June/July 2008

A walk to remember

Genesee mine’s dragline relocation project

By D. Zlotnikov

Hydrovaccing line: A hydrovaccing technique was used to break up the soil around the pipeline.

The logistics of moving mining equipment can be staggeringly complicated. No matter how much engineering ingenuity you have at your disposal, some things just can’t be folded for easy transportation.

The colossal world of mining — where oversized machinery is commonplace — is full of such challenges. Nevertheless, the heavy machinery does make it to its destination, and the work continues apace.

But there’s large, and then there’s 8.5 million pounds’ worth of large. This happened to be the weight of the dragline Prairie Mines and Royalties, Ltd. needed to move at its Genesee mine site. A dragline is a machine used for stripping the overburden from coal.

“People can’t really comprehend what 8.5 million pounds means,” said Genesee mine’s engineering manager Ken Martens. There’s simply nothing a person encounters in daily life that’s this massive. For comparison’s sake, Martens offered the example of a Boeing 747 jet, which clocks in at a measly 380,000 pounds. You’d need about 22 of them (or 607 school busses) to equal the weight of the dragline.

The planning

The original decision to move the dragline was made years back and was part of the mine’s 10-year plan, explained Martens. The original excavation site, mined since 1988, was being depleted, which meant moving operations from the east side of the highway to the west side, to enable excavation on the new seams.

“We have lots of experience moving heavy equipment,” said Martens, so the technical challenges of the move were part of standard operations. But the process was slowed by the presence of a 13-inch, high-pressure gas pipeline. Buried 1.5 metres underground, it parallelled a highway that served as a major artery for the local community and industry. Walking the dragline across the highway required precautionary measures in place to protect the road, the public, and the power and gas lines.

“We went to ATCO Gas, who owned the pipeline and the right of way, and asked if we could have the gas shut off for the duration of the move,” recalled Martens.

”But we were told that the pipeline was supplying not just our power plant but also a number of plants downstream, as well as the local residents, and had to stay on.”

With such a tremendous weight travelling overhead, the danger of a pipe rupture was too great, so any proposal for the move had to demonstrate beyond any doubt that the project could be completed safely.

The design

Genesee contracted Golder Associates, a geotechnical engineering firm out of Calgary, to produce the project proposal. “Due to the nature of our work, we are very experienced with moving large amounts of material,” said Martens. “So we asked them initially how much competent overburden material we’d need to put on top of the existing surface to spread the weight safely. They came back with a figure of between five and seven metres, so that was out of the question.”

Golder went back to the drawing board, and returned with a new design. The revised plan entailed covering the pipeline with 2.4 metres of overburden in combination with rig mats — common reusable surfaces used in the oil and gas industry.

“These rig mats are steel I-Beam frames in-filled with laminated six-inch lumber,” explained Martens. They are often used to get heavy machinery over muddy or swampy terrain. Still, the rig mats themselves could not spread the weight sufficiently, so the pipeline had to be exposed and essentially bridged with the mats.

“We used a hydrovaccing technique,” said Martens, “where a high-pressure water jet is used to break up the soil and a truck-mounted industrial vacuum sucks up the mud.” This technique exposed the pipeline for a length of 90 metres, and the resulting void was filled with a compressible material to keep the trench sides from collapsing under the dragline’s weight. The material — essentially packing foam — did not quite reach the surface of the trench, which meant any weight pressing onto the rig mats would be kept off the pipeline itself.

“The dragline was operating inside a pit that was roughly 15 metres below ground surface,” explained Martens. “It had to be brought up from the first pit and then ramped down into its new operating position. Technically, that was a bigger challenge for us than the crossing.”

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