Leaning Tower of Pisa: Behaviour after Stabilization Operations

Additional foundation stabilization worksIn addition to reducing the inclination of the Tower by half a degree, two other permanent foundation stabilisation measureshave been carried out. During the work of the Committee, a 0.8m thick cement-conglomerate ring was detected in thebottom of the catino around the base of the Tower. This ancient concrete ring is of high quality and it has now beenconnected to the masonry foundation of the Tower by means of stainless steel reinforcement and has been strengthened bycircumferential post-tensioning (Fig. 12). As a result, the effective area of the foundations has been substantially increasedthereby increasing the factor of safety against leaning instability.As mentioned previously, the deduced motion of the Tower foundation shown in Fig. 5 led to the conclusion that theseasonal fluctuating water table in Horizon A during intense periods of rain was the main factor responsible for thiscontinuing movement, see Fig. 6. It is also of significance that, apart from these intense periods of rain, the average groundwater levels close to the South side of the Tower in Horizon A are 200mm to 300mm higher than those to the North, asshown in Fig. 6. This difference generates a small, but not negligible stabilising moment for the Monument which is soclose to falling over. In the autumn and winter, when rainfall events are more intense, the water table rises sharply, reducesthe difference in piezometric level and thereby produces southward rotations of the Tower which are not fully recoverable.To minimise this effect it was necessary to eliminate the fluctuations of the water table and, with this objective, a drainagesystem was installed consisting of three wells sunk on the north side with radial sub-horizontal drains running into themfrom beneath the north side of the catino, (Fig. 13). The water levels in the wells are controlled by the level of the outletpipes. The drainage system was implemented in April and May 2002 and lead to a decrease in the pore water pressure aswell as a significant reduction in its seasonal fluctuations, as shown in Fig. 14. The installation of this drainage systeminduced a further northward rotation of the Tower as can be seen in Fig. 15.Figure 12. Structural connection of concrete ring to the Tower foundation.International Journal of Geoengineering Case Histories ©, Vol. 1, Issue 3, p. 164http://casehistories.geoengineer.org