By 2016, the city of Chino Hills will become home to a 500-kilovolt underground transmission line, a first-of-its-kind project in the United States.

But before the undergrounding — ordered recently by the California Public Utilities Commission — begins along a 3.5-mile section of the Tehachapi Renewable Transmission Project in Chino Hills, Southern California Edison (SCE) must first demolish existing transmission structures and foundations within the project right-of-way.

Sandra Blain, SCE manager, said the five lattice steel towers and 11 tubular steel poles were built to support the interconnection of the Tehachapi overhead transmission line in Chino Hills before the commission’s order to underground.

“SCE employed a four-month, multi-faceted approach to build the structures, but we expect the demolition to take between eight and 10 weeks,” said Blain. “Crews have begun the work to disassemble both the nearly 200-feet-high poles and towers, which will involve separating them section by section.”

When completed, the 179-mile Tehachapi project will run through a number of Southern California communities and will be able to deliver up to 4,500 megawatts of largely renewable energy, enough electricity to power 3 million homes.

Constructed in 2011, the towers and poles along the 3.5-mile route in Chino Hills will be disassembled differently. SCE expects that each structure will take about two days to take down and the cost to remove the structures will be about $4 million.

“Lattice steel towers are more complicated to deconstruct,” Blain said. “When built, the tower sections were bolted together under tension. To disassemble each tower, crews have to slowly remove the bolts connecting the sections before a 250-ton crane then separates each section, ensuring the tension is under control before bringing it to the ground.”

Also erected in sections, removing the poles requires a different approach. The crews will either pull the sections apart, using a large crane, open the seam that was welded on each pole or cut the pole into three sections.

Following each pole removal, a crew will then remove part of the foundation that supported each pole. These foundations range in depth from 45 to 62 feet deep and will be removed to a level to support the underground structures.

Removing transmission structures are not new to SCE. As a precursor to building the new towers and poles, the company had to remove transmission towers in the Chino Hills segment that were part of a previous transmission line, built in the 1940s.

Blain said that safety for this massive project is paramount. SCE’s workforce has established safety zones where the public and others will not be allowed to enter.

“Before starting work each day, crews will gather to review the work to be done, the potential risks and the safety procedures to be implemented,” said Blain. “Additionally, specially trained safety monitors will observe the work site to ensure the safety of the public and work crews

SCE expects to begin undergrounding the 3.5-mile section of the 500-kilovolt transmission line through Chino Hills in early 2014.