SEATTLE (AP) The giant machine digging a highway tunnel under Seattle was expected to resume light work, but officials still don’t know what stopped the giant drill for nearly two months, the Washington State Transportation Department said Jan. 27.

Crews working for the contractors, Seattle Tunnel Partners, planned to finish their underground inspections of the machine’s cutterhead on Jan. 28, the highway department said in a statement. The contractors then planned to have the machine advance 2 ft. (.6 m) so the next tunnel ring could be built. After that, the machine and its operating systems would be evaluated.

The machine called Bertha had been idle for more than seven weeks, since Dec. 6.

The Transportation Department said that inspection crews found a hard object more than 3 ft. (.9 m) wide stuck in the machine’s massive spokes. They also found metal and plastic piping in the machine but have not concluded what exactly caused the problems that halted digging.

None of those things was really sufficient to slow the machine down, department spokeswoman Laura Newborn has said.

Officials previously said the machine also ran into an 8-in. (20 cm) diameter metal pipe that had been left in the ground in 2002 after the Transportation Department checked groundwater in the area.

Working 60 ft. (18 m) underground, Bertha is only one-tenth of the way toward completing a 1.7-mi. (2.7 km) tunnel. The tunnel will carry Highway 99 traffic and allow the removal of the aging Alaskan Way Viaduct along the Seattle waterfront.

The inspection crews spent 134 hours over 10 days checking the cutterhead and the area just in front of the machine, the Transportation Department said.

“While we understand the interest in knowing the reasons why mining was stopped in December, it will take time to review the results’’ of the inspections and consult with tunneling experts, the statement said.

The results of the initial mining and the extent of any required maintenance will determine when more substantial digging resumes, the department said.

The “next critical milestone’’ will come when the machine has tunneled another 500 ft. (152 m). That spot “was proposed by the contractor as a controlled environment for maintenance prior to mining under [the viaduct] and downtown Seattle,’’ the Transportation Department said, adding it plans to close the highway while Bertha is tunneling beneath it.

The Seattle Times reported Jan. 27 that the Transportation Department has already paid $774 million to Seattle Tunnel Partners, or just over half of the $1.44 billion total contract, as of November.

The total viaduct replacement is estimated to be a $3.1 billion project.

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