A rockfall prevention expert is recommending that homeowners of an Aiea Heights subdivision hire a geotechnical engineer to survey the hillside area after a dislodged boulder crashed into a home Saturday, injuring two people.

"There’s definitely things that should be addressed up there," Cliff Tillotson, vice president of Prometheus Construction, a rockfall mitigation firm, said yesterday.

Tillotson assessed part of the hillside at the Aiea Park Place subdivision Monday and yesterday at the request of a friend who lives in the neighborhood and a Realtor who represents a number of homeowners. He determined "root wedging" from a nearby tree dislodged the boulder, which crashed into a two-story home at 99-868 Holoai St. Saturday afternoon.

The boulder split into multiple parts as it fell about 15 feet. One piece pushed a desk on top of a 36-year-old woman. A 50-year-old man was found lying in the hallway, halfway out a bedroom door, as he attempted to scramble out of the way when loud rumbling sounds were heard. The residents were taken to a hospital in stable condition.

Tillotson said tree roots apparently grew into cracks in the boulder. Over time, fractures formed and the growing roots dislodged the boulder. Recent heavy rains also might have caused soil to expand, which is another chief cause of rockfalls, he said.

Tillotson said there’s evidence of root wedging along a 10- to 25-foot band of rocks – some as large as a Volkswagen Beetle – near the area where the boulder fell. "They’re cracking," he said. "With root wedging, things can come down at any time." Boulder removal and safety netting should be considered, Tillotson said.