Transportation officials said Friday that the shaft will be 100 feet deep and 80 feet wide. Other details about the plan for fixing the seals that protect the tunneling machine's main bearing won't be available until next week. That's when experts from the machine's manufacturer arrive from Japan.
The Transportation Department has said it will be months before the huge machine named Bertha starts moving again.
Officials insist the tunnel is still the "most expedient way" to replace the Alaskan Way Viaduct.
The tunneling machine has been stalled since the first week of December. It's completed about 1,000 feet of a 1.7 mile tunnel to carry Highway 99 traffic.