SALT LAKE CITY — The fault lines that run through the Wasatch Front suggest that a deadly earthquake will one day strike the greater Salt Lake area. When it does, work announced Friday could save what the president of The Church of Jesus Christ of Latter-day Saints called "a stunning jewel in the crown of pioneer achievement" and all those who work and serve there.

Welcome to the world of base isolation systems.

The systems are relatively simple to understand. Once the temple is closed on Dec. 29 for renovation, workers will begin a yearlong process of placing hundreds of what look like giant hockey pucks at intervals between the ground and the enormous temple's footings and foundation, said Brent Roberts, the church's director of special projects.

They also will add seismic strengthening to the temple walls and towers. When the work is done, the base isolators will absorb seismic shock wave energy the way shock absorbers in a car isolate passengers from road vibrations.

"It actually will now be the foundation of the temple, so when the earth moves, the base isolation system takes all that movement," Roberts said.

A rendering of the Salt Lake Temple base isolation system.
A rendering of the Salt Lake Temple base isolation system. | The Church of Jesus Christ of Latter-day Saints

Roberts said he's watched striking YouTube videos from Japan where blinds barely move in the windows of buildings with base isolation systems, while surrounding buildings shake violently.

The maximum-strength earthquake expected in the Salt Lake Valley is a 7.3-magnitude temblor, said David Hart, past executive director of the Capitol Preservation Board.

"The base isolators take a lot of the energy out of a 7.3 magnitude earthquake," he said. "It's a really, really efficient way of reducing the force elements that are predicted to hit the building in a major earthquake."

The temple still would feel a 7.3 quake at the level of a 5.1 or 5.2, so reinforcing the walls and towers is an important part of the overall system, Hart said.

Interestingly, engineers first used a base isolation system to restore and protect a historic structure from seismic activity just a mile-and-a-half away from the temple. The Salt Lake City-County Building is one of the temple's contemporaries, constructed between 1890-94. In fact, it was Utah's state capitol until 1915. But in the 1980s it was strengthened with base isolators as part of a $30 million retrofit.

That project became the stuff of engineering journals and textbooks. Some emergency and vital government buildings now are constructed with base isolation systems, but they are also used to protect and strengthen landmarks.

The Utah Capitol is another example. Workers placed 265 base isolators underneath the building during a $273 million renovation between 2004-08. When they were done, Hart exhaled, knowing the Capitol was safe.

Now when an earthquake hits the Capitol, it can sway up to 2 feet in any direction as its isolators absorb the majority of the seismic forces.

Hart estimated the base isolators installed at the Capitol cost about $5 million, but he said the overall installation, including the excavation and strengthening walls and the Capitol dome, cost $65 million to $70 million. The Capitol dome itself weighed 28 million pounds, Hart said.

Visitors can see a cross-section scale model of the base isolation system in the Capitol's Visitors Center and take tours to see the base isolation systems at both the Capitol and the City-County Building.

The Capitol was built between 1912-16 with 800 railroad carloads of quartz monzonite, the same granite-like stone used for the Salt Lake Temple from the same quarry in Little Cottonwood Canyon, according to the Utah Geological Survey.

Roberts said that engineering estimates for the project have been revised, so he couldn't say exactly how many base isolators will be needed underneath the temple, its granite foundation and its sandstone sub-foundation.

"It's literally hundreds," he said. "It's a large edifice, and of course a lot of it has to do with scope, size and weight, because they have to carry the weight of the temple."

The project will be nearly as big as the temple itself. Workers will have to excavate well below the temple's base.

"We're decoupling the footings and foundation from the raw ground," Roberts said. "So it is the footings, foundation, base isolators and the ground. When an earthquake comes, depending upon the magnitude and depending upon the shaking, it follows that shaking in a very slowed-down manner. So the earth moves underneath, the base isolator stays still and they will gradually move at a certain parallel with the waves."

The late President Boyd K. Packer, president of the Quorum of the Twelve Apostles at his death in 2015, said in a 1993 general conference talk that 16 inverted granite arches were built into the Salt Lake Temple's foundation.

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"If someday perchance there be a massive force wanting to lift the temple from beneath, then we shall know why they are there," he said.

Roberts said he talked with President Packer about the arches before his death and said workers will not lift the temple during the renovation.

"They were put in there really to hold the weight of the temple," Roberts said. "The engineering of those was remarkable and exciting, but it doesn't help" with an earthquake. They help with the vertical forces of the temple's weight and gravity, but the base isolators will help with the horizontal waves of seismic activity.

The church completed a seismic upgrade of the Tabernacle on Temple Square from 2004-07 by stabilizing it with supplemental framing and adding reinforcing bars to the walls and 150-foot-wide roof. Base isolators are not necessary at the Tabernacle, Roberts said.

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