After 13 months of digging the New Irvington Tunnel in the state of California from four separate tunnel headings, miners from two of the headings have met up with each other underground.
Called a ‘hole-through’ in tunneling terminology, the road header teams from the Irvington Portal in Fremont and the Vargas Shaft 4,500 feet away shook hands somewhere under the mountainside and formally completed this section of tunnel excavation. The New Irvington Tunnel, a critical component of the Hetch Hetchy Regional Water System, will stretch 3.5 miles from the Sunol Valley to Fremont, CA, when it is complete.
”This is a tremendous accomplishment, said SFPUC General Manager Ed Harrington. “Every foot of tunnel they complete brings our water system one foot closer to increased reliability after an earthquake.”
The new tunnel will provide an additional seismically-designed connection between water supplies from the Sierra Nevada Mountains and the Alameda Watershed to Bay Area water distribution systems. Not only does it provide a seismically-sound alternative to the existing tunnel, the new tunnel will allow the SFPUC to take the existing tunnel out of service for much needed maintenance and repair.
The tunnel is excavated using conventional mining methods including a road header (a tracked tunneling machine) and, in sections of hard rock, controlled detonations. In May 2011, a 55-ton road header was lowered down the 115-foot Vargas Shaft and began to grind its way west. A month later, miners at the Irvington Portal in Fremont began tunneling east and set the two crews on a collision course.
Prior to the hole-through, the crews stopped within 9 feet of each other. The road header from Vargas then advanced until its spinning, titanium-studded knuckle broke through to the other side. Miners then made their way around their machines to shake hands and congratulate each other.
Following the hole-through, miners will prepare the tunnel for the installation of 102-inch diameter steel pipe. The pipe, manufactured in California, is installed in 50-foot sections and welded together inside of the tunnel. The much longer 14,400-foot Alameda West-Vargas tunnel segment is currently being excavated and is expected to hole through in fall 2013. Construction of the entire project is scheduled for completion in fall 2014.
“We’ve now excavated fifty percent of the tunnel and have another 9,000 feet to go,” said SFPUC Project Manager David Tsztoo. “Challenging work including more hard rock tunneling in high groundwater conditions lay ahead.”
To view a video and photos from the hole-through event, visit www.sfwater.org/sunolvalley.