Mine operations technician Ben Medina installs a rock bolt in the Waste Isolation Pilot Plant underground. Rock bolts assure ground stability and worker safety in the underground transuranic waste repository. Photo Courtesy DOE/EM
Department of Energy EM Waste Isolation Pilot Plant (WIPP) crews have marked a significant achievement by installing more than 13,000 bolts this year to slow the natural movement of salt rock in the underground repository where radioactive waste is emplaced.
That tally continues to grow as 2020 progresses. Bolting controls the movement of salt rock — known as salt creep — in the WIPP underground. The waste repository’s walls, roof, and floor can move three to six inches a year. The salt rock seals fractures and naturally closes all openings, permanently encapsulating the waste.
In an average week, WIPP crews install 100 to 150 bolts. In late April, crews installed 344 bolts, a single-week record.
“The mining crews, underground maintenance, and underground facility operations’ performance has been exemplary,” Mine Operations Deputy Manager Kenny Padilla said.
WIPP’s comprehensive ground control system ensures a safe environment for employees working 2,150 feet below the surface in the WIPP underground. The system includes installation of bolts using bolting machines, and monitoring and evaluation of the underground conditions. Wire mesh on the walls corrals small pieces of salt that break free. Engineers walk the underground’s drifts daily, looking for broken bolts or new rock movement.
The effort is bolstered by WIPP’s geotechnical department, which performs electronic monitoring and measures salt movement manually. If the rate of salt movement in an underground area is too high, bolts are added.
The salt in the WIPP underground is a series of layers divided by clay. Holes are drilled from one salt layer into the next, and steel bolts, some as long as 14 feet, are inserted. Steel plates are tightened on to the bottom of the bolt with a nut to cinch it up against the salt, and a lanyard is added to limit movement of the bolt when it eventually breaks.
The bolts are designed to withstand 60,000 pounds of pressure from the advancing salt. But they do break, at an average of two or three per day out of the thousands installed, and the bolting crews install new ones.
“The mine management team worked with the mining crews to refine the roof bolting process so we could improve the safety factor in the mine roof,” Padilla said. “The mining crews, coupled with underground facility operations personnel, met the challenge and improved the roof bolting operation.”