How the Leaning Tower of Pisa survived earthquakes decoded

The survival of the Tower can be attributed to a phenomenon known as dynamic soil-structure interaction.

The soft foundational soil beneath the Leaning Tower of Pisa in Italy has helped it stand through numerous strong earthquakes since 1280, say scientists who unveiled the mystery behind how the structure remained undamaged, despite leaning at a precarious five-degree angle.

The results from the study will be formally announced at the 16th European Conference in Earthquake Engineering taking place in Greece next month.

Given the vulnerability of the structure, which barely manages to stand vertically, it was expected to sustain serious damage or even collapse because of moderate seismic activity. However, this has not happened in spite of at least four strong earthquakes that have hit the region since 1280.

Soil structure studies

After studying available seismological, geotechnical and structural information, the research team, a group of 16 engineers including those from University of Bristol in the UK concluded that the survival of the Tower can be attributed to a phenomenon known as dynamic soil-structure interaction (DSSI).

The considerable height and stiffness of the Tower combined with the softness of the foundation soil, causes the vibrational characteristics of the structure to be modified substantially, in such a way that the Tower does not resonate with earthquake ground motion. This has been the key to its survival. The unique combination of these characteristics gives the Tower of Pisa the world record in DSSI effects.

“Ironically, the very same soil that caused the leaning instability and brought the Tower to the verge of collapse, can be credited for helping it survive these seismic events,” said George Mylonakis, a professor at the University of Bristol.