This month, two massive steel A-frames will be erected on anchor blocks some 100m to the north of Pisa's Leaning Tower. This is the first stage of the latest plan to stabilise the tower and form a semi-permanent solution to the problem of its lean.
Over the next few months, steel cables will link the frames to the tower, forming a safety system for the main works. These will involve using an innovative method of soil extraction to gently induce subsidence on the northern (up-tilt) side of the tower which it is hoped will reduce the inclination by about 10%.
Without its Leaning Tower, the future of tourism in the historic Italian city would be bleak. Its appeal is such that in 1989, the year before it was closed to the public, 700,000 visitors climbed to the top of the bell tower. Even today, with the immediate area cordoned off by security fencing and hoardings, tourists from across the globe still flock to see the apparently gravity defying structure.
Before 1993, the tower's average rate of increase of inclination was about 1.5mm per year. During 1993 and 1994, 600t of lead weights were placed on the north edge of the foundations, which reduced the lean by about 10mm. Since then the annual increase of inclination has almost ceased.
But this year, the tower is showing signs of returning to its previous annual pattern of behaviour. 'We have been expecting this,' says John Burland, professor of soil mechanics at Imperial College in London and a member of the commission deciding the tower's future.
But despite the immediate problem of saving the tower, its historical importance and commercial value has meant that any planned remediation is always subject to lengthy debate.
The most recent attempt to stabilise it caused some worry. In September 1995, while excavations for an anchored concrete ring around the tower's foundations were being carried out on the southern side, it suddenly lurched by 2.5mm overnight. Work had to stop and contractors were forced to place a further 230t of lead ingots on the existing 600t stack on the northern side to prevent any further movement.
The root of the problem lies in Pisa's geology. 'The whole area is underlain by soft clayey and sandy silts,' explains site director Paulo Heiniger. Underneath this layer lies a thicker layer of the highly compressible and normally consolidated Pancone Clay, between 10m and 25m depth, which in turn overlays denser sand at depth.
In the past it was generally believed that the clay was causing the continuing inclination, but it is now thought that the lengthy construction of the tower allowed the clay to compress, so it is the upper 5m to 6m of silt directly below the tower foundations, combined with fluctuations of the high water table, that are the main cause of the long term tilt.
In 1990, the 15th commission for deciding the tower's future was formed. Much of the work under the chairmanship of Michele Jamiolkowski, professor of geotechnical engineering at the University of Turin, has come in for heavy criticism, with detractors saying the temporary safety measures are unsightly. But even if not aesthetically pleasing, the stacks of lead ingots and the steel cables circling and protecting the most stressed masonry, have kept the tower standing, albeit at a dangerous angle of 5.5degrees.
The commission's latest attempt, which started at the end of the summer, has been used successfully in Mexico City where many historical buildings have been affected by differential settlement caused by earthquakes and subsidence. A trial of the method on a 7m diameter test footing near the tower has already shown that it should be suitable for the real thing.
The two steel A-frames will be erected on anchor blocks some 100m to the north of the tower, behind buildings flanking that side of the Campo dei Miracoli. The foundations of these blocks have already been installed, each consisting of 14 vertical and inclined 300mm diameter micro piles, 32m deep.
The A-frames will be linked to the tower by steel cables that will be attached to further cables wrapped around the southern side of the tower at the second level. 'These are only a safety measure,' confirms site director Dr Paulo Heiniger. They will not (as has been reported elsewhere) be used to pull the tower back as the soil is removed but will prevent movement should anything untoward happen. To this end, the load on the cables is adjustable, with two systems, a manual weight system and an hydraulic one. Each frame is capable of exerting a pull of 150t.
The 12 cables fitted around the southern side of the tower will be connected by steel plates to the main cables. Heiniger explains that to prevent damage to the tower's masonry, the stress will be spread by wooden plates fitted between the cables and the stone - 'there is no direct contact and so there will be no damage'. The cables were due to be fitted this month, with work on the main anchors and cables following.
Once all the cabling is finished, the main works can start. Twelve cased boreholes will be drilled at various locations underneath the northern edge of the foundations down to about 7m or 8m below ground level, a few metres below the foundation base. Small amounts of the soil will then be extracted using a method similar in concept to Italian geotechnical contractor Trevi's cased hollow stemmed flight auger piling system. As soil is removed, the cavity created will gently close, inducing settlement and should, with repeated extraction, cause the tower to rotate back northwards in a controlled manner.
While the commission has not set a limit on the amount of tilt reduction, work will be carried out for an initial three month period and
the commission will then decide whether the method is effective and if it should continue.
Soil extraction is due to begin in early January with the target of reducing the tilt by 10%, equivalent to around half a degree. This should result from removing a 0.25m thick layer of soil beneath the foundations.
This should, the commission hopes, ease the tower back to safety, reducing foundation instability and masonry stressing enough to extend the life of the tower by perhaps a couple of centuries. As with everything associated with the tower, only time will tell.