Riddle of the Leaning Tower

Programme Outline

The story of the latest engineering challenge to save the famous Leaning Tower of Pisa, and the long history of past engineers’ mistakes.

00.30—04.00
Overview of the recent history of the Leaning Tower of Pisa. The computer model that reveals that, according to conventional engineering wisdom, it should already have collapsed.

04.00—07.00
Collapse of another Italian bell tower during an earthquake. The furious debate among members of the expert committee charged with tackling the problems of Pisa’s Leaning Tower.

07.00—12.00
The structural engineering perspective — the flaws in the tower that seem to break all the rules of engineering. Computer modelling of the stresses in the tower reveals the critical point where a fracture could lead to total collapse. How the tradition of maintenance by simple replacement of blocks no longer offers a solution.

12.00—20.30
The search to discover what caused the tower to lean in the first place — and how staggering the building over two centuries allowed the tower to stabilise, which is why different levels of the tower have walls built at slightly different angles. The surprising fact that the tower originally leaned in the opposite direction!

20.30—26.30
Detailed monitoring of the tower reveals that it reacts to changes in the environment — particularly hot sun and heavy rain. A model of a tower spun in a huge centrifuge reveals the critical role of the alluvial soil (silt) beneath the tower.

26.30—34.30
Old and new schemes for solving the problem. The fact that the tower is no longer sinking but, due to the degree of lean, is now capsizing. A radical scheme to remove soil from beneath the northern side of the tower is rejected by the committee of experts, in spite of being shown to work in a large-scale test with a specially constructed tower.

34.30—42.30
The nineteenth-century trench dug around the base of the tower that caused the tower to lurch half a metre. A late twentieth-century scheme to anchor the base of the tower into the rock deep beneath leads to another, fortunately much smaller lurch — but the tower shifts as much in nine hours as it has moved in the previous 12 months.

42.30—end
The collapse of another bell tower in an earthquake galvanises the Pisa team, who now decide to give subsoil extraction a try after all. The first extractions cause a 6mm move towards the upright, a small start in the right direction. The aim is to reverse the last three years of lean — but it will take two years of drilling to get to the half-metre target.