WorldWide Drilling Resource

Miners Win Race Against Time in San Francisco Adapted from Information Provided by Mott MacDonald and the San Francisco Public Utilities Commission In the late 1920s and early 1930s, miners used drilling and blasting to con- struct the 3.5-mile Irvington Tunnel through faults between Sunol and Fremont, California. The tunnel became a crucial link in San Francisco’s Hetch Hetchy Regional water system. Approximately 85% of San Francisco’s water has travelled through the tunnel; but this water supply was under threat. According to Julie Labonte, director of San Francisco’s $4.8 billion Water System Improvement Program, “Although the Hetch Hetchy water system is truly an engineering marvel, some critical elements are seismically vulnerable. It’s not a matter of if, but when a major earthquake strikes - we are truly in a race against time.” Since water from the original tunnel is essential to the Bay-area population, the tunnel has not been dewatered or taken out of service for a physical inspection since 1966. A new tunnel was absolutely necessary; but gassy ground conditions, groundwater inflows, and seismic zones with squeezing ground made con- structing another tunnel no easy task. The New Irvington Tunnel was designed to run parallel to the original tunnel, with a steel liner and finished inner diameter of 8.5 feet. Due to the number of fault zones, it was excavated using a roadheader and drill-and- blast methods. The project required miners to be quick on their feet, and ready to adapt in uncertain ground conditions. According to Construction Manager Dan McMaster, “There have been days when we have started with drill-and-blast, and by that afternoon we have been into softer Class III and Class IV ground.” After two-and-a-half years of digging hundreds of feet underground, miners from Fremont, California, used a roadheader to grind through hard rock from one tunnel to another, meeting fellow miners coming from Sunol. It was the last of four reaches to be mined. The breakthrough hole signified the end of tunnel ex- cavation on the project, a significant milestone. McMaster said, “It was a real team effort, as well as having multiple tools in the toolbox to use to excavate and deal with the highly variable ground and groundwater that was encountered.” Construction on the project reached substantial completion in September 2015, and the New Irvington Tunnel is now of- ficially in service. Not only does it provide a seismically sound alternative to the existing tunnel, the new tunnel will also allow the original tunnel to be taken out of service for much-needed maintenance and repair. The new tunnel will carry 90 million gallons of water a day on average during normal operating conditions when both tunnels are in service. An additional 90 million gallons of water per day will also flow through the existing tunnel. The New Irvington Tunnel was the last of three tunnels built to create a crit- ical water lifeline able to withstand earth- quakes on the Hayward, Calaveras, and San Andreas faults. 15 WorldWide Drilling Resource ® AUGUST 2016 October Issue Deadlines! Space Reservation: August 25 th Display & Classified Ad Copy: September 1 st