How do you move a historic artifact seven storeys high, two-thirds the length of a football field and weighing as much as 1,900 automobiles? This was the unique engineering challenge posed by Dredge No. 4 in the early 1990s.
This project presented a number of problems; for one thing, no one had ever attempted to move a decaying derelict dredge this big before. To make it even more difficult, the dredge was filled with silt and locked in frozen ground.
Built near the mouth of the Klondike River in 1913, and operated by Joe Boyle’s Canadian Klondyke Mining Company, No. 4 worked its way slowly down the Klondike valley. The aging dredge was disassembled in 1940 by the Yukon Consolidated Gold Corporation (YCGC), and the old machinery was installed on a new superstructure at the lower end of Bonanza Creek, a few kilometres outside of Dawson City. Overseen by a team of engineers, the reconstruction involved modifications to enable it to dig deeper.
The dredge worked its way up Bonanza Creek until it was shut down at the end of the season in 1959. The following spring, a dam collapsed upstream, sending a wall of water down the creek. Dredge No. 4 was spun around and submerged. Year after year, the spring floods washed over the hull, filling it up with silt until the dredge was buried in almost six metres of sediment. The ground in, around, and beneath the hull froze.
The operations of the YCGC came to an end in 1966 after 60 years. The dredge was donated to Parks Canada by the company in December of 1969 and was subsequently designated as a national historic site.
During the 1970s, cracks began appearing in the massive structure. The tailings stacker and the stern section appeared to be moving upward, but the cause of the problem was buried from view. A series of engineering studies recommended that the dredge be moved to higher ground.
Parks Canada approached 1 Construction Engineering Unit (1CEU), a Canadian military unit stationed in Winnipeg, which agreed to oversee the project. Mechanical means of moving the dredge were explored. In the end, the engineers chose a hydraulic lift method. But before it could be undertaken, a number of questions had to be answered: would the dredge survive such a move? What was the condition of the hull – would it allow the dredge to float once more? Finally, would there be enough water available to make such a move possible?
After soil and structural tests were conducted and feasibility confirmed, 1CEU reaffirmed the hydraulic lift as the most cost-efficient method of moving the dredge. The hull would be dug out and the silt and ice cleared from inside the hull, which would then have to be made watertight.
Next, a dike would be built to enclose the dredge and the area to which it was to be moved. There was plenty of material available at the site to do this. To reduce water loss through the porous ground, a rubber membrane would be laid down inside the enclosure to create a waterproof liner.
This would be done in the first year; in the second year, during spring run-off, water from Bonanza Creek would be pumped into the reservoir enclosed by the dike, and the dredge would be floated to its new location and sunk onto its new foundation. The foundation, a heavy timber crib structure, was built 85 metres away and eight metres above the ground upon which the dredge had rested for 30 years.
Once that was accomplished, the water would be drained, the dike removed, and the former resting place filled in.
The work commenced in the spring of 1991 under the command of Major Gareth Jones. Major Jones, a graduate of engineering science from Durham University in England, and Master Warrant Officer Mac Torrie, the project superintendent, oversaw the work of a small crew of military and civilian employees and contractors.
At first, a slurry pump was employed to churn up and draw away the material surrounding the hull, but the abundant roots and organic material constantly clogged the pumps. Finally, to remove the silt, a backhoe was employed. To reach the depth of the bottom of the dredge, the backhoe had to make three successive passes around the hull.
Chainsaws were used to remove some ice from inside the hull. Thawing with hot water was considered to remove the rest, but calculations proved the amount of energy required was too costly, so cold water was employed instead. Massive volumes were flushed through the hull, and the silt suspension was removed with an improvised airlift pump.
Removal of the silt from the inside of the hull occurred at the same rate as the removal from around the outside of the hull, to equalize the forces on both sides of the hull. One of the surprises of removal of the frozen silt was the exposure of thousands of tools and parts that had been left on the main deck and in the hull when the dredge was abandoned. These perfectly preserved items were systematically removed from the dredge and recorded while work continued.
In preparation for winter, the question was asked: how to leave the site over the winter? Local miners had some advice to offer the engineers of 1CEU: if the reservoir was kept water free during the winter, then “glaciering,” the ongoing freezing of water upwelling from the water table during the cold weather, could fill the reservoir with ice. To prevent this and eliminate the costly melting of the ice in the spring, the pond was filled with water for the winter and allowed to freeze over.
In the spring of 1992, the two-metre skin of winter ice was removed and the pond was drained to allow final hull repairs. There was, however, another problem that had to be overcome. The bottom of the hull seemed to be stuck to the underlying soil, either because it was frozen solid, or because of suction. A network of pipes and hoses was attached to holes that were drilled in the bottom of the dredge to pump water under the hull. It was hoped that in this way, the suction would be broken and the ice holding the hull in place could be thawed, but in the end, it proved unnecessary.
The pond was refilled with water while pumps on board the dredge kept the hull empty. Dredge No. 4 gently floated free on June 11, 1992, after which the pond was filled with water nearly to the top of the dike. Then, the massive machine, while connected to four bulldozers by cables, was slowly inched over the spot where the foundation lay below four metres of murky water. She was correctly positioned, the pumps were shut off and the hull filled with water, allowing the dredge to sink slowly onto her new resting place.
This unique project was safely completed ahead of schedule and $100,000 under budget – a proud moment for the engineering profession.
This article was provided by the Association of Professional Engineers of Yukon.