The Enigma of the Leaning Tower of Pisa‘?’

Sumit Shokeen

CBP Government Engineering College GGS Indraprastha University, New Delhi.

Basic facts about the structure

• Height of the structure: 60 meters• Diameter of the foundation: 19.6

meters• Inclination of the tower: 5.5° to the

south• Estimated weight of the structure:

14,500 tons

Cross section through the tower

.

Soil Profile beneath the tower

Construction timeline• Construction work began on 9th August, 1173• About quarter construction was achieved by

1178.• There was then a halt in the construction for

unknown reasons, but had the construction continued any further, the foundation would have experienced a bearing capacity failure.

• The work recommenced in the year 1272.

.

• In year 1278, the tower’s construction was completed up-to the seventh cornice & then there was again a halt due to the military action.

• Next recommencement was in 1360 and the whole construction was finished in 1370.

• It took nearly 200 years to finish the construction with two intervals of nearly 100 years each which gave the soil to gain strength.

Unpleasant Surprise• In 1838 an architect, Allesandro

Della Gherardesca excavated a walk-way around the foundation of the tower known as Catino.

• It was intended to expose foundation steps and column plinths.

• As the excavation was under the water table, it caused an inflow of water which gave the tower an additional tilt of nearly half degree to the south.

History of Tilting• The commission for the stabilization was headed by Prof.

John B. Burland.• Due to the absence of any documentary evidence all the

clues for the history of tilt were to be inferred from the adjustments made to the masonry.

• He put himself in the place of a mason and asked himself, “What is the most practical thing to do when you arrive at a given floor and find that the tower is out-of-the plumb.”

• A widely accepted hypothesis is that the masons would always try to keep the masonry layers horizontal.

Shape of the axis of the tower deduced from relative inclination of masonry layers

Temporary stabilization of the tower

• There were two distinct problems that threatened the stability of the tower:

1. Strength of the masonry.2. Stability of foundation against overturning.

Temporary stabilization of the masonry

• On the south side of the tower, due to the change in the cross section the masonry blocks were stressed. The exact assessment of stresses was not possible but its effect would have been catastrophic.

• This problem was tackled by using plastic covered steel tendons which were bound around the first cornice upto the second floor which helped in reducing the crack sizes and prevented the tower to fail due to buckling.

Temporary stabilization of the masonry with light circumferential

pre-tensioning

Temporary Stabilization for the possible overturning failure

• This was achieved by placing the counter weight on the northern side of the tower is four phases so that the response of the tower could be recorded.

• Such solution would not have been considered if it had not been recognized that the leaning instability rather than the bearing capacity failure was controlling the behavior of the tower or the north side of the foundation was settling.

Counterweight installation to prevent the overturning of tower

Observed change of inclination of the tower during application of the

counter-weight

Choosing the suitable and Permanent Stabilization

• For bureaucratic and financial reasons the temporary stabilization took longer than expected.

• Because of the severe restraints that have been discussed already measures involving application of concentrated loads to the masonry or underpinning operations beneath the south side of the foundation were ruled out.

• Moreover aesthetic and conservation considerations required that the stabilization effect should have an absolute minimal appearance.

The three possible approaches for the permanent stabilization

After careful consideration of number of possible approaches the commission chose to study three in detail:• Construction of the ground pressing concrete slab to the

north of the tower which was to be connected with the post-tensioned concrete ring to be placed around the periphery of the foundation.

• Consolidation of the Pancone clayby means of carefully devised electro-osmosis.

• The technique of soil extraction which was at that time already in practice in Mexico to reduce differential settlement of number of buildings.

Approach #1: Ground pressing slab

• ✗ Numeric modeling of the ground pressing slab showed that the tower’s response was uncertain and if anyhow it gave positive results the results were not large enough and the settlements were huge & it required costly civil engineering to proceed with.

• RULED OUT.

Approach #2: Electro-osmosis

• ✗ Full scale model showed that the ground conditions at Pisa were not fit for this method.

• RULED OUT

Approach #3: Soil Extraction

• ✔ This method was best suited and was finally adopted for the final large scale work.

• In this method small amounts of soil from predefined locations are removed with the help of inclined drills.

• This leaves a cavity at the drilled location and which is gradually filled with the soil in immediate vicinity of the cavity due to the overburden pressure.

Schematic representation of the under-excavation process

Change in the North-South inclination after the permanent

stabilization

References• The Enigma of the Leaning Tower of Pisa, Prof. John

B. Burland, 1998• http://casehistories.geoengineer.org• http://home.eng.iastate.edu/