For more than half a century, the 115-kilovolt transmission line running through Colorado’s Dolores Canyon provided good service. But with the regional population quadrupling over the years and modern demand for electricity far exceeding what was needed in the 1960s, engineers at Tri-State Generation and Transmission Association knew it was time for an upgrade.

That led to a massive engineering and construction effort to double the capacity of the line and build one of the largest transmission spans of its kind in North America.

The ambitious effort was part of Tri-State’s recently completed Montrose-Nucla-Cahone project, an eight-year, $105 million undertaking to upgrade transmission line and substations over an 80-mile corridor.

“There were nearly a dozen local, state and federal agencies involved, including the U.S. Bureau of Land Management, the U.S. Forest Service, the U.S. Fish and Wildlife Service, the Colorado Public Utilities Commission and Colorado Parks and Wildlife,” says Chris Pink, Tri-State’s vice president for transmission engineering. “The project also included more than 300 miles of access improvements and significant vegetation management along nearly 60 miles of federal land.”

For the canyon crossing, old wooden H-frame transmission structures, in service since 1958, were replaced with taller, more robust steel towers. The new towers were designed to support 8,000 feet of conductor and optical ground wire weighing 100,000 pounds, some 1,100 feet above the canyon floor and the Dolores River.

“That required a span of 6,516 feet between the two lattice towers on the north and south rim of the Dolores canyon, and the conductors were dead-ended on steel tubular poles on concrete pier foundations,” Pink says.

Specialized conductor and hardware were needed to accommodate the length and tension of the crossing, he adds, so “an additional set of all material was ordered and is stored if maintenance is ever needed on those components.”

Accessing the project area and minimizing disruption of its ecologically sensitive terrain presented some logistical challenges.

A transmission line helicopter was hired to make a series of flights carrying lead ropes, which were brought back across by heavy-duty winching equipment. Other special gear included six bucket trucks designed to extend 100 to 150 feet, two cranes with lift capacity of 30 to 50 tons, and a pair of water trucks for fire control and dust mitigation. Two drum pullers rated at 6,000 and 30,000 pounds and a pair of 72-inch wheel tensioners were used to help adjust and properly calibrate sag between the spans.

“Tri-State made five crossings of the canyon to complete the work,” Pink says. “Three of those involved deployment of conductor and the other two were for optical ground wire that’s now part of the regional fiber optic communications backbone.”

The finished spans extend more than 1.5 miles between the dead-ends and drop about 400 feet from the terminal connections, allowing for the wind and ice loading that comes in winters with an average snowfall of 66 inches and wind speeds that often gust to near 40 mph.

“With the support of trained personnel, good equipment, planning and preparations, we were able to complete the canyon work in 12 days,” Pink says. “Besides offering our member distribution co-ops greater operational capacity and flexibility to accommodate future load growth, it will also support Tri-State’s Responsible Energy Plan and expanding opportunities to interconnect with renewable generation projects.”