversize. A beginning charge of 57 grams (2 ounces) is placed in the center of the lower two-thirds of the borehole and secured with a borehole plug of some kind. Another plug is placed approximately one-third the distance down from the borehole collar and the borehole stemmed to a maximum of 30 centimeters (12 inches).

SUBMARINE BLASTING

Underwater blasting is carried out for harbor and channel deepening, reef removal, demolition blasting of underwater structures (See figure 37.3), and blasting for civil construction projects such as powerhouse intakes, etc. Underwater blasting presents special challenges as the weight of water over the shot creates greater confinement, requiring heavier explosives loading. Often specialty explosives are required and protection of the environment plays a large role in blast design.

Limitations

Drilling for submarine blasting is often carried out in water that is too murky to see clearly, or the water depth is too great to be able to see the bottom. Underwater blasting may require drilling operations from a barge (See figure 37.4) In these situations, it is necessary to rely on accurate survey techniques and laser systems for drill alignment. Grade tubes and drill casing systems are used to provide stability to the drill string. It may also be necessary to add drill cuttings from topside down the grade tube in order to ensure stabilization of the grade tube while drilling. If the rock is loose or fractured, it may be necessary to set casing in the rock to provide stability to the borehole before inserting the drill bit into the rock to depth. It may then be necessary to sleeve the hole using PVC casing to keep the boreholes open for loading.

Underwater blasting requires precision drilling to achieve the designed grade. Subdrill depths should equal the borehole spacing. The borehole diameter is limited by the size of drill casing and drive shoes available, the drill capacity, and the maximum charge weight per delay dictated by environmental regulations. Most underwater blasting projects are carried out using 64 millimeter to 83 millimeter (2.5 to 3.25 inch) drill holes in each borehole diameter.

Often, drilling is carried out from a spud barge anchored over the blast site. Drills are often mounted on rails so that they can be easily shifted from hole to hole. Often multiple drills are used to increase. Divers are sometimes required to assist with drill hole alignment.

For situations with relatively shallow water depths, it may be more economical to construct a drill pad using fill material and drill through the fill using a drill casing system. This technique is generally faster than working from a barge and often benefits with Drydock cordials as well as environmental controls.

In some cases such as underwater demolition of reinforced concrete structures, it may be possible to use divers with hand held rock drills. For economical reef removal in shallow water, it may be possible to use hand drilling during the shallowest tides of the year, plug the holes, and come back to load and shoot during the highest tides of the year. The added water depth can be used for flyrock control.

Risks and Hazards

A high degree of drilling accuracy is required for underwater blasting. In rare cases, it may be necessary to obtain a variance from the regulatory authorities to allow "hot" loading of the boreholes as they are drilled. In these cases, there can be a risk of drilling into a loaded borehole if misfakes in drill alignment are made. Every effort must be made to ensure the greatest degree of drilling accuracy. Drilling vertically occurs from spud barges as shown in Figure 37.4.

Great care must be taken to avoid misfires. Boreholes should be double-primed. It is often recommended to use cast boosters for priming, even with cap-sensitive explosives. Care emit be taken with lead lines to ensure that they are not damaged during retrieval of drill casing. Floating debris in the water may also damage load lines. Ensure that lead lines are not tangled, and label each line with the hole-to-charge number.

Underwater blasting presents a risk to marine life through both concussion and overpressure effects. Small fish with swim bladders are most affected by blast overpressures. Where salmon are present, blasting may not be allowed from March 1st until August 15th, the period where salmon fry are most at risk. Larger fish are somewhat less sensitive to overpressure effects, and invertebrates such as crabs, clams, and oysters are the least sensitive.

Detonation of explosives underwater not only creates a pressure pulse in the water, but the gas bubble that is created tends to oscillate, collapsing back upon itself. It is the negative impulse that is most damaging to the swim bladders in fish. Confinement of the explosive charge within a borehole and use of good quality stemming can help control overpressure effects. It is poor practice to load explosives to the collar of the blast hole. As in surface blasting where limiting the charge weight per delay can control blast vibration intensity, similarly in underwater blasting restricting the charge weight per delay can be used to control overpressure intensity.

Peak overpressure and oscillations of the gas bubble from underwater blasting can also be damaging to civil structures. When blasting close to control doors for water intakes for example, great care must be taken to protect the structures from damage through limiting the charge weight per delay and installing a "bubble curtain" in front of the structure.

Explosive Selection Criteria

Submarine blasting requires the use of special explosives. The explosives must have high strength, excellent water resistance, maintain sensitivity under high hydrostatic pressure, be resistant to dead pressing, and resist propagation from hole to hole in the blast. This is one of the most demanding of environments for explosives and only the best quality products should be used for underwater blasting. For larger projects, bulk pumpable explosives are often used. It is important that emulsion explosives are manufactured using high-strength microballoons to resist dead pressing of the explosive due to the hydrodynamic shock loads experienced in submarine blasting.

Special care also needs to be taken with selection of detonators. Not all detonators leave the same resistance to hydrodynamic and dynamic pressure effects. Use detonators with the working depth compatibility. A standard 600 microsecond delay element and the manufacturer's recommendations for the safety.

If using electric detonators, special care needs to be taken to insulate and waterproof connections to avoid current leakage. Nonelectric shock tube detonators or electronic detonators are better choices. The lead lines to these products are generally channels for using waterproofed tape so that they can be recovered easily ready for reuse. The ends of detonating cord must be sealed to prevent water intrusion. Liquid tape is handy for this.

Blast Design

Powder factors used for underwater blasting will depend on the rock type, hardness, and most of all, the water depth. For most application, the powder factor used in underwater blasting will be double that used on surface, with an even higher powder factor used for depths greater than 12 meters (40 feet). High-density explosives and close drill patterns are required to attain the required fragmentation.

Adequate sub drill is always required to ensure that excavations of the shot rock can be carried out to grade. It is extremely costly to re-shoot high spots of the shot does not pull to grade.

The charge weight per delay used till depend on pressure limits imposed by the regulatory authority having jurisdiction. For instance, in Canada, the Department of Fisheries and Oceans limits blast overpressure to a maximum of 100 kilopascals and will impose even tighter restrictions when close to a spawning habitat. Formulas for predicting blast overpressures can be found in Explosive Engineering, Construction Vibrations, and Geotechnology by Lewis Oriard et al in the Guidelines for the Use of Explosives in or Near Canadian Fisheries, Flam Department of Fisheries and Oceans.

Overpressure Mitigation Measures

The measures listed in table 37.1 can be taken to control blast overpressure.

Overpressure Mitigation Measures for Submarine Blasting

Table 37.1 – Overpressure mitigation measures for submarine blasting.

Caution The bubble curtain may frighten fish and deter them from reentering the blast area.

Use of a properly functioning bubble curtain can result in up to 10 times reduction in overpressure. A simple bubble curtain (See figure 37.5) can be made using bull hose from an air track drill. Figure 37.6 shows a bubble curtain being pressurized.

ADDITIONAL RESOURCES

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