More than 100 firefighters battled two brush...

More than 100 firefighters battled two brush fires that charred 20 acres in Irvine Regional Park on Tuesday. The fires started when a runaway boulder knocked down power lines.

The 13-foot boulder was accidentally set loose by a bulldozer working on the Eastern Transportation Corridor, county Fire Capt. Dan Young said.

At 3 p.m., park employees and construction workers reported the fires, about half a mile apart, east of Santiago Canyon Road and south of Santiago Creek.

About 120 firefighters worked for three hours in 90-degree heat to contain the blazes. Five helicopters doused the fires with red detergent, also known as Phoscheck, and water from nearby Irvine Lake.

Officials evacuated about 40 park visitors. No homes were threatened.

“The creek limited where the fire could go north or west,” Young said. “And the areas south of the flames were just land cleared out for construction of the Eastern Transportation Corridor.”

Workers with Sukut Construction, a Santa Ana subcontractor, told officials they were clearing land when a boulder was set airborne by a bulldozer.

The boulder, which Young said was larger than a car, went tumbling down the hillside, cutting through trees and several power lines. Some 440-volt lines swayed, slapped against each other and fell, igniting the fires.

“This was an accident by construction,” Young said. “With a boulder that big, I don’t know how anyone could’ve stopped it.”

No one was cited and the company may be accountable for some of the damage costs, he said.