The Leaning Tower Of Pisa — Architectural Marvel Or Engineering Failure?
Just another sunny day in Tuscany, Italy when..
Tourist Guide(TG): Do you know what makes the Tower of Pisa lean?
Tourist 1(T1): It doesn’t eat much?
T1 : Ugh, well how’s the security arrangements around the tower?
TG : They’re pretty leanient. :P
And yes, this is how an unintended tilt due to an engineering fault led to so many unintended and intended puns later on.
The Leaning Tower of Pisa is a magnificent piece of Romanesque Architecture.The tower gained much publicity due to its southward incline of roughly 5.5 degrees. But restoration officials and engineers did not want the tower to meet the same fate as the tower in Pavia. Without the tilt, this tower would lose most of its tourists and hence even the officials did not dare to completely remove the tilt, but instead reduced it to 3 degrees.
Originally named Torre di Pisa based on its location in the central Italian province of Pisa — the heart of the Italian Renaissance, this bell tower is one of UNESCO’s world heritage sites, thanks to the inadequate foundation (which was only 3 meters thick) and a bad choice of sub-soil (really soft soil).
Getting familiar with the jargon..
The portion of the building/structure below the ground that is not visible to us is called the FOUNDATION and the portion above the ground that’s visible to everyone is called the SUPERSTRUCTURE. The foundation is often neglected, but in reality it is as significant and dangerous as the underside of an ice berg.
The foundation of the Tower of Pisa was built on soils having fractions of sand, clay and silts in proportions that result in a poor bearing capacity because of the sensitive and compressible nature of the resulting sub-soil.This layer is not strong enough to support the huge load coming from such a heavy structure.
Due to weather and precipitation fluctuations, the water table rises up and down. It has been observed that the water level rises more on the northern side leading to more settlement on the southern side.
Now, you might be a bit confused seeing these alien words like Settlement, Bearing Capacity and Sensitivity. Let me break them down for you.
Settlement is the response of the soil (movement in the downward direction) when vertical loads are transferred to it. Settlements under foundations maybe of two types-
- Uniform — When the whole structure settles uniformly to more or less the same depth.
- Differential — When different parts of the structure settle to different depths.
Thus, differential settlement results in an angular shift of the structure as is apparent in this case.
Settlement can be tolerated to some extent depending on the circumstances but differential settlement is the culprit in this case. The water table shifts northward and the water seeps into the clayey layer, thus strengthening the layer on the northern side.But once the water dries off, the soil loses its strength and settlement occurs on the weaker section.
Due to the stage wise and intermittent construction, the soil was subjected to various cycles of loading and unloading, leading to gain of strength with time (thixotropy) and loss of strength due to cyclic loads (sensitivity). Had the construction been completed all at once, it must have collapsed as there wouldn’t have been any time for the soil to regain any lost strength.
STABILISATION EFFORTS:
Now having a clearer insight to the reasons, geotechnical engineers started the stabilization works.
600 tons of weights were applied to the north side of the tower to temporarily stabilize it.
Soil was excavated from below the Northern side of the tower (opposite to the tilt), so that it fell to the same level as the Southern end. This was made feasible with the help of diagonal borings as shown.
Thus, the tower did keep leaning and was no longer in danger of toppling over.
So, with both the parties satisfied, it is a happy ending after all.
This article was originally written by Shradha Panda for the original Pragyan blog.
