Stabilising the Leaning Tower of Pisa

The Leaning Tower of Pisa doesn’t just lean, it is curved. Built over two centuries (1173–1372) in three phases, it began to lean shortly after construction began, and as stories were added, they were built to be vertical with respect to the tilt at the time. By 1990, the lean had reached 5.5° and the tower was believed at risk of collapse and closed to the public. Starting in 1993, 870 tonnes of lead counterweights were installed to counter the force causing the tilt, and a project of excavation was begun under the high side of the foundation, eventually removing a total of 38 cubic metres (70 tonnes) of soil. This reduced the tilt of the tower to 3.97 degrees. The tower was declared safe and stable for another three centuries, and was re-opened to the public in December 2001. In May, 2008, after further work to strengthen the foundations, engineers reported the tower’s movement had ceased completely for the first time since its construction.