Consultant's Role in The Project
Fukunaga & Associates, Inc. (FAINC) served as the prime engineering consultant and civil design engineer for the project. In this role, FAINC was responsible for overall project management and served as the primary liaison with the project owner, the City and County of Honolulu, including the Department of Design and Construction (DDC) and the Department of Environmental Services (ENV). FAINC also led coordination efforts with the sub-consultant team to ensure seamless project execution.
Project Description
The Ala Moana Wastewater Pump Station (WWPS) and Force Main (FM) system is the largest sewage conveyance facility in Honolulu, serving over half of the metropolitan area. As part of the City's ongoing efforts to upgrade and enhance the reliability of its wastewater collection system, two parallel, 7,500-linear-foot, 63-inch diameter sewer force mains were constructed. These mains were installed within 81-inch diameter steel casings using microtunneling methods to increase raw sewage conveyance capacity from the Ala Moana WWPS to the Sand Island Wastewater Treatment Plant. Designed for a peak capacity of 225 million gallons per day (MDG), the system typically conveys approximately 45 MDG. To maintain continuous operation during construction, a temporary bypass system capable of handling over 60,000 gallons per minute was implemented.
A key component of the project involved crossing beneath the only entrance to Honolulu Harbor, where 1,580 linear feet of sewer and casing were installed at a depth of 85 feet below mean sea level (MSL) using 100-foot deep jacking and receiving shafts. The design team faced significant geotechnical challenges including very soft to very loose fine sandy silts and silty sands beneath the harbor, highly variable coralline and limestone formations with voids, a 20-foot thick dense basalt lava layer, and loose lagoonal soils. Additional complexities included existing pier piles that could interfere with tunneling operations, strict limitations on work hours, the requirement to keep the harbor entrance channel unobstructed during construction, and compliance with stringent EPA Consent Decree deadline.
Extensive geotechnical and geophysical investigations were conducted along the sewer alignment to assess subsurface conditions, establish a geotechnical baseline, and develop a design strategy aimed at minimizing construction risks. These studies informed critical design parameters, including shaft construction, groundwater control, and microtunneling requirements for both bidding and execution. Key challenges identified during the design and construction phases included the risk of steering loss during microtunneling, potential uncontrolled settlement of the Microtunnel Boring Machine (MTBM) in extremely soft and loose soils, and significant buoyant forces acting on the jacked steel casings during installation. In response to these complex conditions, several innovative technologies were evaluated during design and successfully implemented during construction.
A state-of-the-art 3D finite element modeling (FEM) program was used to analyze potential deflections of the jacked steel casing during various stages of underwater microtunneling beneath Honolulu Harbor, where subsurface conditions included liquefiable, very loose silty sands. At the time, this application of 3D FEM for microtunneling analysis was considered unprecedented. Tritech of Singapore supplied the modeling software and supported the analysis through peer review.
The project specifications provided detailed technical requirements for the construction of the pipeline and deep jacking/receiving shafts, while allowing flexibility for innovative construction methods. FCCC, the contractor, proposed the use of a Vertical Shaft Sinking Machine (VSM) manufactured by Herrenknecht, a global leader in mechanized tunneling. The VSM, a remote-controlled vertical excavator with slurry handling, had only been used once previously in the U.S. This project marked its first application within a design-bid-build framework and involved the most abrasive ground conditions it had ever encountered- dense, unweathered basalt with a Moh's hardness of 8 to 9.
See Project Gallery for additional photos and descriptions.
Vertical Shaft Sinking Machine (VSM)
Cast-In-Place Shaft Wall Placement
Completed Deep Launch Shaft
Completed Deep Launch Shaft