Case Study: The Leaning Tower of Pisa - Madrid Engineering Group

On August 9, 1173, construction of the Tower of Pisa begins, and it takes two centuries to complete. From the article:

"LEANING TOWER OF PISA-BRIEF CONSTRUCTION HISTORY

The Leaning Tower of Pisa is perhaps the most well-known architectural oddity in the world. The construction of the tower occurred in three phases, spanning nearly two centuries, as war and social unrest mired the construction. The evident tilt of the tower was first noticed during the initial phase of construction which began in 1173 AD. Engineers tried to compensate for the tilt by making the columns and arches of the third story slightly taller on the sinking side; however, political unrest halted construction before they could continue to the fourth story. Additional adjustments were attempted on the fifth and sixth stories, but construction was once again halted before they could continue to the seventh. Nearing the completion of the tower in 1372, the builders made a final attempt to compensate for the lean by angling the eighth (top) story bell chamber. Regardless of these attempts to correct for the lean, the tilt continued to worsen throughout the centuries. Careful monitoring, however, didn’t begin until 1911, when measurements revealed that the top of the tower was actually moving at a rate of around 0.05 inch a year. While the periods of unrest were no doubt a bane to the tower builders, it is very likely that had these construction respites not occurred and allowed time for the underlying soils to settle, the tower would have toppled well before completion.

LEANING TOWER OF PISA-THE CAUSE

The exact cause of the tilt was not fully understood until 2001, when a serious stabilization effort (which began in the 1990’s) was completed. It was known prior to the start of this stabilization effort that the tower had been built atop an inadequate foundation (which was only 3 meters thick); and was constructed on very soft silty soil. Had these been the only factors at work, uniform settlement of the tower could have been expected; and the city of Pisa would play host to a significantly less famous (albeit more vertical) tower. The 800 year old mystery was finally solved by John Burland, an English geotechnical engineer, who discovered that the primary cause of the tilt was a fluctuating water table which would perch higher on the tower’s north side, causing the tower’s characteristic slant to the south."