Underwater Blasting: A Summary  

Joshua Taylor Drake  South Dakota School of Mine and Technology

Abstract

This report answers specific questions concerning underwater blasting considerations, and blasting safety. The blasting considerations discussed consist of basic blasting principles, and new variables to be accounted for. Blasting safety will include the safety of personnel and structures. The environmental considerations will also be discussed with mitigation methods. To be most effective, this document requires the reader to be familiar with key fundamentals of explosive engineering. In order for a blasting professional to benefit from this report, the following questions will be asked: 1. What are the special considerations for underwater blasting? 2. What are the environmental concerns and how do blasting professionals mitigate them? 3. What are the proper safety considerations? In order to answer these questions, the author used an array of research resources. The bulk of the information used comes from reports gathered from industry professionals on a global scale. Secondary research was gathered from blasting journals and online publications. Examples and figures will be provided to aid the reader. Three visual examples of blasting mitigation will be provided. This report provides the reader with information on special considerations for blasting; environmental concerns and ways to mitigate them; and proper safety etiquette while blasting underwater. The paper is a basic overview and will not cover everything involved in underwater blasting.

 

 

Introduction Underwater blasting allows for construction in places normally protected by bedrock, such as harbors, slips, and waterways. This paper is written to inform blasting students and professionals about certain aspects of Underwater Blasting. These aspects include special considerations, environmental impacts and how to mitigate them, and underwater blasting safety. Special considerations of underwater blasting include new variables that can make blast design more challenging. If these new variables are ignored, it could cause undesired effects to the blast. As with land-based blasting, underwater blasting affects the environment and the aquatic life that resides in it. Pressure and vibration from an underwater blast also affect the environment. Timing, fish removal, fish deterrents, and bubble curtains are mitigation methods that will be discussed. In any blasting situation, safety is paramount. With underwater blasting the safety of any personnel and vessels must be accounted for. Lightning is also a chief concern when blasting underwater due to the sensitivity of the explosives used. Precautionary requirements will be cited from the United States Occupational Safety and Health Administration. Considerations Basic Principles Blasting on land requires many sets of equations, ratios and variables. Equations such as the Ash Method or Konya’s Method, found in the Dr. Konyas’ Rock Blasting and Overbreak Control, allow blasting engineers to calculate the proper burden, explosive charge, and type of explosive to use. The equations are based on a known set of ratios and variables that include stemming, spacing, and sub-drilling. These sets of numbers allow explosive engineers to properly model a blast to minimize the chance of detrimental results from initial blasts. Special Considerations When blasting underwater, new variables are encountered; such as aquatic life, water overpressure, and a few new safety hazards. Underwater blasting poses a unique challenge to explosives engineers to design a blast that minimizes the effects on animals in the area. The impact to endangered species is hard to completely control. Furthermore, pressure from a blast is not only magnified underwater, but refracts off of almost any surface. This makes modeling an underwater blast pressure pulse extremely difficult, if not near impossible. Underwater blast designs are in two dimensions instead of three; this is reflected in the powder factor units. Underwater blasting is “blind” in that a blasting engineer cannot physically see each hole; this makes drilling and loading extremely difficult. When blasting underwater, a blaster can get many blowouts due to the lack of knowledge about the underlying geology and rock faces. Normally during a blast, safety perimeters are established and a written and agreed safety procedure is followed to ensure a safe blast. An underwater blast gives safety a whole new dimension. When blasting underwater, a land perimeter as well as a “wet” perimeter must be maintained. As with land based blasting safety concerns include lightning. These include special rules regarding explosives on a vessel, as mandated by the US Coast Guard.

 

 

Environmental Response Concerns Aquatic Life Aquatic life can be affected when an underwater blast occurs. How it is affected is greatly dependant on size of the animal and its proximity to the blast. Although vibration from a blast can impact life in the embryonic stage, the resulting blast pressure is, overall, more detrimental to fish and marine mammals. Injuries received from underwater blast pressures are called barotrauma. The most common types of injuries recorded are trauma to swim bladders, hemorrhage, and embolism. For physical signs of barotrauma see Figure 1. In Kolden and Aimone Martin’s Blasting Effects on Salmonids, the authors reviewed available literature, which suggested the minimum peak pressure pulse to harm salmon is 10 psi (69 kPa). Fish tissue with high water content has a density close to that of water; this would allow pressure pulses to pass through the tissue relatively undisturbed. However, different tissues such as sensory systems consisting of nerves and connecting tissues have different densities associated with them (Glass, 2009). When the pulse encounters a gaseous void, such as a fish’s swim bladder, it rapidly changes the pressure of the enclosed gas. This change causes the swim bladder to expand or contract violently, damaging itself and other tissue/organs around it. Sometimes the swim bladder can even burst causing internal bleeding. This is a common type of barotrauma in fish with swim bladders and can be lethal. With damaged organs and/or a burst swim bladder, a fish is either killed instantly or made easy prey to predators. Hemorrhage can occur internally or externally and is seen often in fish subjected to underwater explosions. Hemorrhaging can occur in the gills, kidney, brain, and heart as well as other organs, tissue, and fins (depends on which study one references). If severe hemorrhage occurs it can be fatal to the fish. Embolism is also caused by extreme pressure changes. It is the formation of gas bubbles in and outside of blood vessels. This is observed to be common in the heart, gills, swim bladder and kidney. This injury can also be fatal (Kolden, Aimone-Martin, 2013).

Figure 1 Diagram showing physical symptoms of barotrauma (Habermann, 2013).

 

 

Vibration The main purpose of an underwater blast is to fracture and displace rock allowing for some form of excavation or recovery. This mechanism coupled with shock waves, compression and tension waves cause rock breakage. Unfortunately, vibration can be detrimental to the environment. At certain frequencies and magnitudes it can cause damage to nearby structures. Sometimes vibration in substrate under a body of water can impact fish in the nearby area. Studies have shown that salmon in certain embryonic stages can be susceptible to vibrations from underwater or near water blasts. Determined experimentally through observation and testing, the peak particle velocity below which some salmonid embryos are unaffected is 5.8 in/s (147 mm/s) (Kolden, Aimone-Martin, 2013). Other fish species may have different tolerances. Note that the vibration of the water above a substrate cannot be measured. Pressure Detonation of an explosive underwater generates three types of pressure. First is the primary pressure pulse, then the secondary pressure pulse, and finally air bubble pressure. The primary and secondary pressure pulses can affect structures, vessels, fish and marine animals in the vicinity of a blast. These pressures must be taken into account when designing an underwater blast. 1. Primary Pressure Pulse

The primary pressure pulse is the initial compression wave created by the displacement of water by the explosion. It is the most destructive of the pressure pulses and, in the case of an omnidirectional charge, propagates outward in all directions. However, certain media like bubble curtains, or thick mud/sediment absorb much of the energy. Hard surfaces, such as rock faces and steel or concrete bulkheads, reflect the pressure pulse which will attenuate with distance from the blast.

2. Secondary Pressure Pulse (Reflection of Primary)

The secondary pressure pulse is the reflection of the primary pulse off of the surface. It can also be reflected off of the bottom, if the bottom is made of a relatively hard rock with little sediment cover.

Mitigation Because blasting has such a profound effect on the environment, mitigation must be a central part of a blast. Although there are many different forms of mitigation for underwater blasting, only the top four mitigation measures with regards to fish will be discussed below: timing, fish removal, fish deterrents, and bubble curtain. Please note that mitigation can be achieved through blast design methods such as lowering charge weight and hole sequence timing. 1. Timing

The easiest way to avoid affecting fish and other life within a blasting area is to select a season to minimize effects of the blast. This strategy utilizes the absence of fish (or aquatic life) at certain times throughout the year. An example of this is; when blasting where salmon migrate, a blaster would wait until a time of year when salmon are no longer in the area.

2. Fish Removal Another method of mitigation is physically removing any and all aquatic life from the blast area. This would be accomplished through trapping, netting, electro-fishing, and dewatering. This method is easiest when the blasting area is small and shallow. It is important to note that this approach can be impossible in some terrain. Fish removal normally has a high cost associated with it. Training may be needed to prevent improper handling of fish that would cause unnecessary harm.

 

 

3. Fish Deterrents Scaring the fish away from the blast site is another viable approach to mitigating loss of aquatic life during a blast. There are several types of deterrents ranging from scare charges to acoustic deterrents. Scare charges are nonlethal explosive charges that are set off before a blast to “scare” away any aquatic life within the proximity of the blast. The effectiveness of this method has not been measured. Acoustic deterrents use sounds at certain frequencies to repel fish and have been utilized with some success. Most notable of the success stories are acoustic “pingers” used to scare dolphins away from fishing nets as seen in Figure 2. Other methods include hazing fish with boats and visual deterrents such as strobe lights.

Figure 2 Diagram of “pinger” used to scare dolphins away from fishing nets (Cornwall Life Trust, 2010).

4. Bubble Curtain

Bubble curtains are designed to create a barrier of bubbles in a column of water to prevent fish and other aquatic life from entering a blast area. Note that this form of mitigation has a high cost, takes time, and is sometimes difficult to install. This method works well for static water but has been proven ineffective for swift turbulent water (Kolden, 2013). An example of bubble curtains is shown in Figure 3, where it is being used to isolate a drilling platform.

 

 

Figure 3 Photo of a bubble curtain in use to isolate drilling platform from fish (Admin, 2012).  Safety Drill/Blast Boat Safety In any blasting situation, the safety of personnel is paramount. In order to be safe during an underwater blast, sets of safety practices have been developed. The Occupational Safety and Health Administration suggest precautions such as: • “A blaster shall conduct all blasting operations, and no shot shall be fired without his approval.”

• “Loading tubes and casings of dissimilar metals shall not be used because of possible electric transient currents from galvanic action of the metals and water.”

• “Only water-resistant blasting caps and detonating cords shall be used for all marine blasting. Loading shall be done through a non-sparking metal loading tube when tube is necessary.”

• “No blast shall be fired while any vessel under way is closer than 1,500 feet (457 meters) to the blasting area. Those on board vessels or craft moored or anchored within 1,500 feet (457 meters) shall be notified before a blast is fired.”

• “No blast shall be fired while any swimming or diving operations are in progress in the vicinity of the blasting area. If such operations are in progress, signals and arrangements shall be agreed upon to assure that no blast shall be fired while any person is in the water.”

• “Blasting flags shall be displayed.”

 

 

• “When more than one charge is placed under water, a float device shall be attached to an element of each charge in such manner that it will be released by the firing” (Safety and Health Regulations for Construction).

Lightning Lightning, as with land blasting, is a special concern with underwater blasting because if it were to strike near the blasting area, the result could set off the explosives. This is a safety hazard to people, structures, and vessels in the surrounding area. Therefore, safety precautions must be taken. These include furnishing, maintaining, and operating lightning detection systems during the project. When lightning is detected, the blasting contractor should: 1. Notify the Coast Guard of the hazard.

2. Clear the exclusion area, marked by buoys, of all vessels and personnel.

3. Terminate blasting activities and return explosives to the magazine.

4. Monitor the blast area to prevent anyone or anything from inadvertently entering the blasting area during the hazard.

5. Remove lightning detector from drill/blasting barge and monitor the hazard from land until it passes.

6. After sounding an “All-Clear” signal, notify the Coast Guard.

7. Resume operations (Konya, 2003). Security In order to be safe during any blast, security must be maintained to prevent anyone from entering an unsafe blast area. Buoys should be placed with warning signs around the blasting area and should be legible at 200 ft (61 m). When working on waterways, security is vital. At least two patrol boats should be used to ensure security of the waters around the blasting area. Both vessels should be equipped with a radio, spotlight, and horn. Land-oriented security is also essential in preventing unauthorized access. Prior to firing, the blast area should be inspected thoroughly to ensure there is not any person or vessel within the buoyed work area (Konya, 2003). Conclusion This report has discussed special considerations of underwater blasting, environmental impacts and how to mitigate them, and underwater blasting safety. While blasting underwater, new variables are encountered which have an effect on the blast design. Pressure and vibration from an underwater blast affect the environment but can be mitigated. Safety is paramount, key safety factors include lightning and the safety of the vessels and personnel involved. In order to best represent professionals in this field, the utmost care must be taken in the above-mentioned portions of an underwater blast.

 

 

References

1. Admin, C. A. S. (Photographer). (June 05, 2012). Big Bubble Curtain HY75 [Print Photo].

Retrieved from http://www.hydrotechnik-luebeck.de/big-bubble-curtain-hy75?lang=en

2. Cornwall Life Trust. (Artist). (Dec. 13, 2010). Protective Signal Highlighting the Presence of Nets to Dolphins [Web Photo]. Retrieved from http://www.cwtstrandings.org/news/pinger_trial/pinger_trial_131210.htm

3. Glass, M. L., & Wood, S. C. (2009). Cardio-respiratory control in vertebrates: Comparative and

evolutionary aspects. Dordrecht: Springer.

4. Habermann, R. (Artist). (May, 2013). Physical Signs of Barotrauma [Print Graphic]. Retrieved from http://www.seagrant.umn.edu/newsletter/2013/05/let_em_down_easy_returning_a_fish_to_deep_water.html

5. Kolden, K. (2013). Management considerations for blasting near fish and fish habitat. Juneau,

AK: Alaska Seismic and Environmental LLC.

6. Kolden, K., Aimone-Martin, C. T. (2013). Blasting effects on salmonids . Douglas, AK: Alaska Department of Fish and Game.

7. Konya, C. (2003). Guide specifications for surface and underwater blasting. Montville, OH:

Precision Blasting Services.

8. Konya, C. (1985). Rock blasting. S.l.: U S Govt Printing Office.

9. Safety and Health Regulations for Construction: Underwater Blasting, Standard Number: 1926.912. (n.d.). Retrieved August 5, 2014, from https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10818