FORD ISLAND WATERLINE REPLACEMENT

INNOVATIVE APPLICATION OF NEW AND EXISTING TECHNIQUES

Horizontal Directional Drilling techniques are now fairly well developed and have been applied to numerous projects throughout the world. However, in this project, an extension of conventional HDD techniques was applied to address several unique conditions facing the design team. The first, and potentially the most difficult and costly challenge, was the under channel crossing beneath Pearl Harbor. From shoreline to shoreline, the crossing would need to cover over 5,000 linear feet. It would not be possible to make this crossing in a single pull using conventional HDD methods with thermoplastic pipe.

Alternatives considered included using HDD methods of pulling a steel casing beneath the channel and then performing a second pull to insert a plastic pipe through the casing. Then the annular space between the plastic pipe and steel casing would need to be pressure grouted to support the plastic waterline. Non- HDD alternatives included conventional underwater pipe laying by dredging a trench along the channel bottom and installing a pipe, then backfilling the trench underwater. This operation would almost certainly cause significant disturbance to the channel bottom, and raised the concern of unleashing potentially hazardous sediment into the Harbor waters. Implementation of mitigative measures and water quality monitoring would add significant costs to thisalternative. Furthermore, the proposed crossing alignment was adjacent to the new Admiral Clarey Bridge that connects Ford Island to the landside.

Impacts of dredging, pipe laying and backfill operations could jeopardize the stability of the bridge foundation system. Since the channel would need to accommodate some of the largest ships in the U.S. Navy�fs fleet (aircraft carriers), the pipeline would need to be at least 10 feet below the channel bottom. This meant significant underwater excavation and backfill.

The selected installation method was developed through a joint effort of FAINC and their geotechnical/HDD subconsultant, URS. A dual bore operation and underwater splice was considered as an alternative early in the conceptual design development for the project. However, there were concerns that creating a midcrossing high point underwater could potentially trap residual air bubbles and cause operational problems. Air trapped in the pipeline would reduce the carrying capacity of the transmission line and possibly cause air-binding that could prevent water flow through the pipe.

Careful inspection of a bathymetric survey of the harbor bottom identified a distinct drop-off after a shallow shelf on the northeast shore of Ford Island. By using two separate bores, with one starting from the Ford Island side (Bore No. 2), exiting at the toe of the steep drop-off, and the second starting from the Halawa shore (Bore No. 1) exiting just past the navigation channel, a continuous, sweeping profile through the underwater splice would be possible without creating a high-point under the channel. (The design profile is illustrated in Figure 2.) The underwater splice was backfilled with clean, crushed aggregate and covered with an articulated concrete matting system. This was done to protect the pipeline, and also to provide ballast to keep the pipeline permanently anchored in place. Providing the proper ballast volume and weights for the pipeline were crucial to insure stability throughout the underwater installation process, as well as for the permanent installation.

The remainder of the waterline network on Ford Island was installed utilizing both conventional HDD and open trench operations. In all, approximately 19,550 linear feet of pipeline was installed on Ford Island. The total project construction cost was $12.0 million, which was well below the estimated construction cost (ECC) program budget of $12.6 million.

TECHNICAL VALUE TO THE ENGINEERING PROFESSION

This project demonstrates how existing technologies can be extended beyond conventional applications to provide effective solutions. The two bore horizontal directional drilling and pipe string method used in this project exceeded conventional single point bore applications. The two bore method allowed the installation of a pipeline twice the length expected under conventional HDD operations. Of course, the use of dive operations for the underwater splice connection was required; however, this underwater work was substantially less than would be required for typical underwater pipe laying construction.