(Image: CHS)

Passersby were surprised by a spouting geyser of grey, foamy water shooting from the street at the intersection of E Pike and Boylston just after noon Thursday.

Witnesses tell CHS that the water suddenly shot high into the air, sending a manhole cover tumbling and covering a few cars parked within range of the plume with icky grey mud.

Seattle Fire quickly arrived but the water continued to pour as they worked to shut off the flow. In the meantime, the grey sludge-y water flowed down the nearest storm drain — and gathered a large lunchtime crowd of onlookers.

Seattle Fire had no immediate information on what specifically had broken and what caused the incident to occur.

Last week, CHS reported on another peculiar moment of a Capitol Hill infrastructure breakdown when a Metro pole on 15th Ave E used to support the wires that power the city’s electric trolleys snapped in two and closed the street for hours.

Thanks to reader Laura for the picture!

(Image: CHS)

(Image: CHS)

1:00PM – Seattle Public Utilities confirms the water main break but has no further details yet.

1:23PM – Grey geyser bursts forth on E Pike in apparent water main break – No significant construction or maintenance was underway in the immediate area of the break — though as we know the Hill is busy with construction above and below ground these days. We have been told to contact Sound Transit for more information but have not yet heard back. The agency’s light rail tunnel project is underway in the area but we have no confirmation that the work had an impact in this incident.

1:31PM – CHS has learned that the cause of the geyser was not a water main break. That grey sludge might have looked familiar to anybody watching Capitol Hill’s light rail tunneling process closely. We’re told the tunneling in the area accidentally reached an “observation well” drilled along the route to monitor the tunnels stability. When the shaft was reached, the grey mixture of soil, water and grout was spewed to the surface and into the street above.

1:35PM – We have documented Sound Transit’s Capitol Hill tunneling efforts extensively here. The grout mixture is reportedly mostly harmless and is being trucked away by the cubic yard daily from the light rail construction site at John and Broadway.

1:36PM – We have not yet heard back from Sound Transit on how the observation shaft was reached by the tunnel boring machine but will update when we know more.

2:30PM – According to Sound Transit’s Bruce Gray, the geyser of grey was indeed caused by light rail tunneling ninety feet below but it’s not like the high-precision tunnel boring machine got off course or clipped some piece of underground infrastructure. Instead, Gray tells us the observation wells along the route are supposed to have been capped — this one wasn’t. UPDATE: Sound Transit clarifies that there was a cap on the well but it hadn’t been “decommissioned” — filled with concrete and grout — before the TBM made its pass. The result was the soil conditioning agents spit out by the boring machine as it digs were sent shooting up the well and out onto the street above. Gray says the agency hasn’t seen anything like this before and will check out the four wells of this type along this part of the route to make sure there won’t be any more geysers. As for any dangers posed by the grey sludge, Gray said not to worry but “We are going to be washing some cars up here.”