Large voids found below transmission line project

By Ray K. Erku

Rawlins Times

Via Wyoming News Exchange

RAWLINS – Large, subterranean voids scattered throughout two defunct mining sites in Carbon County discovered by experts with Brierley Associates, a Denver-based geotechnical consulting firm, pose bit of a problem.

On Tuesday, Brierley representatives reported to Carbon County Commissioners that these voids, found in the old Hanna No. 1 Mine and the Nebraska Mine six miles southwest of Rawlins, rest directly in the proposed path of the Gateway West Transmission Line Project.

Headed by Rocky Mountain Power, the project is set to connect 1,000 miles of new high-voltage transmission lines between the Windstar Substation near Glenrock, Wyo. and the Hemingway Substation near Melba, Idaho, according to online records.

Brierley, who was first tasked in 2015 by the Wyoming Department of Environmental Quality with executing Abandoned Mines Land Subsidence Mitigation Program investigations, reports that they found “needles in the haystack” when it came to potential problem spots in prevalent coal-mining areas.

As such, Hanna, which was once the site of one of Wyoming’s largest coal mine, was discovered to be a problem spot.

According to Brierley representative Robert Mascarenas, several large voids ranging from seven to 27 feet were left in the town of Hanna when thousands of tons of sand slurry, a mixture of sand, water and cement used for foundations, were injected underground.

The injections somehow dissipated underground, causing a weakness to propagate toward the earth’s surface. In other words, this has caused subsidence, or potential sinkholes.

Construction reports from Midwest Mining Company reveal that about 780,000 tons of sand slurry were injected around the area, said Mascarenas.

This means if a transmission line is installed without proper mitigation, which involves injecting grout, a material paste for filling crevices, it could be in danger of sustaining structural damages.

“(It’s) a little concerning,” said Mascarenas.

Meanwhile, the same problems were discovered around the former Nebraska Mine, a substantially smaller site that was used between 1908 and 1951.

It was reported that this mine, which is also directly in the proximity of the proposed Gateway route, has encountered voids between 30 to 170 feet below the earth’s surface.

A proposed summer mitigation plan would involve adding between 20,000 to 30,000 cubic yards of grout in the area, Brierley reported. Officials also said the contract would be 150 days and that they anticipate 10-15 people for the job.

In addition, both construction parties and the county will need to facilitate a road use agreement for usage of 20-Mile Road. AML funding for potential road maintenance will likely be allocated.

Being the Nebraska Mine is in open country, officials anticipate finishing the proposed summer mitigation plan by October. For Hanna, however, which is a lot more degraded and is in closer proximity to houses and other infrastructure, it’s mitigation is anticipated to take a bit longer.

Officials hope to bid for projects sometime next month.

As for Gateway West, its construction, which will start north of the county, will likely not be delayed by mining mitigation plan.