Sinkhole fix complicated by ‘a lot of unknowns’

A sinkhole in Kakaako that led city officials to close the intersection at Halekauwila and Cooke streets in October is months away from a fix.

An emergency repair project will start in mid-­December and likely take three to six months, city officials announced at a news conference Wednesday at the intersection near Mother Waldron Neighborhood Park.

“We really don’t know how severe the voids are, the size of the voids. Once we start digging, we’ll have a better idea,” said Haku Milles, acting director of the city’s Department of Design and Construction. Mayor Rick Blangiardi added that the timeline is a “conservative” estimate “because there are a lot of unknowns.”

City officials also plan to excavate and fill roadway in a mauka direction, up the block to Queen Street, to address further underground erosion discovered in an investigation. The city anticipates spending $8 million to $10 million on repairing the intersection alone, with $20 million set aside for the whole project.

The sinkhole resulted from a leak in an underground box culvert that eroded the soil around it, Milles said. The erosion occurred 14 feet beneath the surface, according to a city news release.

Assembled in 6-foot-long interlocking concrete sections that are 12 feet wide and 8 feet deep, the culvert conveys storm drain water to the ocean, allowing the tides to influence it, Milles said. What caused the joints to separate? “Vibration and movement,” he said. “From cars driving over it, heavy vehicles.”

The city sent in divers from the firm Sea Engineering, and they found more leaks, from Halekauwila Street to Kapiolani Boulevard, Milles said. The most severe gaps — 2 inches to 6 inches — were between Halekauwila and Queen streets, and large enough for the tides to move water in and out. “The significance of the gaps is new to us,” he said.

After fixing the intersection, the city plans to seal the gaps in the culvert from the inside up to Queen Street, Milles said. The city then intends to fill the underground voids left by the erosion outside of the culvert. There’s no plan to seal the leaks between Queen Street and Kapiolani Boulevard at this time, Milles said, because “those voids aren’t as severe,” ground- penetrating radar found.

Upon completing that work, “We’ll continue on with a full investigation into what more may need to be done, continuing up past Queen Street and farther down Halekauwila to the ocean,” Milles said.

The city has installed signs reminding passersby that nearby shops are still open. Also, the city’s Office of Economic Revitalization deployed a team of business and constituent educators to help out local business owners and their employees. “We understand that Cooke Street and Halekauwila are heavily trafficked areas and the surrounding businesses depend on people having access,” Blangiardi said,

Nadine Leong, who owns the Sake Shop on Cooke Street, said the road closure has hurt her business.

“It has been helpful that the city addressed our concerns of making the store more visible to customers,” Leong said, pointing to a sign the city posted on the street. “But to be honest, our sales in October dropped more than they did last year.”

The closed intersection deters customers who would otherwise drive by and decide to stop in, Leong said. “It’s not convenient anymore,” she said, adding that access to her shop is now limited to the mauka section of Cooke Street. Still, she’s grateful to the city for its help.

“I’m glad they’re listening,” Leong said.

The culvert system was installed in 1989, according to a state database. The city inherited the system from the state, Milles said.

When a fluid moves beneath surface land, it can carry off soil and other material, hollowing out a pocket underground. That space can grow until the surface above collapses in, leaving a sinkhole.

“Most of the sinkholes, but not all, in urban Honolulu seem to be associated with water main breaks or sewer main breaks, and the removal of material is caused by the flowing water or sewage that gets out of the broken pipe and erodes soil,” said Stephen Martel, a professor emeritus of geophysics at the University of Hawaii at Manoa. In this case, the water leaking from the culvert eroded the surrounding material.

“It’s a reminder all of the stuff under the road is old,” said newly elected City Council member Tyler Dos Santos-Tam, who lives nearby. The underlying ground was once used for disposal of dredging, garbage and old sugar mill waste, and has “extremely fast permeability,” according to the University of Hawaii’s Soil Atlas.

“All this is sand, coral and fill, so that requires an extra degree of care,” Dos Santos-­Tam said. “With sea level rise, it could get worse,” he said, pointing out that the area already gets “tidally inundated.”

With 3.2 feet of sea level rise, pockets of inland Kakaako would likely be exposed to ocean water, according to the Sea Level Rise Viewer, UH’s Pacific Islands Ocean Observing System. Hawaii could see ocean rise to that mark by the end of the century, the state’s 2017 Sea Level Rise Vulnerability and Adaptation Report said.

Martel said he’s not familiar with any academic work suggesting rising seas create more sinkholes. However, he noted, “It’s not out of the question that sea level rise could cause sinkholes by redirecting acidic fresh water to new places,” as the elevated sea level causes the water table to rise.

What’s more, the groundwater beneath Kakaako is heavily polluted, according to Chip Fletcher, the interim dean of the School of Ocean and Earth Science and Technology at the University of Hawaii, Manoa.

“Some areas have floating oil and diesel fuel, sewage from cesspool and septic tanks as well as leaky sewage pipes,” Fletcher wrote in an email to the Honolulu Star-Advertiser. With sea level rise, he said, polluted runoff would be lifted to the surface. “We also need to worry about increased corrosion” of cement, rebar and other building foundation elements, Fletcher said.