Leaning Tower of Pisa Secure For 300 More Years

Ponca City, We Love You writes "The tower of Pisa began to lean five years after its construction began, in 1178, and by 1990 it had tilted more than four meters off its true vertical. Conservationists estimated that the entire 14,500-ton structure would collapse 'some time between 2030 and 2040.' Now the Leaning Tower of Pisa has been stabilized and declared safe for at least another three centuries. The stabilization, which cost $30M, was accomplished by anchoring it to cables and lead counterweights, while 70 tons of soil were removed from the side away from the lean, and cement was injected into the ground to relieve the pressure. The tilt has now returned to where it was in the early 19th century. Nicholas Shrady, author of Tilt: A Skewed History of the Tower of Pisa, says that the tower was destined to lean from the outset because it was built on 'what is essentially a former bog.' Shrady adds that the tower previously came close to collapsing in 1838, 1934, and 1995. (The commission convened in 1990 to study the tower's stability was the 17th such.) Although Galileo Galilei is said to have dropped cannon balls from the tower in a gravity experiment, Shrady says the myth is the result of 'the overripe imagination of Galileo's secretary and first biographer, Vincenzo Viviani.'"

Hmm.

[apodyopsis]

Of course if they straightened it totally it would be worse, because the top leans the other way slightly as the builders attempted to compensate.

Re:That's just wrong...

[thermian]

It couldn't be straightened anyway, it wasn't finished before it began to lean, so the upper levels were built to be level with the amount of tilt present at that time.

Re:70 tons?

[maxume]

The concrete is probably mostly serving to make the apparent size of the hard foundation larger, so that it is pushing down on a great deal more soil, thus pushing down on each bit of soil quite a bit less.

70 tons of soil is something like 70 cubic meters of soil(on the low end, that's at density of 2g/cc and assuming ton means 1000 kg (where it either should mean either 900 or 2000, I didn't read the article)), which is 'only' a pad that is 12 meters by 12 meters by 0.5 meters. If you go with 0.25 meters, you get something like 25 meters on a side. If you start by figuring that the soil that was there was barely strong enough to do the job, it isn't surprising that a bit of concrete goes a long way.

Re:Aye, the Europeans be fit

[Anonymous Coward]

Pretend like the moderation system doesn't exist and everything will take care of itself. People who pay too much attention to mods and their whims either don't last too long, take a serious karma hit for constantly complaining about the system, or end up like this guy http://slashdot.org/~RageTroll+9000/ [slashdot.org].

The end goal is more quality content (subjective), but plenty of individual unfairness happens too (also subjective).

If you fall into the demographic that enjoys slashdot, keep posting and pretend karma doesn't exist.

The degree of lean

[Huntr]

According to the article,

By 1990 it had tilted more than four metres off its true vertical

Then it says

The tilt has now returned to where it was in the early 19th century, with a lean out of true of 3.99 metres

According to Wikipedia [wikipedia.org], they moved it about 45 cm, meaning 45 cm is the difference between toppling in the next few decades vs the next few centuries.

Re:70 tons?

[Skagit]

The tower is built on alluvial silt, and that's pretty nasty stuff to build on. Modern techniques for such poor soils rely on very large and very stiff concrete mats, like Chicago's skyscrapers, or on piles driven to bedrock like at the beach. One of the temporary stabilizing measures they used for the tower was to stiffen the soil by pumping liquid nitrogen through pipes to freeze the groundwater in the silt to prevent it from subsiding more on the side of the tilt until they figured out a more permanent solution.

If you look at pictures of it (I guarantee, pick any geotechnical book and Pisa will either be the cover or in the first chapter), you'll notice a subtle banana shape. The builders over time knew it was tilting, so they started correcting as they were building.

Another fascinating thing about the tower is that the walls are built of rubble clad with marble facing. The rubble over time subsided, and now the entire weight of the tower is bearing on the thin marble. Some of the tilt-side masonry is under enormous stress, and the very fine joints keep most stress concentrations low.

The book cited in the the summary, Tilt, was an excellent history of Pisa, because the history of the city is completely entwined with the history of the tower. A very fine read, though the hardback book is cut at an angle, so the book, when shelved , tilts back into the shelves.

Re:Wow, just like Superman III!

[priegog]

Ah, most of us prefer to act as if the third (and fourth!) movie never actually happened.