Just 40 miles long and 26 miles wide, Oahu is not the biggest island in the Hawaiian archipelago, but it serves as the seat of government and is home to the state's largest city, Honolulu. It also is home to Honolulu Harbor, the largest and most singularly important of the state's commercial harbors. According to the Hawaii DOT, “Its success as a world-renowned port is responsible for the evolution of an ancient Hawaiian village into the state's capital city.”
So, when a 50-year-old sewer that crosses under the harbor had to be replaced, it was a job of utmost importance. It was pulled off without a hitch when the city and county of Honolulu completed one of the longest and largest sewer force main installations in the United States using horizontal directional drilling (HDD) and microtunneling trenchless technologies.
The Honolulu Hart Street Wastewater Pump Station Force Main Replacement project consisted of the installation of a new force main to replace an existing 48-inch and 42-inch main some 5200 feet long. It connects the Hart Street Wastewater Pump Station to the Sand Island Wastewater Treatment Plant, the largest treatment plant in Hawaii.
The original Hart Street force main, a concrete cylinder pipe, served about one-third of the Honolulu area population. This includes a 29-square-mile area, nearly one-half of urban Honolulu. An average of 17.9mgd of wastewater passes through the force main from the pump station to the treatment plant. In the years just prior to the construction of this project, major line breaks were repaired on three separate occasions. With no backup and the force main nearing the end of its useful life, the construction of a replacement became a top priority.
The primary challenge for the replacement force main was to minimize environmental and economic disruption while crossing beneath the main channel of Hawaii's major commercial port, and across the largest container yard in an island state where 98% of goods arrive by ship.
The job was complicated by the alignment crossing under the busiest docks in Honolulu Harbor and a major coastal highway. Facing these challenges, an all-trenchless installation was planned.
For the replacement force main's 3200-foot crossing beneath Honolulu Harbor, HDD was used to install two separate 46-inch steel casings. Then, 36-inch diameter high-density polyethylene (HOPE) pipes, which serve as the force mains, were pulled through each casing.