Com Ed’s West Loop project had two main objectives: to add capacity and improve reliability by bringing 345-kV power underground into Chicago’s business district and to convert its transmission system to a network model (see Projects in Progress, 4-25-08). Both objectives have been achieved, but one major obstacle stood in the way – the Chicago River. The West Loop Substation is on an island in the North Fork of the Chicago River, called Goose Island. Two 345-kV circuits had to be brought to the island under the river.
As explained by Bruce Whiteway, program manager, “Construction vaults approximately 15 ft by 30 ft by 60 ft deep were dug on either side of the river. We lowered a boring machine into the vault and used it to tunnel from one side of the river to the island, a distance of approximately 300 ft. Pushing reinforced concrete pipe casings behind the drilling unit created a 5-ft inside diameter tunnel – one for each of the two transmission circuits.”
The pipe casings were then filled with 16 PVC pipes that run the full length of the tunnel (inside the concrete casing), held in place by bore spacers. The spacers support the PVC pipes in proper position to hold the 345-kV solid-dielectric transmission cables. The space between the pipes was backfilled with grout to make a solid duct package. The cables, once pulled through the ducts connect on one side to the gas-insulated switchgear on the island and into the ducts on the mainland that run under Chicago’s streets. Two more casings were also drilled and installed for future use.
“At one point we hit solid rock and had to blast,” said Whiteway, “but the delay only cost us three or four days with no serious consequences.”
Whiteway cited getting under the river four times with the casings as a major accomplishment. “The engineering and implementation of getting these tunnels under the river went very well. The contractor performed well. I was very pleased.”