Egypt's Great Pyramid of Giza, also known as the Great Pyramid of Khufu, has stood for more than 4,600 years, even through powerful earthquakes. Now, new research explains why: The structure is remarkably resilient to vibrations.
The pyramid has lost only about 33 feet (10 meters) of height since its construction during Egypt's Old Kingdom (2649 to 2150 B.C.). That's despite experiencing strong earthquakes over the millennia, including one with an estimated magnitude of 6.8 that struck south of Cairo in the city of Fayum in 1847 and a temblor with a magnitude of 5.9 in 1992 that knocked some of the topmost stones to the ground.
The new study of vibrations throughout the pyramid's structure suggests that certain architectural features, such as a series of chambers known as the pressure-relieving chambers above the chamber where the pharaoh Khufu once rested, dampen seismic movement toward the top of the structure.
"[T]he study highlights the extraordinary practical engineering knowledge of ancient Egyptian builders, who developed highly effective construction practices through centuries of experimentation and refinement," study co-author Asem Salama, a geoscientist at the National Research Institute of Astronomy and Geophysics in Cairo, wrote Live Science in an email.
Salama and his team placed vibration sensors in 37 sites in and around the Great Pyramid and recorded ambient vibrations when no tourists were inside the pyramid. "In heritage conservation, this information can provide important insights into structural stability, hidden vulnerabilities, and long-term preservation strategies while fully respecting the integrity of the monument," Salama explained.
The team found that throughout the pyramid, vibrations were remarkably similar, ranging from about 2.0 to 2.6 hertz. This was quite different from the frequencies on the ground nearby, which were typically around 0.6 hertz. This gap in frequencies means that during earthquakes, the pyramid sits apart from the vibrations traveling through the ground, perhaps contributing to its resilience, the researchers reported Thursday (May 21) in the journal Scientific Reports.
Other features contributing to the pyramid's stability include its massive base, strong limestone foundation and symmetrical geometry, Salama said. The vibrations do increase toward the top of the structure, which is typical in most buildings (imagine a skyscraper built to sway in an earthquake), but this pattern is interrupted by the pressure-relieving chambers that sit about 200 feet (61 m) high inside the pyramid. These chambers lie over the King's Chamber and are thought to have been built to take some of the weight off the pharaoh's final resting place. They also, it turns out, dampen vibrations that otherwise might travel toward the structure's apex.
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The researchers plan to take more measurements at the Great Pyramid and hope to use similar methods at other important Egyptian archaeological sites. It's likely that some of the same features of the pyramidal shape help protect the other pyramids at Giza, but each structure is likely unique, Salama said, as ancient Egyptian architects evolved their methods over time.
"Earlier pyramids," he said, "show evidence of experimentation and structural evolution, including changes in slope geometry and internal layouts."
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