OPEN CANAL SOLUTION FOR ADDRESSING THE STABILITY OF AN UNSUPPORTED WATER CONVEYANCE TUNNEL

A non-pressurized water conveyance tunnel on a critical irrigation canal in the lower Sierra Nevada foothills exhibited signs of crown instability, and raised concerns regarding the potential for a large-scale failure that could completely block canal flow, resulting in millions of dollars of crop losses. The tunnel is on the Upper Main Canal, a 70 cubic meter/second canal that conveys water from the Tuolumne River to Turlock Lake for agricultural irrigation in California’s fertile Central Valley. The approximately 110-meter-long tunnel was originally constructed in 1891, and was later enlarged in 1914 to a typical cross section of 9 meters wide and 7 meters tall. The tunnel was excavated primarily within a sandstone unit of the Mehrten Formation. The concrete-lined invert and sidewalls performed well with no evidence of instability, but a large domed cavity had formed in the unsupported crown near the center of the tunnel. The cavity extended approximately 5 meters above the original crown, exposing an alluvial paleochannel filled with a low-strength conglomerate unit. Detailed crown mapping combined with six exploratory borings and seismic refraction surveys indicated that the conglomerate unit was unfavorably positioned within 4.5 meters above the crown along much of the tunnel. Tunnel rehabilitation options considered included stabilizing the tunnel crown via mechanical support, and daylighting (open cutting) the tunnel. Internally-braced crown support systems, such as steel ribs and lagging, were evaluated. However, given to the position of the conglomerate unit, the daylighting option was found to be best suited to meet the owner’s objective for a long-term reliable solution. Following extensive environmental permitting efforts, the tunnel crown was removed during an annual outage in late 2012. The contractor opted to excavate the crown using conventional grading equipment rather than blasting to minimize damage to the canal lining. The tunnel crown was removed over five weeks with minimal damage to the canal lining and within the outage window. This project presents a unique case study of addressing the potential vulnerability of a critical irrigation canal to tunnel failure by daylighting, and the lessons learned may translate to similar projects throughout the western United States.