Both due to the slope and due to the original architecture, from 1173 to the present day, the tower is the object of close attention. Efforts are constantly being made to make the tower more stable. For example, collapsing columns were replaced several times. Nowadays, mostly underground work is being carried out to strengthen the foundation.
Under the southern part of the Leaning Tower of Pisa, the soil is more muddy and clayey than under the northern one, which is why it began to tilt as soon as they began to build it. It would be very difficult to keep the tower from falling with the help of external supports, and, moreover, any frame would inevitably spoil the view of the monument. Nevertheless, the fall of the Leaning Tower of Pisa was stopped, and moreover, it was slightly straightened, reducing the slope by half a degree. All of this is the result of unique work carried out under and around the bell tower in the 1990s.
As a temporary measure, on the north side (let us recall, the tower falls to the south), a block of lead blocks was installed on concrete beams, which, acting as a counterweight, stabilized the structure. Then, the nonlinear processes taking place in the soil under the tower foundation were investigated, after which a series of experiments followed with a concrete model foundation specially erected not far from the monument. Experiments have confirmed that the tower can be straightened and stabilized a little if you select a piece of hard soil from under the northern part of the foundation, that is, carry out the so-called sidework. It must be understood that the part-time job had nothing to do with ordinary earthworks – the soil was selected literally millimeter-by-millimeter through a system of casing pipes, inside which a rotating auger drill was placed.
The calculations turned out to be correct, and the Leaning Tower of Pisa, having deviated slightly to the north, sagged a few centimeters and stabilized, which made it possible to remove both lead counterweights and temporary supports placed for safety.
Once again, on December 15, 2001, the tower was opened to tourists.
In 2008, scientists once again announced that the further fall of the Leaning Tower of Pisa had been stopped. Prior to this, despite numerous efforts to “freeze”, the tower continued to deviate from the vertical at a rate of 1 mm per year.
In 2002-2010. the restoration of the tower was carried out, as a result of which its angle of inclination was reduced from 5 ° 30 ‘to 3 ° 54’.
Guided tours in Pisa are here https://excurzilla.com/en/italy/pisa