Primary power duct banks and vaults in the street

The job

A new building on First Hill needed primary power, secondary power and communications brought in from existing utility vaults across two streets and up an alley. The stack of conduits was tall: feeder, primary, secondary and comm ducts together, encased in red-dyed concrete, with three feet of cover on top. That put most of the trench six to nine feet deep.

What made it hard

The route crossed gas mains, other utilities and street tree roots. We potholed every crossing with a vactor truck before trenching, and used the vactor to dig where roots ruled out a bucket. The trenches were shored for their full length and covered with steel plates at the end of each shift. Dump trucks stood by in the street so spoils left as they came out.

No two bends in the duct bank were alike. We bought a pipe bender for this project and formed and threaded our own 4 in. galvanized sweeps, each one made for its location and for its own conduit within the stack.

What we did

• Saw-cut and broke out concrete street panels and aprons.
• Built about 500 ft of primary power duct bank and about 350 ft of communication duct bank, 1,860 ft of conduit in all.
• Set eight vaults, each in its own shored excavation: two 712 vaults for Seattle City Light, five 444 vaults for City Light and the franchise communication utilities, and one 264 communication vault.
• Built the duct banks with spacers and 68 custom formed and threaded galvanized sweeps, supported the power poles next to the trench, and ran risers up the pole.
• Encased the ducts in red-dyed concrete, backfilled with CDF, and capped the trench with temporary asphalt.