ENGINEERING INSIGHTS | How Do You Save a Tower That Won't Stop Leaning?

The goal wasn't to make it straight. It was to stop it falling over.

And part of the solution was to remove the ground beneath it.

For centuries, the Leaning Tower of Pisa has been admired for the very thing that threatened its survival. Behind the familiar tourist photographs is an extraordinary engineering challenge: how do you stabilise a leaning building without causing the collapse you are trying to prevent?

Construction began in 1173, on weak, compressible ground. The tower began leaning before it was finished. Remarkably, long interruptions in construction helped it survive, giving the underlying clay time to consolidate and gain strength before more weight was added.

By 1990, the situation had become critical. The tower was closed to visitors, and an international committee was brought together to safeguard its future. Disturbing the ground beneath its lower, southern side carried an unacceptable risk. Even conventional approaches such as underpinning or grouting could have made the situation worse.

So engineers worked beneath the higher side instead.

Through a process called underexcavation, they carefully removed small quantities of soil beneath the northern side of the foundation. This allowed that side to settle, gently rotating the tower back towards a safer position. The work took place between 1999 and 2001, following extensive analysis and testing.

Imagine trying to adjust a 14,500 tonne masonry tower by controlling how the ground beneath it settles.

Every extraction depended on how the tower had responded to the previous work. Engineers reviewed measurements of settlement and inclination, then issued instructions for the next operation. Defined trigger levels told the team when movement required intervention.

The soil removal reduced the lean by approximately half a degree. Groundwater control also played an important role, with drainage introduced to address seasonal changes that affected the tower's movement.

The tower kept the lean that made it famous, while gaining a safer future.

For anyone working with existing buildings, there is a powerful lesson here. Before deciding what to change, we need to understand what is moving, why it is moving, and how the whole system will respond when we intervene.

At Pisa, the engineering achievement was knowing how much correction was enough, and controlling every step needed to get there.

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