January 30, 2024
This article originally appeared in the November/December 2010 issue of DEMOLITION.
Located 30 miles southeast of downtown Seattle, Washington, Snoqualmie Falls is an extremely popular tourist and visitor destination — the second most visited in the Pacific Northwest with over 2 million visitors per year. Whether it is a day trip to simply observe the 300-foot natural waterfall or a weekend stay at the four-star Salish Lodge and Spa, many visitors are not aware of the mostly subterranean century-old engineering marvel that quietly produced hydroelectric power up until last spring when Puget Sound Energy completely shut down the two powerhouses to accommodate a major $120 million plant upgrade and rebuild.
The Snoqualmie Falls Hydroelectric Plant consists of two power plants: Plant 1 and Plant 2. Plant 1, constructed in 1898, is the world’s first completely underground powerhouse. At 270 feet below ground, the Plant 1 powerhouse’s rock cavity accommodates five hydroelectric turbines and generators capable of producing 12 MW of power. Plant 2 powerhouse was built in 1910 and later added onto in 1956. Plant 2 is located on the riverbank below the falls and has two turbines and generators capable of producing 33 MW of power.
J. Harper Contractors Inc. was initially contacted by Barnard Construction Inc. in October 2009 about the possibility of providing a sub-bid for the demolition work on the Snoqualmie Falls project. After an initial site visit, it was apparent that this would be a substantial and extremely challenging demolition project, which instantly grabbed the attention of the Harper estimating staff. Harper proceeded to complete the demolition estimate and made the trip to Barnard’s Bozeman, Montana, offices for in-depth estimate reviews and strategy meetings. Bids were submitted in late January 2010, and contract awards were made later that spring.
The project kicked off with the wrecking of the old visitors' observation platform that had been determined to be structurally unsafe due to corrosion of the structural steel support beams due to constant mist from the falls. This proved to be no simple task and not for those with discomfort with heights, as the platform cantilevered out over a 300-foot cliff drop off to the bottom of the falls below. Additionally challenging to the platform removal was the fact that new landscaping and sidewalks had been installed on all sides of the platform, prohibiting access to large equipment, thus requiring the removal of the visitors' observation platform to be accomplished in large part by hand.
Next was the simple removal of the original machine shop (it was tough to find parts in 1898) and the transformer building, both heavy brick and timber buildings. Only one would think simple, as this is where things got interesting due to the owner’s aversion to recycling concrete and brick that had been painted since 1898 with lead paint. It took considerable meetings with the owner and the help of NDA’s Mike Taylor, Jeff Kroeker’s U.S. Army Corps of Engineers study on concrete crushing and a positive letter from the Washington State Department of Ecology advocating recycling over landfilling to convince the owner that recycling concrete and brick that possesses a TCLP was accepted national industry standards. This was an important process on a project that had been bid to recycle 16,000 tons of concrete and brick.
With the initial stages of demolition and clearing behind J. Harper, the tough, time-sensitive work began once the in water work permits were in place. The first time-sensitive piece of demolition involved removing the top 5 feet off the 350-foot concrete diversion dam. This work was necessary because the dam was over 100 years old and the fact that lowering the dam allowed for the river level to be lowered, facilitating new construction.
The diversion dam was removed utilizing three Atlas-Copco 7,000 ft/lb hydraulic breakers attached to Kobelco 330 and 350 excavators. Over 2,000 cy of concrete was removed from the dam in less than two weeks.
The Plant 2 powerhouse demolition proved to be a challenging part of the project with the contract calling for the removal of the entire superstructure while leaving one of the large turbines and generators in place. Protecting the turbine that remains was accomplished by a combination of the mechanical contractor installing a climate-controlled silo over the turbine and generator coupled with J. Harper installing shoring towers and heavy planking over the silo to project the tin structure.
As additional protection, the existing overhead crane, which would be eventually demolished with the superstructure, was moved into place above the shoring and was also planked with salvaged glue-lams, providing a third layer of protection from falling concrete roof debris. The superstructure was wrecked utilizing a Kobelco 480 excavator equipped with an 80-foot Jewell high reach stick and a Stanley LaBounty UP20 shear. After wrecking the Plant 2 superstructure, the turbine, which was installed in 1910, was demolished along with separating and removing half of the powerhouse substructure, which consisted of a solid 30x30x30 concrete foundation.
Selective demolition began on the Plant 1 powerhouse after Barnard completed the river diversion work on the south bank of the river, consisting of blasting 1,000 feet of riverbed and installing a steel coffer dam. Plant 1 shaft demolition work consisted of removing the shaft elevator and two 7-foot diameter 250-feet vertical steel penstock pipes along with 60 cubic yards of 3-foot-thick concrete pipe collars. This work was accomplished with two cranes, one supporting a man basket for workers to use torches for hot-cutting concrete anchors in the shaft. The second crane was used for picking up to 40-foot sections of penstock pipe and elevator equipment from the shaft.
Selective removal work in the Plant 1 cavern (located 250 feet below the upper ground surface) included the removal of the subterranean control room, the 300-foot, 7-foot diameter horizontal penstock and all the header and valves for the original 1898 existing turbines, which will eventually be refurbished and put back into operation. The major challenge or the demolition work in Plant 1 is that access for the removal of the scrap steel and concrete rubble out of the cavern was up and out the 250-foot elevator/penstock shaft.