Two vehicles hit the 1,000-pound boulder while trying to avoid it and not one vehicle as initially reported by emergency crews yesterday. The drivers, both women, were seriously injured.

Leigh Ann Siaosi, older sister of University of Hawai'i quarterback Tim Chang, was one of the injured, a relative said. She underwent surgery yesterday.

Grace Pacific Corp., an asphalt paving and quarry company, said through a spokesman that the boulder, which measured 5 feet in diameter, fell around 8 a.m. yesterday while crews were working to refortify the quarry's lower rim wall.

"This was a rock that we dislodged," said Sidney Quintal, vice president for marketing at Grace Pacific Corp. "We deeply regret it. We're conducting a full-scale, in-depth investigation."

Quintal said this was the first time a rock has fallen from the quarry onto the freeway. The digging, excavating and bulldozing at the quarry yesterday were meant to prevent rockslides after the recent heavy rains, he said. He said Honolulu police have not contacted the company about the incident.

The boulder landed in the westbound lanes of the H-1 Freeway about a half-mile before the Makakilo off-ramp and was struck by the SUV, firefighters said.

Paramedics took the victims — Siaosi and a woman whose name was not released — to The Queen's Medical Center in serious condition, said Donnie Gates, assistant chief of the city's Emergency Medical Services.

Fire Department spokesman Capt. Kenison Tejada said the victims suffered leg injuries. Neither victim was pinned in the SUV, he said.

The boulder fell at 8:13 a.m. Westbound traffic was backed up for about an hour. Grace Pacific crews removed the boulder from the freeway by 9:30 a.m.

It appears that one of the vehicles pushed the boulder before stopping, Tejada said.

State Department of Transportation spokesman Scott Ishikawa said the DOT initially sent out investigators thinking the boulder came from state-owned land.

Ishikawa said the DOT investigation is ongoing and that disciplinary action, if any, is pending. Ishikawa said the state has asked Grace Pacific to provide, in writing, a list of safety measures to be put in place to prevent a similar incident.

Last year, a state Department of Transportation survey identified 66 highway sites on O'ahu that have a high risk of rockfall. Fifty-one other areas were rated moderately high. Cost estimates to mitigate the top 10 sites alone total $89.5 million. The sites surveyed all are on state-owned land.

Ishikawa said that before the boulder hit the freeway, it rolled through a state-owned area that ranks 95th on the list of 112 state-owned sites recommended for mitigation.

"The study was based on natural rockfalls, not those by man-made causes," said Ishikawa.

The DOT already has spent millions to install rock catchment devices at Makapu'u and Waimea and to reshape the hillside at Castle Junction in Kailua.

According to the company's Web site, Grace Pacific purchased the stock of Pacific Concrete & Rock Co. from Baker International in 1984 and, as a result, secured the lease to the rock quarry in Makakilo. Grace merged the two companies, creating Grace Pacific Corp.

Falling boulders have killed people, damaged homes and disrupted traffic on several occasions in recent years.

Reach Mike Gordon at mgordon@honoluluadvertiser.com or 525-8012. Reach Peter Boylan at pboylan@honoluluadvertiser.com or 535-8110.