Environmental regulations and opposition from property owners makes expanding underground infrastructures in densely populated urban areas increasingly difficult. Upgrading the system becomes even more critical when new construction invades expensive residential areas that are adjacent to sensitive wetlands.

Alameda Power and Telecom, the utility serving the city of Alameda, California, U.S., faced this problem when a community at Bay Farm Island required increased power capacity. Since that community constitutes a major part of Alameda P&T's territory, the utility decided to construct a new 12-kV feeder to reinforce service to the area. Presently, the area is served from four submarine feeder cables crossing the San Leandro Channel near the west side of the Bay Farm Island Bridge.

In assessing locations for the new feeder, Alameda P&T determined it would be prudent to install the feeder crossing at a different location across the San Leandro Channel separating Bay Farm Island from Alameda Island. The entry point of the circuit would be located in a park, which is part of a green belt along the channel, adjacent to Brunswick Way and Sea View Parkway on Bay Farm Island. Its exit point would be behind an exclusive residential area on the main island at Broadway and Shoreline Drive.

Residents here enjoy an unimpeded view of the entire San Francisco Bay area, and the channel itself provides habitat for a wide variety of waterfowl, including some endangered species. The California Clapper Rail, for example, inhabits the marshland along the shore and is at risk after the end of January when nesting season begins. Therefore, it was important that construction be completed before the end of January to minimize any disruption to the Clapper Rail and other wildlife along the shoreline.

The Construction Process

Alameda P&T had to obtain several permits before it could begin construction. In accordance with the California Environmental Quality Act (CEQA) requirements, a Mitigation Negative Declaration was prepared and circulated for public review. The utility applied for and received permits from the California Regional Water Quality Control Board, San Francisco Bay Conservation and Development Commission, and the Army Corps of Engineers. The State Lands Commission determined that no discretionary approval was required for the project.

The utility selected Cherrington Corp. (Sacramento, California) to install the new circuit using horizontal directional drilling (HDD) technology. Cherrington installed three 6-inch (15-cm) high-density polyethylene (HDPE) SDR-11 ducts, two of which were used to convey 12-kV cables, with the third held in reserve for future fiber-optic cable use. The crossing was installed 40 ft (12 m) beneath the channel and spanned 2041 ft (622 m). The exit point was located in a city-owned buffer zone between a residential fence line and the edge of the inter-tidal zone of the channel at the intersection of Shoreline Drive and Broadway. Using the HDD technology eliminated any potential concerns of blocked boat traffic, of disturbing the channel bottom and bay mud, or impacting public access to the shoreline.

Cherrington faced the challenge of completing the project within a tightly scheduled time frame, while minimizing any drill fluid fracturing to the surface while drilling. To operate successfully in the sensitive environment and to expedite the work, Cherrington elected to use a method employing two drill strings threaded over one another. The smaller string was used to drill the pilot hole, and the second string was used to wash over the pilot string with a reamer attached. This technique minimized the potential risk of fracturing by increasing the volume of the borehole directly behind the bit. With a larger volume hole immediately behind the bit, drilling fluid moved back up the enlarged hole section rather than into the surrounding formation and possibly to the surface. The pilot hole and the reaming steps were performed simultaneously, reducing the chances of fracturing and eliminating the time to run a separate pass to open the hole after the pilot hole had been completed. The washover technique employed a 3½-inch (8.9-cm) Jet Bit on the end of a 2⅞-inch (7.3-cm) drill pipe to drill the pilot hole. The pilot string was washed over with an 18-inch (46-cm) reamer attached to a 6⅝-inch (16.8-cm) drill pipe as the hole was drilled. Using the washover method eliminated extra trips, helped to meet the deadline for construction completion and provided the ability to free the pilot string in the event it got stuck.

Installation of the HDD equipment began on Dec. 30, 2002, in Shoreline Park on Bay Farm Island, and drilling commenced on Jan. 3, 2003. The HDD rig and supporting equipment were confined to a 110- by 70-ft (34- by 21-m) temporary enclosure with two additional drill spread units stationed just outside the enclosure on Sea View Parkway.

As the drilling proceeded, the Cherrington crew monitored the operation to check on fluid fractures. The risk for “frac-outs” was greatest at the entry point of the hole and at the exit point. While the crew carefully monitored drill mud circulating pressures to minimize the chance of fracturing through the soft overlying formation, two incidents did occur. The crew detected the first fracture at about 150 ft (46 m) from the entry point above the high tide mark; the second was within 75 ft (23 m) of the exit point inside the inter-tidal zone. In each case, the drilling mud was swiftly contained with hay bales and waddles, which were on hand for such an occurrence. The areas were promptly vacuumed and no serious contamination was experienced.

Directional control while drilling was facilitated by using Tru-Tracker® coils on each side of the crossing. A coil was laid on the channel bed using a small “John-Boat” and was positioned precisely in a loop extending 700 ft (213 m) into the water from both the entry and exit sides. A steering tool, located just behind the drilling bit, sensed an artificial magnetic field from the coils when the coils were energized. Surveys were made every 15 ft (5 m) to monitor the actual trajectory and compared with the planned trajectory.

During each survey, pilot hole drilling was halted while the magnetometers in the tool were used to determine the precise lateral location of the drill string. Accelerometers in the tool provided data to determine the relative vertical position. This technology is used routinely in the HDD industry to guide the drill string along its prescribed path. The accuracy of the system was demonstrated when the drill bit was redirected just before exiting to within 4 ft (1 m) of the planned target to facilitate the duct pullback operation. Pilot hole drilling and reaming were completed in 10 working days.

In preparation for the pullback phase, an assembly area adjacent to Shoreline Drive on Alameda Island was set up to fuse and bundle more than 6000 ft (183 m) of 6-inch (15-cm) HDPE ducts. The ducts were positioned for pulling into the borehole, despite the fact that the duct had to be pulled around a tight corner that was not in line with the axis of the borehole. The pullback phase was successfully completed when the duct bundle was pulled into place on Jan. 17, 2003.

Project Completion

As with all projects of this kind, planning was an important part of the job and regular communications between Alameda P&T and Cherrington contributed to the on-time completion of the work. Both companies exercised great care to operate within the strict set of guidelines established before construction started, therefore, local residents registered few complaints. Cherrington also restored both the entry and exit sites to better than their original condition. In particular, the HDD technology proved to be a good alternative to conventional underground construction methods.

Acknowledgment

The author would like to thank Ken Weeks of Cherrington Corp. (KWeeks@CHERRINGTON.com) for his assistance in preparing this article.

Kalpana Dulip Singh is an electrical engineer with Alameda Power & Telecom, a local utility providing electrical and telecom services to the city of Alameda. She has a master's degree in electric power engineering from Rensselaer Polytechnic University (Troy, New York, U.S.). She worked for United Technologies and Parsons Brinckerhoff before joining Alameda P&T in 1996.

ksingh@alamedapt.com

Juan M. Ulloa is an electrical engineer with Alameda Power & Telecom. He graduated from California Polytechnic State University (San Luis Obispo, California) in 1993. Ulloa worked for a petrochemical company as a design engineer prior to joining Alameda P&T in 1999.

julloa@alamedapt.com