Earlier this month, Devil’s Slide tunnel diggers tapped into the biggest underground reservoir they’ve encountered to date, according to Caltrans. A steady gush of water did not slow the project, however.

Caltrans workers were probing the northbound tunnel face to get a sense of hydrology in the mountain ahead when they struck a water pocket – all part of the process, says chief spokeswoman Lauren Wonder. What’s unique about the pocket is it rendered a 150 gallon-per-minute flow, by far the fastest to date. Typical groundwater pockets workers have found produce flows ranging between 5 and 60 gallons per minute, Wonder said.

When asked if the heightened flow indicates workers have tapped into a larger water body, Caltrans spokesman Bob Haus answered, “Pretty much.”

“It’s the difference between punching a hole in a small bucket of water versus a large bucket,” he said.

Diggers punctured three holes to drain the reservoir, capping them with hoses that channel the water to a sump pump that empties into a storage tank outside the tunnels. After treatment, the water is funneled into the ocean.

The discovery didn’t hinder diggers – they directed their focus to another part of the tunnel while the reservoir drained. Wonder said flow is expected to dissipate to about 5 gallons per minute by this weekend.

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