On-Grade Contracting out of Jamestown, California, did this recently on a job site for the city of San Francisco-Public Works Bureau. The work was at the base of the O'Shaughnessy Dam at the Hetch Hetchy Valley Reservoir within Yosemite National Park.
"Fifteen years ago, a couple of rock slides occurred in the Tuolumne River just below the dam causing the river to backup and flood the dam's weir tunnel," said Mike Johnson, owner of On-Grade. "The flooded weir tunnel has prevented routine seepage monitoring of the dam." One of the rock slides rose some 20 feet from the river bed, but Public Utilities officials were hoping heavy water flow from periodic dam releases would dislodge enough boulders to clean out the rock slide deposits. O'Shaughnessy Dam reservoir has provided water for the city of San Francisco since 1923.
One bidder for the contract wanted to set up operations on top of the dam and set a crane to haul the rocks aloft, said Johnson, a more expensive and lengthy process than the one his company figured on.
"This is a very unusual project in that the canyon where we worked is very narrow and extremely steep, with sheer bedrock slopes," he said. "We thought we could get down to the work and haul the rock up from the canyon edge by taking a cue from the logging industry. We decided to use a 'swing yarder,' which has a boom that supports the heavy weight of machinery and buckets filled with material from below on a series of pulleys, a winch and cables, anchored into the rock of the opposite side of the canyon wall."
"We thought we could get set up in about a week, but it ended up taking us about one month," Johnson added. Each bucket load weighed in at approximately 10,000 pounds — close to 2.5 cubic yards per load. Twenty separate anchors, at $1,000 each, were drilled into the opposite canyon's side wall and epoxied in place
"We finally settled on a technique used in the 1920s," said Johnson. "We got calculations from a structural engineer to use 1-1/8-inch rebar bent into a horse shoe shape and fitted into the canyon wall to use as our anchor." During the work, the anchors across the canyon were checked for safety twice each day.
"This was a very dangerous job," Johnson acknowledged. Just winching down the excavator was a challenge. There was only a small rock pile below that was a good place to "land" the equipment and start removing material. When the time came to drive the excavator to the canyon edge while connected by cable, Johnson decided to do that himself, because in his words, "How do you pay someone else to do that?!" And it was done at night with flood lights aimed over the canyon.
When excavation work started, the main cable was anchored across the canyon. It was then slacked to drop the haul bucket down to be loaded with a CAT 321C excavator, then the yarder tightened the main cable, lifting the filled bucket, and pulled it back to the canyon edge by another cable. On a level place above the canyon, a Case 9020B excavator then tipped the bucket until emptied, and loaded the material onto a series of Peterbuilt 10- wheel trucks to be off-hauled and stockpiled.
"The yarder worked just about at its capacity," said Johnson. "But we were able to get our cycle time per bucket-load to just a bit over three minutes from start to finish. When the job was completed, we had hauled out and stock-piled some 8,000 tons of rock that city of San Francisco may use in the future for other projects."
"Some rocks weighed 5,000 pounds," said Johnson. "With the tight work space we needed an excavator that could swing within its tracks. Holt Caterpillar out of Stockton supplied us with a rental of their new 321C LCR. I don't think there's another machine out there that could do what this one did. And I'll say this about Holt — our yarder's engine, an older CAT Model 3304, threw a valve. We were going to order a new engine but CAT Holt's mechanics team urged us to let them fix it. They had the thing up and running by the next morning. Amazing."
"The contract specs stated that we could place an excavator up to the water's edge," according to Johnson. "We were very careful and did everything to ensure we stayed out of the water — only allowing our excavator arm and bucket to dig into the river.
"Also, our turbidity levels were very, very tight," Johnson said. "This was probably our biggest hurdle — keeping the levels below what was allowable there. We had our own turbidity test kits along with an Imhoff sediment cone. Added to that, we had to adhere to four agencies' permits: Yosemite National Park, California Regional Water Quality Board, Fish and Game, and the Army Corps of Engineers. Also, we were closely monitored by the city and county of San Francisco. We made sure we were in compliance. Most environmental lawsuits on jobs usually occur when construction companies don't adhere to the specs on a project."
Work started in October 2006, at a time that allowed the Tuolumne River to be low enough that On-Grade Contracting could do its job. Water flows from the dam were dropped to 35 cubic feet per second. The company had to be out of the area by late February 2007. They finished on time.