The massive machine that’s digging a tunnel along Seattle’s Lake Washington Ship Canal to store sewage and stormwater is no ordinary drill. The powerful device, nicknamed “MudHoney” after the local grunge band, is 18 feet in diameter, weighs 900,000 pounds and is worth about $15 million.

But a boulder that the drill recently smashed into was no ordinary rock.

The huge slab, at least 10 feet tall and 4 feet thick, brought the machine to a grinding temporary halt on its journey from Ballard to Wallingford.


The city agency managing the $570 million project, Seattle Public Utilities, is calling the subterranean encounter “MudHoney vs. MegaBoulder,” with the drill eventually conquering the rock.

Though MudHoney’s interior parts weren’t damaged when it hit the boulder on April 21, a number of blades on its cutterhead had to be replaced to equip the machine for churning through solid rock, an SPU bulletin said last week. Such blades are typically replaced regularly even when there are no boulders in the way, according to the agency.

To make the repairs, workers descended 70 feet, reaching the cutterhead through the tunnel and then through the machine. The inside of the drill is maintained at normal atmospheric pressure, but when workers leave to access the cutterhead at the front of the machine and reenter, they must pass through a special chamber to adjust, like deep-sea divers.


Tunneling was paused to allow for the repairs, then resumed at a slower pace than usual, with extra cutterhead inspections undertaken until the machine broke through the boulder on May 27, according to the bulletin.

SPU knew the drill might hit some large rocks along its 2.7-mile route — but not quite this large, said Keith Ward, the agency’s project executive.

“This got dropped by a glacier about 15,000 years ago.” Ward said. “The bad luck was that it got dropped right where we were tunneling.”

SPU and King County’s Wastewater Treatment Division are digging the tunnel to prevent pollution. In older Seattle neighborhoods, sewage from bathrooms and stormwater from street gutters drain through the same pipes. Today, when heavy rains overwhelm those pipes, some of the dirty mixture gets spilled into the ship canal, causing environmental harm.

The city and county are under pressure to reduce how often that happens, per consent decrees with federal and state authorities. The storage tunnel is supposed to keep 75 million gallons of slop out of the water each year.

The project includes a $100 million pump station in Ballard that will be wrapped in an 80-foot-tall, stainless-steel lattice with shimmering lights.


Drilling began last summer and the tunnel is expected to be operational by 2026. SPU says MegaBoulder delayed the project by weeks but the agency hasn’t yet estimated the cost of the repairs and delay. The obstruction occurred about 2,900 feet in, with about 14,000 feet to go. MudHoney is currently under Ballard.

Its blockage might remind Seattle residents of another incident. In 2013, the machine digging the downtown Highway 99 tunnel to replace the Alaskan Way Viaduct hit a steel pipe and subsequently broke down.

That was a more serious problem. The drill, nicknamed Bertha, was stranded for about two years. Bertha was 58 feet in diameter and the world’s largest tunnel-boring machine at the time.

This coverage is partially underwritten by Microsoft Philanthropies. The Seattle Times maintains editorial control over this and all its coverage.