The state Department of Labor and Industrial Relations will investigate what caused a 6-inch diameter rock to hurl into a worker who was removing debris yesterday at the site of an April rockslide at Waimea Bay.

The injured worker, a 24-year-old "high scaler" from Forest Grove, Ore., was part of a four-person crew using a "non-explosive boulder buster" to break up the rock and make it easier to take down from the hillside, said Chris Ingram, president of the company that employed the worker, Oregon-based Hi-Tech Rockfall Construction, Inc.

The process involves drilling a hole into a boulder, then filling it with water, along with a "booster cartridge similar to a shotgun shell and then pulling the trigger," Ingram said. "The force of the water turns the rock into pieces. It is considered non-explosive. ... We are not using blasting out there."

A piece of the rock bounced off a deflector shield shortly before 9 a.m. yesterday and hit the worker in the head, opening up a 2-inch wound behind his right ear that needed six stitches to close.

It was the first injury in the project to reduce the risk of boulders falling onto Kamehameha Highway since two rockslides have forced the closure of the only road into and out of the North Shore, said Scott Ishikawa, spokesman for the state Department of Transportation.

In April, boulders rained down on Kamehameha Highway, choking North Shore traffic. The rockfall occurred in nearly the same location as a March 2000 rockslide that closed Kamehameha Highway for 95 days.

The injured man was working on an eight-day state contract worth $151,600 to remove hazardous rocks from the hillside.

There were conflicting reports yesterday on the condition of the injured worker.

The man was taken by city paramedics to The Queen's Medical Center in serious condition. Although he was in intensive care, Ingram said the worker's neurologist told Ingram that he was being kept overnight only as a precaution.

But Bryan Cheplic, spokesman for the city's emergency services department, said the man's condition had worsened from serious to critical while he was at Queen's.

Hi-Tech Rockfall's company policy requires workers to be at least 100 feet away when the boulder buster goes off, Ingram said. But he did not know how far away the workers were yesterday when the man was hit.

"It's a freak accident," he said. "They're my ... best crew. They're my A-team. I'm sure they were more than 100 feet away."

A state investigator will visit the scene this morning and interview workers and Hi-Tech Rockfall's management to confirm whether the company has a safety plan and whether workers were adhering to it and following other safety guidelines, among other issues, said James Hardway, spokesman for the state Department of Labor and Industrial Relations.

"We will try to the best of our ability to determine how safe the employer is in making the job safe for the employee," Hardway said.

Out of 30 or so similar companies around the country, Ingram said his company has the best safety record in the U.S. "rockfall mitigation" industry and has experienced only two previous accidents since the company was founded in 1996.

The most serious injury was a broken shoulder, he said.

He said his company has a .81 "mod factor" when it comes to determining workers' compensation premiums.

Bob Dove, CEO and president of HEMIC, Hawai'i's largest writer of workers' compensation insurance, did not know Hi-Tech Rockfall's mod factor but said yesterday that .81 means "that for guys that do what they do, they are relatively safer than the other people who do that work."

A mod factor of 1.00 represents the average for any industry so .81 "is one indicator that they're generally safer than the other guys."

Hi-Tech Rockfall is ahead of schedule and has cleared more debris at the rockfall site than required, Ingram said.

A private consultant determined yesterday that more material has to be removed, so Hi-Tech Rockfall is negotiating to continue the work through Saturday, Ingram and Ishikawa said.

The state Department of Transportation said yesterday that drivers will face intermittent road closures around the work through Saturday.

Reach Dan Nakaso at dnakaso@honoluluadvertiser.com.