Design and material choices help the nearly 5,000-year-old monument resist shaking
Egypt’s Great Pyramid of Giza has resisted earthquake damage for nearly 5,000 years, and scientists finally know why.
The pyramid tends to vibrate at a different frequency in relation to the surrounding ground, which avoids excessive shaking during an earthquake. Combined with the pyramid’s robust shape and mass-distributed internal design, this difference helped keep the structure intact and stable, the researchers wrote May 21 in Scientific reports.
“It’s not surprising that they are very resistant to earthquakes,” given that the pyramids stood for so long, says Sherif El-Tawil, a civil engineer at the University of Michigan in Ann Arbor who was not involved in the research. But the new study offers important insights into why the pyramids are so resistant to seismic damage, he says.
THE Great Pyramidthe last of the Seven Wonders of the World still standing, was built at Giza around 2600 BC as the tomb of Pharaoh Khufu. It contains approximately 2.3 million stone blocks and took more than two decades to build.
Although Egypt generally has low seismic activity, it occasionally experiences strong earthquakes, including a magnitude 6.8 in 1847 and a magnitude 5.8 in 1992. Despite these powerful tremors, the Great Pyramid suffered only minimal damage.
To discover the origins of this lasting stability, geophysicist Mohamed ElGabry and his colleagues monitored subtle vibrations at 37 points in and around the pyramid. Intentionally shaking the pyramid could harm it, so the team relied on tiny disturbances caused by distant ocean waves, traffic or other human activities to vibrate the structure.
At about three-quarters of the measurement sites inside the pyramid, the structure naturally vibrated back and forth at frequencies between 2 and 2.6 times per second. The narrow frequency distribution indicates that stress is distributed evenly throughout the pyramid. The surrounding ground, meanwhile, vibrated more slowly, oscillating a little more than once every two seconds.
Because the natural frequencies of building materials and the ground are different, they are less likely to experience resonance, in which the pyramid absorbs energy from the ground’s vibrations. Resonance would significantly increase the strength of vibrations – but not necessarily their frequency – and the risk of damage during an earthquake.
Spaces inside the pyramid also helped to mitigate the shaking. Ancient Egyptian builders built pressure relief chambers above the king’s chamber, where the pharaoh was buried, to distribute the weight of the pyramid and protect the burial chamber in the event of collapse. These chambers also decreased the strength of vibrations closer to the top of the pyramid, the team found.
Most buildings behave like inverted pendulums, says ElGabry, of the Egyptian National Research Institute of Astronomy and Geophysics in Cairo. Their bases are anchored to the ground, but the tops have more room to swing and shake. In the Great Pyramid, this behavior meant that vibrations were amplified by a factor of four in the king’s chamber, located well above the bedrock and near the center of the pyramid. But in the decompression chambers above the king’s chamber, the vibrations were only amplified threefold. It is not yet clear which room design factors contribute to this effect.
“It’s all really amazing to see from today’s engineering perspective,” ElGabry says. “But it’s more amazing and impressive when you look at the tools and available resources we had 4,600 years ago. [ago].” However, the results cannot confirm whether the ancient Egyptians intentionally designed the pyramids with earthquakes in mind, the researchers write in the study.
Modern builders can use similar strategies to plan projects, choose materials and create durable structures, says ElGabry. “When we design our buildings, we design for 100 years [or] for 500 years,” he says. To apply the principles that make a building last much longer than that, “it’s important to understand how that building survived.”