Project TypeBACKUP ELECTRICAL SYSTEM
LocationSAN FRANCISCO, CALIFORNIA
Length / Hole Diameter396 m and 549 m (1,300 ft and 1,800 ft) approximate length / 305 mm and 76 mm (12 in and 3 in) conduit
DivisionsThe HDD Company
SHORE TO BAY TRANSMISSION INSTALL
San Francisco required a redundant electrical system to provide backup service in the event of an earthquake. Due to the city’s dense and highly populated downtown, it was determined the most efficient and least disruptive solution would be to install the lines along the bay floor, connected through a shore approach using horizontal directional drilling (HDD).
The HDD Company drilled approximately 548.6 m (1,800 ft) at one location, and approximately 396.2 m (1,300 ft) at another. Both sites involved three separate landings and the installation of 304.8 m and 76.2 m (12 in and 3 in) conduit.
All horizontal directional drill paths were shore approaches, from rigs set up in downtown San Francisco. The HDDs were terminated on the San Francisco Bay floor under nearly 80 feet of water. All pipe was fused on the land and loaded onto a cable ship. The piping was laid along the seabed floor and winched up through the hole using the HDD rig. A highly specialized marine diver was required to safely connect the pipe to the winch.
Drillers faced two types of bay mud: “old bay mud” and “new bay mud”. The drilling route called for a vertical and a horizontal curve. Accuracy was required to ensure the pilot holes for the 304.8 m and 76.2 m (12 in and 3 in) lines did not converge. Throughout construction, only one road was partially closed, with one-lane left open to accommodate local traffic.