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Soil extraction method

Pisa; Leaning tower

1. Steel tubes, 225mm in diameter, are installed raking into the ground north of the tower. In the preliminary work, 12 tubes were installed at a 30 degree angle at 500mm centres. For the full soil extraction, 41 tubes will be installed at a shallower, 20 degree angle. This shallower angle avoids the risk of penetrating a clay strata below the tower.

2. Each tube stops just before passing below the tower's foundations, at a depth of about 6m.

3. Augers are inserted into each tube and in a preplanned sequence, are driven into the soil beyond the casing tubes. Once at the desired depth, the augers are withdrawn to leave a void.

4. A probe is placed in the void to monitor the closure of the cavity formed. The tower's response on the surface is observed and the results used to determine future extraction sequence.

5. During the preliminary trial the same auger was used in each tube to remove soil. However, to improve efficiency, in the full soil extraction process, each tube will have its own dedicated auger

6. By extracting first from tubes at the centre of the tower and then from tubes cutting across the edge of the footprint, a so-called 'subsidence cradle' is created which enables the movement of the tower to be 'steered' east and west as well as north.

7. It is estimated that to achieve the desired 500mm of movement at the top of the tower, around 50m3 of soil will have to be removed. The Commission hopes to have the tower in its desired location ready to take visitors by 17 June 2001, the festival of San Ranieri, Pisa's patron saint.

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