Above: CCFRPM pipes were microtunneled under the port docks and the highway to reach the main interceptor. Photo: HOBAS Pipe USA. Left: The single microtunneled pipe and two directionally drilled pipelines were joined with this special FRP 48x36-inch bifurcation (wye) fitting. Photo: Wilson Okamoto Corp.

Under the port docks and the highway to reach the main interceptor, grade needed to be controlled, so 48-inch HOBAS centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipes were microtunneled into place. The pipe used on this job was one of the first uses of a new product that was capable of handling both the axial loads imposed by jacking and the hoop tensile loads caused by internal pressure. This project included 2000 feet of 48-inch CCFRPM pressure jacking pipes and more than a dozen custom designed fittings.

Project designer Wilson Okamoto & Associates of Honolulu (now Wilson Okamoto Corp.) was selected to plan and manage the project. Established in 1947, it is one of the largest multi-disciplinary engineering and planning firms in Hawaii. Wilson Okamoto worked on all the different phases of this project—planning, design, and construction—cooperating closely with Honolulu's project manager, Wes Yokoyama. They also worked with and supported the city's construction manager because of the unusual installation methods used.

Wilson Okamoto specified only two products for the microtunneled pipes— concrete steel cylinder, or prestressed concrete cylinder pipe (the pressure version of regular reinforced concrete pipe), and CCFRPM. The choice was limited because of the demanding application and installation. First, the pipes had to be both strong enough for jacking and capable of pressure service. Second, considering the seawater external environment and raw sanitary sewer flows inside, corrosion resistance was important.

Finally, reliability was paramount because of the difficulty of future access, depths up to 40 feet, and the critical nature of this application.

Pipe Jacking

The general contractor, Modern Continental of Cambridge, Mass., chose HOBAS pipes for microtunneling to install the remaining 2000 feet of the replacement force main from the container yard to the Sand Island Wastewater Treatment Plant. The microtunneling was completed in four drives. The CCFRPM pipes performed 100% reliably in jacking with no breaks and maximum loads of only 200 tons even on the longest push of more than 1000 feet.

Five microtunneling jacking shafts were required to perform four microtunneling drives. One interjack station was inserted on each of the long drives, but neither was ever activated. Modem attributed the low loads to the mostly coral soils and the true outside diameter dimensions of the HOBAS pipes. The true nature of the outside diameter (OD) makes the load lower since you don't get the “snowplow” effect of pushing additional soil out of the way as the OD gets bigger and smaller along the jack. To minimize changeovers, 19-foot-long sections were used.

Jet grouting was used for ground stabilization and groundwater control at selected locations. It was also used at the microtunneling shafts to provide ground support at the launch and reception points and for groundwater control during construction.

HOBAS fabricated a variety of fiberglass reinforced plastic (FRP) fittings to match the required alignment. These included a 48x48-inch bifurcation, reducers to 36 inches, and several 36-inch elbows to connect the two 36-inch HOPE lines to the single 48-inch portion. Flanges and a 48x48-inch wye permitted assembly to two valves and a lateral. The single microtunneling and dual directional drilling pipelines were joined with the special wye fitting fabricated by HOBAS.

Planning and Design

Wilson Okamoto performed extensive preliminary engineering work to identify the best alignment and construction methods for the force main installation. The replacement force main had to accommodate the existing daily flow from the Hart Street Wastewater Pump Station as well as the increases demanded by expected population growth, land use changes, and wet weather as projected through 2020. These criteria imposed a design peak flow of 81.5 mgd. Either a single 48-inch line or two 34-inch lines could support such parameters.