Tower of Pisa

Tower of Pisa in Pisa, Italy

Although the Leaning Tower of Pisa has been barely standing for over 800 years, recent threats to the tower's stability, demanding a mixture of old and new engineering, warrant its inclusion on this page. The tower has begun to lean so far that a computer model cannot replicate the real tower's actual position (5.5 degrees off perpendicular) because the model collapses at 5.44 degrees. Due to this drastic situation relatively new methods are being implemented at a fast pace; mainly removing portions of the earth under the tower's north end (the tower leans toward the south).

The diagram at left illustrates the stages in the tower's construction. Each stage represents an awareness of the tower's leaning, the most obvious being the opposite lean of the belfry. With each delay the structure's weight was able to compress the loose soil and clay which it sits upon: without delays the clay would have failed under the tower's load. The tower's lean is due to the 30 feet of dense river silts it sits on, with the silt layer more compressible on the south side. And while the tilt continues the increase the tower has ceased to sink; the clay now strong enough. Instead the tower is rotating, the north side moving up toward the surface.

John Burland, specialist in soil mechanics in London (responsible for the Jubilee Line extension and a parking garage under Big Ben), saw that the solution to the tower's tilting problem was not above ground, but below. Previous attempts focused on the tower itself: most prominently the strapping of 600 tons of lead weights to the north ground story. Burland concluded that soil extraction was the key to bringing the tilt back to the goal of five degrees. The many tests required would take a very long time so a temporary solution was proposed: "replace the lead weights with 10 anchors buried 180 fee underground, in the firmer sand below the Pancone clay." This procedure required freezing the ground the prevent ground water flooding the walkway at the ground story. Unfortunately this made matters worse as the tower lurched south due to the compression of the soil from the freezing (since water expands when it freezes gaps were created once the water melted). Adding 300 tons of lead weights temporarily halted the southward movement.

Burland and his colleagues were not very popular after that moment, but in 1998 plastic-sheathed tendons were wrapped around the first-floor loggia to prevent cracking and any southward movements from the soil removal to get under way. No problems were experienced with the tendons and the soil removal began in February of this year with almost immediate positive results. About 40 drill pipes dig at a 30 degree angle around the north side of the tower. Each pipe uses ancient engineering: an corkscrew-like tip (an auger) to channel dirt up through its blades. Ten tons of soil has been removed in the first four months, enough that the city's Mayor hopes to open the tower (closed for ten years) on June 17, 2001, the feast of San Ranieri, patron saint of Pisa.