The state Department of Transportation hopes within weeks to complete a yearlong project to prevent large rocks from tumbling from a Makapuu hillside onto Kalanianaole Highway before the rainy season arrives in earnest and aggravates that risk.
It is one of 10 projects planned or under way by state and city transportation officials to stabilize hillsides on three islands.
The La Nina weather pattern, which results in cooler ocean water surface temperatures, is forecast to bring a wetter-than-normal season from January to April, according to hydrologist Kevin Kodama of the National Weather Service.
"We can easily see some rain events that are going to be more of what we’ve been seeing," said Kodama.
The forecast is a red flag for transportation officials as potential rockfalls are typical during the rainy season.
"Anytime we have a big rain coming down, especially over an extended period of time, you’ll tend to see more rockfalls," said state Highways Administrator Ed Sniffen. "When the water comes down, it saturates the ground. If the ground is saturated when more water comes down, there’s no place to go. There’s a high erosion potential. The more saturation, the more potential for rockfall."
Other factors contributing to rockfalls are steep slopes, fractured rocks, vegetation, warm temperatures and earthquakes, according to Steve Martel, professor of geology and geophysics of the University of Hawaii.
Contractors help protect highways on Big Island
The October 2006 earthquake on the Big Island underscored the need for work to keep rocks from falling onto roadways.
Many loose basalt rocks on the highways throughout the Big Island were observed after the quake, warning transportation officials of the high potential for much more rocky material to fall.
Shortly after the earthquake, contractors launched a $1 million project along Hawaii Belt Road on Route 19 near Honomu.
Last month, scaling work started on Routes 250 and 270 after basalt rocks ranging from gravel-sized to volleyball-sized were observed at several sites on Kohala Mountain Road and Akoni Pule Highway. The $2.2 million project is expected to be completed in July.
On Mamalahoa Highway and Hawaii Belt Road on Route 19, contractors plan to drape wire mesh netting at 13 sites from Honokaa to Honomu. Draping also will be done at 11 sites on Mamalahoa Highway from Kalaoa to Keamuku on Route 190.
In the final phase of the hillside stabilization project, contractors will install a 12-foot-high heavy-duty steel wire fence along Kalanianaole Highway by the end of the year.
Workers removed about 10,000 cubic yards of boulders and cobblestones in soil from the cliff and will remove an additional 6,000 cubic yards.
IN THE PAST, gravel-sized to basketball-sized rocks have fallen onto the highway.
"There were a lot of near misses," said Sniffen.
In 2002 the state installed wire mesh netting to catch falling rocks.
Transportation officials later saw a need to reinforce the hillside and install a rockfall impact barrier after they noticed more rocks being caught by the netting.
"We thought it wouldn’t be long before something would happen," said Sniffen. "We wanted to take care of that sooner rather than later."
The $7.8 million Makapuu project ranks in the top five of the Transportation Department’s priority rockfall mitigation projects on Oahu because of high traffic volume and the existence of only one other road to Waimanalo. Sniffen said the limited access to Waimanalo has made the Makapuu project the most challenging.
The planned closure of lanes on Kalanianaole Highway brought complaints from residents who faced longer commutes and business owners who feared a reduction in customers.
After community meetings were held, the Department of Transportation changed the lane closure schedule to nighttime.
"So far, it’s been conducive," said Wilson Kekoa Ho, chairman of the Waimanalo Neighborhood Board. "We’re just watching and seeing what’s going to happen."
On the North Shore, meanwhile, the city is tackling a steep rocky cliff on Pupukea Road. Collins Lam, acting director of the Department of Transportation Services, said the cliff with areas of loose fractured basaltic rock was determined to be unsafe due to natural erosion.
The $1.5 million project is slated to take five months to complete. Contractors of Prometheus Construction Co. will install wire-mesh netting and a rockfall fence along the cliffside.
Pupukea Road is the only thoroughfare for hundreds of North Shore residents to get to Kamehameha Highway. A starting date for the project has yet to be set.