Despite confronting what was initially “unstable” conditions, Mark Suden and the crew of Mining Construction Inc. have made substantial progress in shoring up the timbered sections of Esmeralda Tunnel.
The 1,500-foot-long tunnel was built by PG&E in 1930 to bypass a section of canal that was prone to blowouts. The tunnel and the 22 miles of canal, flumes and tunnels was acquired by the El Dorado Irrigation District in 1999. The 165 cubic feet of water that normally flows down this system by way of a diversion dam on the South Fork of the American River near Kyburz is fed by four alpine reservoirs.
The system — with water rights dating to 1856 — has provided EID 15,080 acre-feet of water since 1919, plus 17,000 acre-feet that was adjudicated in 2003.
The Esmeralda Tunnel had a section collapse Sept. 21, which forced EID to shut down the canal system a month earlier than normal. Normally, the system is dewatered in October for repairs and replacement of flume sections, with completion by December when the power plant restarts.
This year the canal won’t be rewatered until the end of February as the miners work to shore up the timbered sections of the tunnel and dig out the collapsed section.
“Shoring up timbers” is an understatement. What MCI has done is replace timber sets along the walls with 4-inch steel posts bolted into the ceiling timbers. Metal bars at varying heights are temporarily placed between each steel post as an earthquake safety measure to prevent the steel posts from folding like an accordion in the event of a seismic event.
“What surprised everyone was the fault line in the tunnel,” said EID engineer Daryl Noel.
Noel said he was very pleased to have an expert mining contractor like MCI.
In addition to the metal bar blocking between the steel posts, there are a number of safety precautions. A large flexible tube brings air into the tunnel and a string of lights provides illumination in addition to helmet lights. Everyone who enters the tunnel signs in at the construction shack and is briefed on how to use the emergency breathing apparatus in case of a fire. Everyone entering the tunnel wears a belt holding the breather that converts carbon monoxide to oxygen. All entering the tunnel also wear helmets with lights and steel-toed boots. At the tunnel entrance Suden moves a ring from the out- to the in-hook for each person entering the tunnel and reverses that as each exits.
Over the course of the 84 years between the tunnel’s construction and the collapse last fall, the rock ceiling above the 500 feet of timbers has disintegrated to point that the timber sets have been holding up 12 feet of rubble above them.
The timber sections total 500 feet, with a portion on the downstream end of the tunnel and a portion on the upstream section of the tunnel. The remaining 1,000 feet are largely stable rock surfaces that don’t require timbering.
The timbered section of the downstream section of the tunnel is 70 feet long and 350 feet in from the downstream portal, according to Noel, who added that 60 cubic yards of rock and debris was removed from this end of the tunnel.
The upstream end of the tunnel is where the collapse section is closest. To access and remove the collapsed material, MCI dug a second tunnel above the 1930 tunnel. The access tunnel has gone 80 feet in and has another 20 feet to go forward and downward. When the Mountain Democrat toured this tunnel Jan. 7, MCI had just shotcreted the inside of the upper access tunnel. Behind the shotcrete were rock bolts holding heavy steel wire mats.
At the end of the upper access tunnel was a cavern excavated upward until stable rock had been reached. From the top of this approximately 10-foot-high cavern to the bottom of the 1930 tunnel below it is 32 feet. Below the floor of the cavern it is 20-25 feet to the bottom. That is how much tunnel debris the mining company has to remove.
Noel estimated about 55 feet of wooden lagging failed 50 feet in from the upstream portal amounting to 800 cubic yards of debris.
Some tunnel muck was being removed from below by a motorized and rubber wheeled mine loader. Once the shotcrete in the new upper tunnel has set, MCI will begin excavating down the 20-25 feet through the debris fall to the original tunnel below. The upper cavern is 22 feet wide. The cavern will remain as a permanent feature of the tunnel once all the reconstruction is completed.
The ultimate outcome for the timbered sections of the tunnel will be to fill in the bottom and sides of the tunnel with six inches of reinforced concrete. The ceiling will have 8 inches of reinforced concrete. Once the concrete sets, holes will be drilled and shotcrete injected into the rubble to fill voids as wide as 4 feet.
The final outcome will be a box culvert through the timbered sections of the tunnel.
Because the tunnel won’t be ready to take on water until the end of February, work on Flume 42-43 has proceeded slower, since putting a contractor crew on the job 24 hours a day is not necessary.
The work done by Syblon Reid under a $1.98 milion contract included widening the marshalling yard at the end of Rock Crusher Road and widening the bench along the 430 feet of Flume 42-43. That involved constructing a 33-foot-high mechanically stabilized earthen wall. An MSE wall begins with a geo-grid anchored into the rock face and then covered with compacted earth and crushed rock held in a wire cage-work stacked on the downhill side in step-like fashion. Each step is another geo-grid layer that is anchored back into the rock face and covered with compacted earth.
By Jan. 7, Syblon Reid had completed the MSE wall and a concrete base for the new flume and was laying out the base timbers and drilling holes for the retaining plates. Noel explained that the concrete foundation would enable the district to later come in and install cast concrete flumes. Because of timing issues this time, wooden flumes were being erected on the concrete foundation.
In fact, the district’s supplies of flume timbers are being depleted by this job, because the order EID put in for treated timbers is delayed because of the wildfire that swept through Weed on Highway 97 in Northern California.
The widened bench along Flume 42-43 will enable heavy equipment to access the next flume downstream at a later date — Flume 44. Meanwhile EID construction crews at a cost of $314,000, which includes $113,000 of helicopter support, added in replacement supports in sections of Flume 44 where rotten timbers were found. The 430 feet of Flume 44 includes an elevated section 30 feet below the catwalk. Some of the supporting timbers are buried under as much as 10 feet of dirt that washed down from the hillside.
Full replacement of Flume 44 and construction of an MSE wall to allow heavy equipment in is estimated at $2.75 million and is slated for 2017. Next in line after that will be Flume 45, a 1,942-foot-long flume section that has an historic rock wall, such as Flume 41 has. It will get the same treatment by running anchor bolts into the rock face and injecting shotcrete through tubes to “glue” the stacked rocks in place, provided that geotechnical investigation finds the base suitable for that treatment. That is slated for 2017-2018.
For this year, however, the two biggest jobs have been the $1.9 million Syblon Reid work on Flume 42-43 and the not-to-exceed $3.7 million Mining Construction Inc. contract for the Esmeralda Tunnel. Total completion may take a second season for Esmeralda, but it will be in condition to run water through by the end of February.