As specialists in sinking slab repair, we hear plenty of jokes about traveling to Italy and saving the Leaning Tower of Pisa. As the thinking goes, a slab jacking technique that saves a concrete slab should also work on the tower. But in reality, there is a lot more to it than that.
Known formally as Torre di Pisa (the Tower of Pisa) the region’s most famous piece of architecture has been leaning for hundreds of years. At its worst, the angle was about 5.5 degrees. However, stabilization efforts have reduced it to just about 4 degrees.
How was the lean reduced? Through several methods, none of which was slab jacking. We suspect that slab jacking was not utilized due to the nature and condition of the underlying soil. Keep reading to learn more about that.
200 Years of Construction
Many people do not realize that the Tower of Pisa took nearly 200 years to complete. Construction began in 1173 and was not completed until 1372. You might also not know that the leaning did not start after the structure was complete. It actually started just six years after construction began.
Builders had barely begun work on the seventh floor when they noticed the tower sinking on one side. A subsequent investigation revealed the soil underneath was too unstable. With a foundation of just 3 meters and heavy marble being the primary construction material, the tower was too heavy for the soil.
Constructed was halted for almost 100 years in order to allow the soil to settle. Some believe the tower would have eventually toppled had engineers not taken a break in construction.
Fears of Eventual Collapse
Construction was eventually completed in two more phases. The tower was opened to the public and soon became the centerpiece of the region. Over the next several hundred years, it was used for everything from scientific experiments to religious services. It was also used for military operations.
During World War II, the Germans were suspected of using the tower as an observation point. But because it was so important to Pisa’s architectural history, Allied forces decided not to destroy it. Fast forward to the 1960s and engineers began expressing genuine concern that the tower would fall over.
Shoring It Up
This brings us to how the tower was ultimately saved. Engineers knew they had to do something, or the tower would come down. They first tried attaching tons of counterweights to the high side in order to pull the tower back.
That effort produced limited results and the tower was closed to the public in early 1990. Its bells were also removed to take some of the weight off. Finally, engineers hit on the right solution: they decided to remove soil from the high end of the tower. This not only reduced the tilt to 4 degrees, but it also prevented the tower from leaning any further.
It turned out to be the correct solution for a curious reason: the soft soil is responsible for keeping the tower standing despite numerous earthquakes. The same problematic soil causing the tower to lean was also keeping it erect. This is why slab jacking was determined to be inappropriate. Engineers wanted to keep the soil as-is so as to not increase the chances of future earthquake damage.
In the end, removing soil accomplished the same thing as slab jacking, just in reverse. Rather than filling in voids underneath unstable soil, they created new voids on the other side, thus encouraging the structure to level itself. And now you know.