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"They started designing this pipeline in 1969. Personal computers didn't exist. You almost didn't have scientific calculators. It was slide rules and pieces of paper back then, but these guys were really pushing the frontiers of lot of stuff including seismic design."-- Bill Howitt, Alyeska engineer
The Ultimate Test
More than thirty years ago, engineers designed the Trans-Alaska pipeline to withstand a potential earthquake of 8.5 magnitude on the Richter scale. To put this in perspective, the earthquake that leveled San Francisco in 1906 would have notched a mere 7.7 on the Richter scale. The Alaska engineers may not have expected that their design would get the ultimate test. But on November 3, 2002, a 7.9-magnitude earthquake shook the Denali Fault, which sits directly under the path of the pipeline. It was one of the largest tremors ever recorded in the United States, and it rocked boats as far south as Louisiana. The shaking was so violent it left a scar measuring about 200 miles long across the landscape.
Across Three Fault Lines
When plans for the pipeline were first drawn up, it was inevitable that the pipeline would have to cross the Denali Fault. In fact, the pipeline traverses a total of three active faults. It would have been impossible to build a straight pipeline and go around so many big obstacles.
Dealing With Strike-Slip Movement
The Denali Fault is one of the largest strike-slip faults in the world, equal in size to the California's San Andreas Fault, the culprit in the San Francisco earthquake. Strike-slip faults produce earthquakes when tectonic plates on either side of the fault move horizontally against another. But the Denali Fault was not the only challenge the engineers faced. The pipeline had to be built above ground because of permafrost. To protect elevated pipe in the event of an earthquake, the engineers came up with an ingenious plan.
A Moveable Pipe
As the elevated pipeline approaches the Denali Fault, it comes off its vertical support members, the H-shaped pilings that hold the pipe above ground in the permafrost zone. The pipe is placed in a steel shoe, which sits on top of a low-to-the-ground concrete beam. The bottom of the pipe shoe is covered in a slippery, Teflon-coated plate, allowing it to slide across the beam. At the Denali Fault, the pipe can slide up to 20 feet horizontally and five feet vertically. As an added precaution, in several of the pump stations, the engineers installed an earthquake monitoring system, which records and analyzes the severity of the shaking and the potential damage it could do to the pipe.
Put to the Test
When the 2002 earthquake struck, the pipe moved seven and a half feet horizontally and two and a half feet vertically -- well within the range conceived by the original engineers. The shaking broke at least five above-ground cross beams that support the pipeline and two vertical support members, but there were no reported leaks. At the time, Jim Lusher, an engineer on the pipeline, said "Quake damage to the pipeline was right in line with what was expected."