Posted on 09/18/2003 1:12:49 PM PDT by bedolido
RAPID CITY, S.D. - Technology that provides precise data about cracks on the face of Mount Rushmore confirm the rock is moving, but only with changes in temperatures and only slightly.
The National Park Service hired Rapid City engineering firm RESPEC in 1999 to install "linear variable displacement transducers" - or LVDTs - at strategic locations on Mount Rushmore. The LVDTs can detect rock movement measuring a thousandth of an inch.
On Tuesday experts gave their annual assurances that Mount Rushmore won't tumble anytime soon. "I'd bet my life on it," Rushmore maintenance mechanic Bob Crisman said.
Gathering the data is a complicated process. Although Rushmore looks like a solid piece, it's actually a series of 25 to 30 "blocks" of granite, said Paul Nelson, a geological engineer with RESPEC.
Engineers picked four "key blocks" to monitor, he said. Two are on the right side of George Washington's head. The other two are on Abraham Lincoln's head. The devices automatically take measurements every four hours and transmit the information to a computer.
The LVDTs are grouped in small "boxes" that are attached to the rock. Other devices measure rock temperatures.
The data shows the rock moves in increments ranging from five-thousandths of an inch to five-hundredths of an inch. But all the movement in four years of monitoring has been tied to temperature, said Chief ranger Mike Pflaum.
So far, the rock has always returned to its original position.
"It's very, very good rock," Nelson said.
Crisman, who has worked at Mount Rushmore for 30 years agrees.
He believes the most significant maintenance on the sculpture was done in the 1970s, when he and others stopped bigger leaks behind the four heads. Water there was washing out iron dissolved from old equipment and the runoff was making its way onto the faces.
Crisman said the hairline cracks at Mount Rushmore are nothing to worry about. They are so narrow - less than the width of a pencil lead - that they admit hardly any water, he said.
"It's like a mortar joint without any mortar," Crisman said.
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