Caltrans workers were probing into the northbound tunnel face to get a sense of hydrology in the mountain ahead when they struck a water pocket. That much is 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 in San Pedro Mountain produce flows ranging between five and 60 gallons per minute, Wonder said.
 Caltrans says drillers recently hit the largest water pocket yet during the excavation of twin tunnels at Devil'€™s Slide. |
A Caltrans contractor estimates the pocket rendered 2.2 million gallons of water.
Diggers punctured three holes to drain the reservoir, capping them with hoses that channeled the water to a sump pump that empties into a storage tank outside the tunnels. After treatment, the water was funneled into the ocean.
Early in the planning stages for the tunnel project, some on the Midcoast talked of connecting a pipeline from the mountain to the Montara Water and Sanitary District. District directors objected, saying the mountain wouldn’t produce enough water to justify millions of dollars for the water line.
Flow of 150 gallons-per-minute rivals what district directors expect to pull from the district well on Alta Vista Road in Montara, according to district president Paul Perkovic. While some may point to the 2-million-gallon pocket as evidence of poor decision-making, Perkovic calls that amount “barely perceptible” in the long run. The costs would have greatly overshadowed the benefits of the single, 10-day spring, he says.
Instead of routing water to Midcoast residents, Caltrans has found another noble use for it.
In the name of efficiency, Caltrans is engaging in “dewatering” on the northern side of the mountain where the tunnels will eventually punch through. Workers are probing into the rock face along the path of the tunnels to drain as much as they can before diggers inside the mountain reach the reservoirs.
Some of the water is being trucked to the treatment tank at the tunnel site. But some is so pure it’s being funneled directly into ponds underneath the Devil’s Slide bridges to improve habitat for the California red-legged frog. The frog is listed as a threatened species and presents a common obstacle for developers looking to build on the Coastside. Bridge builders took special precautions not to disturb the frog ponds in the valley below during construction.
Diggers are approximately 70 percent through the mountain, with 1,300 feet to go in the northbound tunnel and 1,350 feet remaining in the southbound tunnel. Caltrans expects to punch through the mountain next summer.
“By allowing the water to drain out, we should have better conditions moving forward,” Haus said. “It does not pose an impact to the overall project schedule.”
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