TURLOCK -- If Tunnel No. 2 can hold up for six more weeks, it will have served the Turlock Irrigation District well.
The tunnel, built near La Grange in 1891, is at risk of collapsing because of unstable material in its roof, TID officials said.
Tuesday, the district board voted 5-0 to remove the roof and turn the 360-foot tunnel into an open canal. The work is scheduled to start after the Oct. 10 end of irrigation season and finish in December.
"By the end of 2012, Tunnel No. 2 will have provided the district the last of its 121 years of service," senior civil engineer Brad Koehn told the board.
The cost is estimated at $975,000. The district plans to call for bids next month.
A tunnel collapse would force the TID to get by with the water stored downstream in Turlock Lake, along with groundwater. The much bigger supply from Don Pedro Reservoir would be lost until emergency repairs were done.
District staff last year estimated that a three-week water loss could cost farmers about $4 million in income. After five weeks, the losses could reach about $18 million.
By this time, the TID would be spending about $1 million on increased operating costs as it scrambled to keep the farms irrigated.
The tunnel is one of three along the Upper Main Canal, which carries Tuolumne River water bound for about 146,000 acres of farms around Ceres, Turlock, Hilmar and other areas.
Inspections have found that No. 1 and No. 3 are fine, thanks to the solid rock that makes up their roofs, Koehn said. The native rock in No. 2's roof is "a very weak gravel conglomerate" that is not holding up, he said.
Falling rock from the roof has created a cavity 17 feet higher than the original ceiling height, he said.
In the "daylighting" project, as it is called, heavy equipment will move about 40 vertical feet of soil and rock. The estimated 78,000 cubic yards of material will be deposited in an area previously disturbed by construction along the canal.
The TID is not alone in dealing with old tunnels in the foothills. The Oakdale Irrigation District is upgrading several thousand linear feet.
The Modesto Irrigation District, which shares the Don Pedro supply with the TID, does not have any tunnels dating to the early days. The MID built a milelong tunnel in the late 1980s and monitors its condition.
Bee staff writer John Holland can be reached at email@example.com or (209) 578-2385.