An explosives expert who was called to the scene of a blasting incident which sent rocks flying into the Lobird Park trailer court appeared before a packed courtroom Tuesday to give his opinion of how the event occurred and how it could have been prevented.

Richard Scott Parker brought almost four decades' experience to the stand as he explained to the court what likely went wrong on the May 6, 2008 blast, part of the $15-million Hamilton Boulevard extension project.

The explosion threw jagged, ham-sized rocks into the trailer court, punching holes in roofs, fences and sheds. Several thousands of dollars' worth of damage was done, but no one was hurt in the mishap.

Parker appeared as a Crown witness in the trial of contractor P.S. Sidhu Trucking, the Yukon Department of Community Services and site supervisor Bill Cratty, all of whom are charged under the Occupational Health and Safety Act for permitting an unsafe blast to be detonated.

The trial is being heard by Territorial Court judge John Faulkner.

Parker began by explaining the blasting process, whereby a number of holes are drilled in a pattern, then filled with either granular or water-resistant explosives, followed by a fuse line, then topped with ground-up rocks to force the explosion outward instead of upward.

The idea is to have the blasts go off in sequence, so that each explosion facilitates the next and forces the exploded rocks in a certain direction so they can then be removed.

If one hole doesn't blast as planned, he said, chances are the following holes will also malfunction, sometimes causing the explosion to "rifle”.

"That's when the holes detonate from the bottom to the top,” Parker explained, "very much like a shotgun.”

This can send rocks flying into the air, he said, as they did on May 6, 2008.

Although Parker did not arrive on the scene until eight days after the offending blast, he said the blast site and resulting muck pile (the pile of smashed rocks created by the explosion) had not been disturbed. He did note, as other witnesses also have, that rocks had clearly been moved off a nearby roadway.

Based on the information provided to him, Parker said the unexpected flying rocks were likely the result of a too-shallow hole which didn't detonate as planned, causing the blasts which followed to also malfunction.

He added, however, that "nobody can tell for sure” what has happened in any blast once the detonator has been triggered.

When asked what should have been done to prevent the rocks from sailing into the nearby residential area, Parker would not lay blame.

"Blasting is not an exact science,” he said "... No one can do a perfect blast every time. That's why blasting companies carry insurance.”

The court heard Tuesday that the blast site was not covered with sand and blasting mats which might have stopped the flying rocks, but Parker said the area was too large for that.

"Because of the number of holes, I don't know that it would be practical to cover the site,” he said.

Parker said the blaster could have reduced the number of holes to make sand or blasting mats feasible.

Blasting mats, he added, are often more trouble than they are worth because they can damage the wires which connect the detonators and create a very dangerous situation

where the blaster has to go looking for unexploded holes.

In a dense urban setting, he said, the blaster should lay down sand, then put mats on top.

But this is a very expensive and time-consuming way of doing things, he said, and is only done when blasting right next to a structure. Often, he said, it is more reasonable to protect the structure than cover the blast.

The court heard that 430 holes were blasted in one go on the evening of May 6, the largest single blast done on the whole project.

"Possibly two holes malfunctioned out of 400,” Parker said.

He was asked if, in his opinion, the blaster was the only person on the work site responsible for ensuring a safe blast.

"Collectively, everyone is responsible for safety,” Parker said.

On Monday, the judge heard that the blaster, Peter Hildebrand, did not know how close Lobird Park was to his blast site, but estimated it was at least 1,000 metres. In fact, the trailer court was just 350 m away.

"There should have been enough people on site to tap him on the shoulder and say, ‘The trailer park is right there,'” Parker said.

He also said the site engineer should have provided the blaster with a map of the area and the site inspector who looked at the blast proposal should have noted the park's proximity.

The blaster who designed and detonated the May 6 blast pleaded guilty to the charges against him earlier this month and was fined $1,000.

The trial is scheduled to continue until the end of the week.