Chapter 25: Tying-In and Initiating the Blast

The blast tie-in is likely the most critical phase of the blasting process. It deals with the connection and testing of the initiation system used to begin, sequence, and control the detonation of the many separate explosive charges in a modern commercial blast. Because initiation system tie-in controls the progress and performance of the blast, its correct execution is of primary concern in any blasting operation.

Whether the blast is in a large surface operation, small quarry, construction site, demolition process, underground development, or underground mass blasting, errors or omissions in the tie-in process have been the primary cause of misfires. As initiation systems have become more complex, manufacturers have requirements for more detailed procedures to minimize errors and omissions.

TYING-IN

The tie-in process begins with the timing designs created by the blaster-in-charge, blast engineer, or other technically proficient person. This person must have the end goal of the blast in mind in this design, and have an excellent knowledge of the capabilities and limitations of the initiation system he or she has chosen. These are discussed in chapter 7 and are critical to the execution of the tie-in. The result of this planning should be a tie-in map for use on the pattern or at the face.

Once the tie-in design is finalized and the boreholes are loaded, the best practice process requires that the blaster-in-charge take ownership of the design and its implementation on the blast. This is a critical responsibility in the tying-in process, so it ensures that the succeeding steps will be executed and checked for proper completion. This individual will ensure that the tie-in is organized, executed according to the plan, and checked before the final step of shot firing occurs. A final approval to go ahead with firing the shot should come only from personnel that have assured themselves that all connections and tests have been properly made.

Procedure

Before the tie-in process begins, all personnel and equipment not required for the tie-in must be removed from the area of pattern or face to be tied in. Distractions, interruptions, or obstructions to a controlled tie-in process must be eliminated. These hindrances will greatly increase the possibility of errors and omissions in the course of the tie-in. Only qualified and experienced personnel should be used. If training is occurring during the tie-in, the trainee must concentrate on the trainee only and not attempt to tie-in other parts of the blast.

At the commencement of the tie-in process, the owner of the tie-in will determine if he or she will perform the actual connection of the blasting array or assign others to that task. The best case is that the owner performs the tie-in alone. In the case of complicated or large blasts, a delegate or delegates may be used to execute the tie-in. Whether done by the owner or delegated to others, the tie-in must be followed up by a thorough check by the owner or her delegate.

For all initiation systems, the tie-in should be made to minimize loose or tangled lines of connection. This will reduce the chances of snagging by personnel walking on the pattern or working at the face. This can cause damage that may not be picked up by the later checks and tests, or require blast delay due the need to repair problems shown by checking or testing. An effort should also be made to isolate tie-in lines from loose faces or poles that may collapse and damage the lines.

Depending on the type of initiation system or connections being made, the manufacturer of that system will provide detailed instructions and warnings in its application. The product discussions in chapter 13 provide general use procedures for products. However, the manufacturers' literature and training is the ultimate authority and must be followed.

Check the Tie-In

A critical part of the tie-in and firing process is a check by the blaster-in-charge before the final firing proceeds. Different systems require varying methods of checks. Attempts to rush this final check are often placed on the blaster by operating personnel in an effort to minimize the down time of loading crews. The blaster must resist this request or pressure and make sure the tie-in check is completed properly.

Electronic System Checks

The proper and safe use of electronic detonator systems requires a much higher level of training and understanding than nonelectric or electric systems. The blaster-in-charge (responsible person and owner of the blast) must have extensive and comprehensive training that includes thorough trouble shooting experience.

Electronic detonator systems have extensive and complicated checking mechanisms and routines. Due to the presumed intelligence of the systems, it may be tempting to assume that any faults or missing detonators will be discovered by the "smart" system. Make no assumptions about what the system will discover. There is no substitute for checking or reviewing what the system check has discovered and recorded about the tie-in array. Numbers of detonators and their assigned firing times and sequences should be verified by the blaster.

It is also important to recognize that several levels of energy are used to conduct tests with electronic systems. The blaster must be aware of and understand the difference between the levels of testing (testing individual detonators, testing groups of detonators in a string, and testing the complete array). These may occur at different voltage levels that may alter the possibility of the detonator firing inadvertently. The difference between testing devices and firing devices must be understood and the manufacturers' procedures carefully followed.

Electronic detonator systems have complicated and precise signal codes and voltage levels that are required to function properly. Stray current from leaking power cables or overhead lines can cause communications problems. Remote control devices used on or near the pattern can also disrupt communications. Cell phones or wireless devices are also a potential problem. Familiarity with these hazards and their resolution are necessary for the blaster-in-charge to be able to operate these systems. Have the proper training and references from the manufacturers of the system are necessary.

Nonelectric System Checks

For nonelectric initiation systems the check is visual, and requires confirmation that all connections are made according to manufacturers' specifications and the blast design. This visual check should also confirm that the tie-in was done to the specifications of the design. A count of boreholes or in-hole detonators should be made and used to confirm that the tie-in connected the correct number of boreholes.

Before the fire command is given, the blaster-in-charge must be sure that all loaded boreholes have been connected to the firing circuit in the proper order. This means that the individual should personally inspect each charge for the proper connection.

Electric System Checks

It is assumed that stray current and static electricity checks of the blast site have been made before loading with electric detonators. If not, the connection of several detonators and lead in line can create an antenna that will collect voltage that may be a premature firing hazard or possibility of firing failure.

With electric detonators, the check is both a continuity check and resistance (ohms) check. In addition, the tie-in must be executed in a manner that does not leave any connections exposed to a grounding condition. This includes bare wires contacting the ground or in water. Low voltage continuity and resistance checks may not pick up a current leakage condition caused by bare wires or poor taping that allows leakage. Sequential cables should also be checked and maintained to prevent or repair chafing that wears away insulation between conductors. This leakage will also be missed by the low voltage continuity and resistance tests.

Caution

A "blasters'" multimeter is the preferred instrument to make the final circuit test.

The "Blasters'" multimeter will measure the total resistance of the circuit to compare with the calculated resistance of the circuit load (See Electric Blasting Circuit Design, Calculations, and Hazard Assessment in Appendix F). A variance exceeding 10% should be corrected.

Lead Lines

The layout of lead lines from the blast to the point of safety should be planned to avoid traffic or other potential damage that would interrupt the firing signals. This point of safety will be the location from which the final firing command is given by the blaster-in-charge or his delegate.

Lead lines should avoid being too close to caving or raveling high walls that may roll rock on the lines. Hard road crossings should be avoided. If necessary, the connection across the hard road or other trafficway should be made as late off the blast crew leaves the blast site to fire the blast.

Wire lead lines should be kept on the ground as much as possible. Suspending them in the air will magnify their potential as an antenna to pick up stray current, signals, or other interference. Running lead lines closely parallel to or crossing energized power lines or cables can also cause stray current pick up or signal disruption.

Where permanent lead lines or sections are used underground or on surface projects, these lines must be secured and locked to prevent tampering. This security must be ensured where the lines terminate at or near a power source. They should also be tested before the final blast clearing to make sure no delay or misfire will occur during the actual firing of a blast.

General Precautions

The blaster-in-charge must ensure that all connections are made correctly, regardless of the system used, before firing the blast. There should be confirmation that all boreholes or detonators have been tied in to the initiation array and will receive a firing signal. If there is any doubt about any portion of the blast or tie-in, it should be rechecked and confirmed to be correct.

Special Considerations for Underground Blasting

Due to the limited light conditions in underground blasting (face, stope, shaft, or raise) a special effort has to be made with all initiation systems (including electronic) to assure that all boreholes are tied in. The tie-in lines must be protected from damage from detonating cord or ground movement after the tie-in process.

In nonelectric blasting, the use of detonating cord to initiate the shock tube creates special hazards from the explosive effect of the cord detonating. All connections of shock tube must be at right angles to the run of the detonating cord and made with the manufacturer's connector where possible. The shock tube must then run away from the detonating cord in a straight line to the collar of the borehole it initiates. No shock tube should run parallel to, or across detonating cord.

Where a detonator is used to initiate the detonating cord, the detonator must be far enough from the face or round so that shrapnel from its explosion does not damage the detonating cord tying-in the round or shock tubes in the round.

As noted above, a detonator count is critical to the final check of any initiation system in underground blasting. A final check of loaded detonators versus the number of those tied in will prevent misfires. In most underground applications, the failure to tie-in caps is the primary cause of misfires. With electronic systems, the blaster may feel more secure that the system itself will do the cap count. However, if the initial number loaded versus the number recognized by the system is not known, the result will be misfires.

INITIATING THE BLAST

The firing of the blast must be as soon after the tie-in is completed as possible. As operations are usually suspended before and during the firing of the blast, it should be scheduled at a time that will coincide with a pause in normal operation – lunch, break, or shift change.

Consideration should also be given to surrounding neighbors. It may be necessary to interrupt highways and transportation routes for the blast and coordination with local authorities will be necessary. Shot firing should be avoided during early morning, or late afternoon or evening. These times will find neighbors at home and not expecting disruptive due to noise and vibration. These times will also escape traffic rush hours if roadways or ramps are not in the perimeters.

Many blasting plans require filing of blasting schedules with local authorities. The above factors should be considered when writing the blasting plan.

Safety Perimeters

As the first step in firing the blast, a perimeter distance must be established for estimated or likely flyrock. This perimeter circle around the blast is often based on the history of previous blasts of the same design in the same geology. This is the responsibility of the blaster-in-charge and is a further confirmation of the need for this person to have extensive experience and training. Please refer to chapter 15 for recommendations on setting this perimeter distance (clearance zone).

Once the estimated flyrock perimeter has been established, the equipment safety perimeter can be made. This is the distance from the blast that mobile or stationary equipment (loading equipment, pumps, ambulance, crushers, etc.) can be moved to avoid the potential of flyrock damage. This perimeter is usually just outside the flyrock perimeter.

The last perimeter to be established is the personnel limit. This is normally at least double the estimated flyrock distance, but may be more with unusual circumstances. Special personnel limits must be used in the case of blast boulders, loose or cracked material, seams in the rock, or wet and inadequate stemming.

Blasters often fire their blasts from inside the personnel perimeter. In this case, protection from flyrock must be used. This has to be protection from rolling, ricocheting, or direct flying material. As flyrock is the most often occurring cause of injury or death resulting from blasts, it is imperative that it be protected against. If fired within the personnel perimeter, even with flyrock protection, the firing point should be in front of the blast. This is the most likely path of any flyrock from a blast and should be avoided.

Blasts should be initiated from approved blast shelters, which may be a dedicated shelter or equipment.

Clearing the Blast Area

After the blast is loaded and tied-in, the next step is to clear the blast area according to the values established in the flyrock and personnel perimeter limits. The blaster-in-charge has the responsibility for making sure equipment and personnel are in a location of safety from flyrock. The blaster-in-charge must also block all paths or methods of access closer than the personnel perimeter. To do this may require the posting of guards to prevent access. This clearing process should be delegated by the blaster-in-charge to another competent and experienced person.

After sealing the blast area from unauthorized access, a clearing or sweep of the blast area should be conducted. This will ensure that all personnel are indeed clear of potential flyrock. Because this blocking, clearing, and sweeping operation can be extensive and complicated, a checklist is recommended for the person conducting the final clearing. All known personnel, equipment, and access locations should be noted. Also the areas or equipment to be cleared are recorded. A positive method of communication must be established with anyone responsible for clearing or blocking. This would normally require wireless or radio contact.

Once the final clearance has been completed, the blaster or blast crew can begin the final connection of the blast to the firing point. In the case of initiation of a non-electric blast with an electric detonator, the electric detonator can now be attached to the lead line and then to the blast. Always attach the detonator to the lead line first in case the lead line has picked up a charge that may fire the detonator. For electric and electronic blasts, the lead line connections can now be made from the blast to the firing point. If a remote system is being used, that process can now begin.

Blast Warnings

A method of audible, and if possible radio broadcast, warning of the impending blast should be established. This warning can be a horn, whistle, or other very loud device with a distinguishable sound and established signal pattern. A system of preblast, blast, and all clear signals should be published and conveyed orally. A preblast signal (can vary) to begin final count down before the blast is to fire, followed by a signal 1 minute or less before the blast, and a signal immediately before the fire command is given. The system outlined in table 25.1 is an example of such a system. These warnings can each be a different number of tones that can be identified as to their purpose. After the blast has been fired and checked by the blaster-in-charge, an all clear signal or message should be given.

Caution

Certain jurisdictions may mandate a particular warning scheme.

Typical Warning System Signals

Table 25.1 – Typical warning system.

Security

The blaster-in-charge has the ultimate responsibility for firing the blast. As such, he should maintain custody of the blasting machine, blasting switch, or control device during the tie-in and preparation to blast. Only the blaster-in-charge, or his designee should have access to the control equipment.

Final Testing

Before any blasting operation is started, all control and power equipment must be tested for condition and level of charge. Nonelectric firing devices should be cleaned and tested. Electric equipment should be tested for charge or battery level, and tested for output. The blaster should also assure that the electric equipment has the capacity to fire the required number of detonators with the planned test-in and electric load. Electronic equipment must also be checked for charge level and test routines run if available. It is also suggested that spare back up equipment be readily available.

Caution

All lead lines should be secured to prevent vertical tram cables from being jerked in contact with overhead power lines.

The completion of the blasting circuit from the blast to the point of safety for firing should not occur until it has been confirmed that all personnel are clear and access points are blocked. At this time the testing of the final circuit from the point of safety can be conducted. An electrical blasting test will require a final continuity check is made with a "Blasters'" multimeter, blasting galvanometer, or blasting ohmmeter. For nonelectric systems, manufacturers' recommendations for the tie-in and checking procedure should be followed.

When using an electronic system where final testing is done by the control equipment, no connection or testing should be done until the blast area is definitely cleared and sealed. After all testing is complete, there is generally no additional connection, and the initiation can occur.

As stated above, the final initiation should be conducted at the point of safety and under shelter from flyrock hazard.

POST BLAST ACTIVITIES

Immediately after the blast has fired, the blaster should disconnect the firing line from the blasting machine or control device, and secure the equipment or switches. The blaster should remain under cover for a short time to make sure all flyrock has landed.

It is the blaster-in-charge's responsibility to give the all clear signal that personnel can return to the blast area. Before this can be done, the blaster must inspect the blast site for any misfires, explosive gases, or other hazards created by the blast. When making this inspection, the blaster may have to wait until any toxic gases, dust or dust clouds have been cleared from the blast. Underground, the time required for proper ventilation and gas exhaust must be considered before re-entering the blast area.

This post blast check will look for any signs of unexploded charges, burning explosives, or misfunctioning nonelectric surface tie-in. Also inspect for overhangs, back break, loose rock, or other hazards. In underground situations, loose material on the ribs and back must be checked and corrected. And the blaster must look for collection of toxic gases in low spots around the blast site. Unbroken or unmoved areas of the planned blast area are of special concern and should be closely investigated for misfire or explosives malfunction.

This post blast inspection or rest combined later is an excellent opportunity for the blaster to evaluate the blast performance. Often, clean up after the blast by optimizing movements and other cover signs of back break, flyrock, or operational results. Before these are altered or eliminated, the blaster has the chance to see what appeared to happen during the blast and explain any visible results.

ADDITIONAL RESOURCES

Atlas Powder Company. 1987. Explosives and Rock Blasting, Atlas Powder Company. Dallas, TX.

Dyno Nobel. 2003. Dyno Nobel Blast Site Safety Manual. Dyno Nobel, Salt Lake City, UT.