The federal government this week authorized nearly $185 million in construction on Honolulu’s planned 20-mile rail line, but the city so far hasn’t been given the green light to start erecting the concrete guideway that will sit atop the support columns it can now begin building.

The Federal Transit Administration on Monday authorized the city to pour foundations and build support columns for the rail guideway along Farrington and Kamehameha highways, but won’t yet allow the city to spend $21.8 million to fabricate concrete sections of the guideway at what is called a precast yard.

Toru Hamayasu, interim executive director for the Honolulu Authority for Rapid Transportation, said work related to the precast yard was excluded from the FTA’s authorization letter because the city hasn’t finalized a site for the yard.

The city is still investigating what environmental work would need to be done on potential sites, but does not expect that an environmental assessment or impact statement would be required for the proposed precast facility sites, he said.

However, "we need to confirm that, and the FTA needs to concur with it," Hamayasu said Wednesday. The potential precast sites include one in Campbell Industrial Park, and already have appropriate zoning or are being used for similar precasting operations, he said.

The city plans to use the precast yard to fabricate the 30-foot-wide elevated concrete guideway, which will be erected on top of concrete support columns to create an unobstructed overhead channel for the trains.

The contractor would fabricate sections of the guideway at the yard, and then haul the individual guideway sections to the construction site to assemble them on top of the support columns.

The delay in federal approval of plans for a precast yard will not add to the cost of the rail project because the city expects to obtain FTA clearance in time for casting activity to begin as scheduled this summer, Hamayasu said.

The cost of the precasting work is included in a $501 million West Oahu/Farrington Highway guideway contract awarded to Kiewit Infrastructure West Co., but that contract has been dogged by delays.

At the time the West Oahu contract was awarded to Kiewit in 2009, city officials said they hoped to begin construction in December of that year. However, delays in the approval of the federal environmental impact statement stalled the entire rail project.

The city did not begin preliminary work relocating utilities in the path of the rail line until last year, and the FTA only approved heavy construction of the support structure for the first segments of guideway this week. The city gave Kiewit a notice to proceed with heavy construction work this week.

Those delays resulted in a change order that increased the Kiewit contract price by at least $15 million. City officials say the final cost of the change order will be resolved later through negotiations now that Kiewit has been cleared to begin heavy construction.

The letter from the FTA this week granted the city approval to immediately begin up to $184.7 million in construction and other activities.

Included in the authorization letter was permission to begin erecting columns and performing other work on the $372 million section of guideway from Pearl Highlands to Aloha Stadium, and authority to do site grading and foundation work on a $195 million train car maintenance and storage facility.