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O’Neil Tunnel Improvements Project

Oneil tunnel

Tuolumne Utilities District (TUD) has started the O’Neil Tunnel Improvements Project with improvements to the O’Neil Tunnel, which is located on the historic 1852 Table Mountain Ditch, one of thirteen ditch conveyance systems the District operates. The O’Neil tunnel is located at the end of the Table Mountain Ditch at O’Neil Reservoir near Jamestown.  The District owns and operates a total of 71 miles of ditch, flume, pipe, and tunnel infrastructure that diverts water from the Pacific Gas and Electric (PG&E) system at various locations.

Overtime the O’Neil tunnel has shown significant degradation. The total length of the tunnel is approximately 476 feet.  In 2017, Condor Earth Technologies, Inc., hired by TUD, investigated the downstream 206 feet of open tunnel section and prepared a hazards study.  The hazards study revealed several concerns and risks about the tunnel and the level of deterioration that has taken place since it was constructed in the 1850’s.  The O’Neil Tunnel Improvements Project started in July 2019, consists of the installation of approximately 200 linear feet of 24” High Density Polyethylene (HDPE) pipe and 40 linear feet of 30” steel pipe inside of the O’Neil Tunnel. At the Board meeting held on July 23, 2019, the Board of Directors awarded a contract to the Sierra Mountain Construction, Inc. (SMCI) in the amount of $236,315.00 to complete the project. The project is estimated to be completed by January 2020, weather permitting.

As a historical ditch conveyance system, TUD staff has included multiple design features to preserve the historic nature of the tunnel. Features include avoiding rockwork that is located between the tunnel and O’Neil Reservoir, covering modern looking plastic pipe so it cannot be seen, and using metal pipe where the pipe is visible. The metal pipe will be allowed to rust naturally in order to enhance the historic look. The project is located on Rawhide Road in Jamestown.

The history of the Table Mountain Ditch begins during the Gold Rush period in Tuolumne County. The Table Mountain Ditch, also known as the Rawhide Ranch Ditch, was constructed by the Tuolumne County Water Company (TCWC) in 1852 to take water to the diggings on the west side of the Table Mountain district. At that time, it primarily served the Rawhide Mine, with its two periods of prosperity: from the 1850s to 1867 and from 1891 to 1905 (Gudde 1975:285). The ditch brought water from the Columbia system through the Deadman Ditch to the Springfield Weir, then down along Mormon Creek to the Slum Dam Reservoir, constructed in 1853 by the TCWC to slum out the tailings carried down from the Columbia placers. The dam was rebuilt in stone in 1900 and in 1916 was described as a small rubble masonry structure, built directly on the bedrock of the creek. From there the Table Mountain Ditch was diverted through a flume and carried along the base of Table Mountain to Rawhide Flat (Rhodin 1916). About 1856 a tunnel was driven through Table Mountain connecting the Rawhide Ditch (Table Mountain Ditch) with the Montezuma Ditch at a reservoir on the mountain’s west side (present O’Neil Reservoir), purchased from Allen Oliver in February 1856 (Rhodin 1916; USGS 1900; Wallace 1862). (Reference from the Tuolumne Utilities District Ditch Sustainability Project Historic Resource Evaluation Report, completed January 2012).

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