A controlled commercial blast is over in far less time than it takes to read this sentence. Yet the technical planning and preparation can take hours, sometimes days. The elements of this procedure are important to Maine Drilling & Blasting.

As a subcontractor, the blaster follows specifications set by the contractor, developer or government agency. The blaster also must adhere to the most restrictive of federal, state or local laws concerning the amount of explosives allowed and the magnitude of the resulting blast.

Pre-Blast Preparation

• The blasting superintendent evaluates the nature of the rock that needs to be removed and the general scope of the job.
  
  
• The safety specialist performs a pre-blast survey, inspecting and recording the homes, businesses, utilities and other structures in the area local to the blast site.
  
  
• As part of a community awareness program, the safety specialist distributes informational leaflets and discusses the possible effects of the blast on neighboring homes and businesses.
  
  
• The safety specialist and the blasting superintendent share their information and the blaster tailors the plan to create the least disruption while assuring safety.
  
  
• On a more complex job, a computer program may be tapped to help the blaster design the blast and customize the plan. In any case, a test blast is set off and monitored. The resulting data, measured by seismic and other monitoring equipment, is analyzed and integrated into the blast plan.

Designing a Blast
To ensure safety while still fragmenting the rock to the specification, the blasting superintendent has several choices to make and variables to manipulate. They include:

• The appropriate size of the blasts.
  
  
• The number of holes to be drilled, the depths and diameters of the holes, the pattern of the holes and the spacing between them.
  
  
• The sequence and pattern of multiple explosions. He determines the number of small explosions, thousandths of a second apart, that comprise a single blast. Designing the sequence adds to the amount of control the blaster has.
  
  
• The amount of explosive material needed and how much of it to load into each hole.

The Blast

• Blasting crews set the blasting caps, load the holes with explosives, stem the holes with crushed stone, and connect each hole within the shot.
  
  
• Sometimes, to prevent rocks from flying, the blaster may cover the site with mats. A typical mat is made of recycled tires tightly compressed with steel cable. A 12-foot by 12-foot mat weighs about 6,000 pounds.
  
  
• Up until the time of the blast the shot area is secured.
  
  
• The site is evacuated and traffic, if any, is stopped.
  
  
• A horn is sounded three times to mark "Five Minutes Until Detonation."
  
  
• The horn is sounded twice to announce the one-minute mark.
  
  
• A blaster initiates the blast.
  
  
• The blasting crew inspects the site to make sure that all the explosives have been properly detonated.
  
  
• The horn is sounded once to signal "All Clear"