As the city plans to start building a $200 million, 3.2-mile wastewater tunnel between Kaneohe and Kailua this year, some residents living near the project are worried about the potential impact on their properties.

Residents whose communities have experienced soil instability and cracks in building walls in the past fear the vibrations from tunnel work could aggravate sensitive ground conditions.

"There’s going to be cracks in our walls and foundations," said Collette Madden, Healani Gardens Homeowners Association board president. "All of the community is concerned. … We want some indemnification."

Madden said a study done by the condominium association found the area sits on a caldera where the soil is susceptible to shifting.

Madden said the association sent the study to the city, warning it about the potential problem, but never got a response. City officials said the association’s study was received long after the project’s environmental review was completed.

The project’s design engineer, Wesley Yokoyama, said experts hired by the city have done extensive geologic tests of the soil along the underground route and are confident the tunnel will have virtually no effect on the adjacent communities.

"Experts feel there will be very little risk to structures," said Yokoyama, who is also the city’s wastewater design branch chief.

Yokoyama said the tunnel will be bored from the Kailua Regional Wastewater Treatment Plant site to an average depth of 100 feet below ground through a layer of basalt rock. The tunnel itself, about 101⁄2 to 12 feet in diameter, has an estimated life of 150 years.

He said roughly 50 truckloads of extracted material will be hauled away during workday hours.

Under the plan, the tunnel will not only carry sewage water by gravity flow from Kaneohe to Kailua, but will also serve as storage for larger wastewater flows, greatly decreasing above-ground sewage spills.

The tunnel is being built in part to comply with a 2010 consent agreement involving the federal Environmental Protection Agency, the state and environmental groups critical of the city’s handling of sewage in the Kaneohe-Kailua region.

Construction from the Kaneohe Pre-Treatment Facility to the Kailua Regional Wastewater Treatment Plant is expected to begin in December and be completed by June 30, 2018.

Yokoyama said that in the long run, the 3.2-mile tunnel is less expensive and a better alternative than building the original consent-decree sewage force main across Kaneohe Bay, along with above-ground, multimillion-gallon storage tanks.

The city said eliminating the storage tanks will result in significant cost savings.

Yokoyama said his office is working with consultants to schedule briefings with communities about the project. He said the city’s project team provided presentations to the Kaneohe Neighborhood Board in May and the Kailua Neighborhood Board this month.

Yokoyama said a city community liaison has walked through the Kailua and Kaneohe neighborhoods in the vicinity of the project and invited residents to call to schedule a meeting about their concerns about the project.