Sinking feeling could save Pisa

The tower of Pisa tilts more precariously each year. But this gradual toppling will be stopped in its tracks if the latest engineering scheme is successful.

The tower tilts to the south and the plan is to drill at a shallow incline alongside and beneath its northern foundations and remove small cores of soil. The tower should settle back into these cavities, collecting the lean by about 10 per cent. Engineers I hope this will be enough to stabilise the structure and prevent future tilting.

"A 10 per cent reduction in lean is about half a degree, says John Burland, head of soil mechanics at Imperial, College and London, and a member of the commission charged with protecting the tower. «It's quite a big step, but it should stabilise the masonry and the ground».

The commission opted explore the subsidence scheme at a meeting last month. Since 1990, this international team of specialist has already implemented several plans to temporarily improve

stability.

First they fitted the second storey of the tower with a corset of steel tendons to stabilise the masonry and prevent the tower collapsing its own weight. They then hung 600 tonnes of lead from the base of the tower on the north side, which gradually pulled the structure upright by 50 arc seconds and reduced its tendency to tilt by 14 per cent.

The next plan was surround the foundations with a concrete ring beam and anchor this collar in place with cables running deep into the ground. "This would maintain the status quo without the lead weights," says Burland. He estimates that it could stabilise the tower for 20 years.

During construction of the ring beam the surrounding ground had to be frozen to prevent water leaking into the excavations. It was vital that this process did not extend to the soil beneath the tower itself-wet clay expands as it freezes, so it could destabilise the monument.

But no one realised that the towers foundations were "tied" by a set of steel pipes to the walkway, or catino, that surrounds, reduced its tendency to tilt by 14 per cent, its base. As the ground beneath the catino froze, thawed and shifted, so did the tower. Work had to be stopped and more weights added to the foundations to arrest the movement. This brought the total weight of lead to over 750 tonnes. Since work stopped in September, the tower has tilted a total of 11 arc seconds before stabilising.

Burland says that adding more leads unlikely to be an option if anything else goes wrong. "We can't just keep on increasing the weight," he says. So the commission will first anchor the tower to the north by running two steel cables from half way up, down to the ground some distance away.

Burland says that test drilling will probably start in two to three months. "We'll begin with a very gentle incremental operation using six to twelve holes, " he says. The tower should sink into these holes and straighten up by around 20 arc seconds.

"You can never be 100 per cent sure of success until you try with an operation of this kind," warns Burland. Other engineers are therefore continuing to design a revised version of the ring beam and anchor solution, in case the test drilling fails.

If the subsidence scheme works, the next step will be to tackle the fluctuating water table beneath the tower. According to Burland, the water level can rise by up to 0-5 metres in one day, especially during the rainy autumn. 'The tower's movement also accelerates in the autumn," he says. He thinks they may be able to solve the problem by filling a trench around the tower with impermeable material. The enclosed area would be equipped with a system to pump out excess water. Anyone concerned about the impact that a straight tower could have on the tourist industry can rest easy. The tower began to lean during the 200 years it took to build, and various attempts were made to compensate. The resulting banana-shaped building can never stand truly upright.

Дата добавления: 2015-11-14