Ala Moana Wastewater Pump Station_ _ _ _ _ _ _ _ . . Force Mains No. 3 & No. 4

The Ala Moana Wastewater Pump Station (WWPS) and Force Main (FM) system is Honolulu’s largest sewage conveyance facility serving more than half of the metropolitan Honolulu area. In its continuing efforts to upgrade and improve the reliability of its wastewater collection system, the City has constructed two parallel 7,500 linear feet (LF) 63-inch sewer force mains installed within 81-inch diameter steel casings jacked by microtunneling methods to increase raw sewage conveyance capacity from Honolulu’s Ala Moana WWPS to the Sand Island Wastewater Treatment Plant. The force main system is designed to handle up to 225 million gallons per day (mgd), while typically conveying an average flow of 45 mgd. The Ala Moana system had to remain operational at all times, and construction of the improvements required provisions for temporary flow bypassing capable of handling over 60,000 gallons per minute.

To cross under the only entrance to Honolulu Harbor, 1,580 LF of the sewer and jacked steel casing was installed at a depth of 85 feet below mean sea level (MSL) with 100-foot deep jacking and receiving shafts. The unique set of challenges faced by the design team included the presence of very soft to very loose fine sandy silts and silty fine sands under the harbor entrance channel, highly variable coralline and limestone layers containing cavities, a 20-foot thick extremely hard basalt lava flow within the coralline formation near the base of one deep shaft invert, and very loose lagoonal soils. Compounding these difficulties were the presence of pier piles along the harbor entrance that could obstruct microtunneling, the harbor entrance channel could not be blocked by the construction activities, restricted construction work hour periods, and a strict Federal Environmental Protection Agency (EPA) Consent Decree deadline.

Comprehensive geotechnical and geophysical investigations were performed to explore subsurface conditions along the sewer alignment to develop a geotechnical baseline and design approach with the intent of minimizing construction risks, and establishing minimum shaft design/construction, ground water control, and microtunneling requirements for bidding and construction. Some of the key concerns during design and construction were the potential loss of steering during microtunneling operations, uncontrollable settlement of the Microtunnel Boring Machine (MTBM) in the very soft to very loose sediments, and effects of high buoyant force on the jacked steel casings during installation.

Due to the unique combination of design and construction challenges, the following new/innovative application of technologies were considered and evaluated during design, and implemented during construction:

Use of three dimensional (3-D) state-of-the-art finite elemental model (FEM) analysis computer program and method to evaluate probable microtunneling and jacked steel casing pipe deflections during varying stages of underwater microtunneling drives under the Honolulu Harbor entrance channel through liquefiable, very loose silty fine sands. It is believed that the use of the 3-D FEM to analyze such microtunneling situations was unprecedented in 2009. Tritech of Singapore provided the 3-D FEM program and also assisted with peer review of the 3-D analysis.

The project specifications provided fairly extensive technical information and requirements for the Contractor to incorporate into the construction of the pipeline and deep jacking and receiving shafts. The specifications allowed the Contractor to utilize innovative methods in the project that would safely meet the project objectives. FCCC proposed the use of an innovative vertical shaft sinking machine (VSM), provided by Herrenknect, an international company specializing in mechanized tunneling technology. The VSM is similar to a vertical “road-header” excavator type tunnel equipment with remote control and slurry spoil handling. The VSM had only one prior use in the U.S., and this project was the first time the VSM was included in design-bid-build documents in the U.S. This was also the hardest and most abrasive rock (unweathered dense basalt with Mohr’s hardness of 8 to 9) that the VSM had ever encountered.

See Project Gallery for additional photos and descriptions.

FAINC and the design team were fully engaged in support of the project from the beginning, through conceptual development and design phase, and to final completion. The long-term benefits to the City & County of Honolulu is the enhanced system reliability. The new force mains provide for sufficient wastewater capacity, system backup capability, and flexibility in service and maintenance of the system.

The project qualified for low interest financing through the State of Hawaii Department of Health – State Water Pollution Control Revolving Fund (SRF). As a result, $111.0 million of the project’s original $116.7 million bid price was funded through the SRF program.

The design and construction team completed the project within the City’s programmed budget and met all requirements under the Consent Decree with EPA and State of Hawaii – Department of Health (DOH). The project was put out to bid in July 2011, with construction commencing in November 2011. The project was substantially completed in August 2015, meeting all mandated deadlines and successfully placed in operation.