By Will Dunham
May 21 (Reuters) - If ever there were a structure built to stand the test of time, it surely would be the Great Pyramid of Giza, a transcendent monument to human imagination and ambition. Since being erected during the Old Kingdom period of ancient Egypt, it has stood impassively as time marched on and civilizations rose and fell.
Researchers now have discovered one of the reasons for its astonishing durability - it was designed and built with structural features that have helped it withstand
the destructive energy of earthquakes since its construction about 4,600 years ago as the tomb of the pharaoh Khufu.
The scientists assessed its structural dynamics by using devices called seismometers to record ambient vibrations - continuous subtle background shaking generated by natural forces and human activity - at 37 locations in and around the pyramid. It exhibited a remarkably homogeneous and stable structural response to these vibrations despite its size and complexity.
The four sides of the pyramid, located in Giza right outside the Egyptian capital Cairo and built of massive limestone blocks, each measure about 755 feet (230 meters) at the base, and it covers about 13 acres (5.3 hectares).
It originally stood about 480 feet (147 meters) tall. Natural erosion over time and the removal centuries ago of its smooth outer casing stones for building material left it at its current height of approximately 455 feet (138.5 meters). It was the world's tallest structure for some 3,800 years.
The scientists identified several characteristics that gave the pyramid earthquake resistance. It has an extremely broad base with low center of gravity, highly symmetrical geometry, a gradual reduction in mass toward the top and sophisticated internal design including internal chambers that blunt vibration amplification. It also was constructed on strong limestone bedrock.
"These elements together create a well-balanced, coherent structure," said seismologist Mohamed ElGabry of the National Research Institute of Astronomy and Geophysics, or NRIAG, in Egypt, lead author of the study published on Thursday in the journal Scientific Reports.
"Ancient Egyptian builders clearly possessed practical knowledge related to stability, foundation behavior, mass distribution and load transfer," NRIAG seismologist and study senior author Asem Salama said.
The researchers found that most vibrations recorded within the pyramid had a frequency that indicated that mechanical stress was evenly distributed throughout.
"So while I would hesitate to claim that they intentionally designed the pyramid specifically for earthquake resistance, I do think they developed architectural and geotechnical solutions that naturally produced structures with exceptional long-term resilience," Salama said.
This was learned over time through trial and error, as some flawed pyramids that preceded this one demonstrate.
The researchers collected seismic data from various passages and chambers built inside the pyramid, including the primary burial room called the King's Chamber, as well as the surrounding bedrock and soil.
They found that vibration amplification increased with elevation inside the pyramid, a normal phenomenon for tall structures. But they observed a reduction in amplification within five special chambers built above the King's Chamber, despite their higher position.
"This suggests these chambers effectively help dissipate seismic energy and protect the King's Chamber - one of the most critical areas - from excessive shaking," ElGabry said.
The most recent earthquakes in the region included notable ones in 1847 and 1992, both of which heavily damaged thousands of buildings and the latter of which killed more than 560 people. The pyramid experienced scant damage.
It is part of a large complex, alongside other pyramids and the Great Sphinx of Giza - all drawing throngs of visitors since ancient times.
"The Great Pyramid is not only an extraordinary engineering achievement but also a profound work of art and human vision. Its perfect symmetry, monumental scale and elegant proportions create a timeless beauty that continues to inspire awe even after 4,600 years," ElGabry said.
"Beyond its physical beauty, what impresses me most is the incredible project management and organizational mastery it represents. Building such a monument took approximately 20 years and required sustaining a clear, long-term vision, an extremely complex supply chain and the coordination of tens of thousands of skilled workers, engineers, and administrators," ElGabry said.
This would have included managing human resources, training specialized labor forces, ensuring a continuous food supply for the workers, and logistics for massive quantities of stone.
"It reminds us what human civilization is capable of when vision, science, organization and determination come together," ElGabry said.
"They really did," Salama said, "build 'one for the ages'."
(Reporting by Will Dunham in Washington; Editing by Daniel Wallis)
