The LEANING TOWER of PISA
PHASES of RESTORATION
by Vittorio Novelli
1st PHASE: Anchorage of the Tower by means of Steel Cables
Introduction 1) Structure of anchorage: The plan foresees the construction, at about 90 m from the center of the Tower (North side in plan direction of the maximum inclination of the Tower), of a plinth of concrete reinforced for the anchorage of the cables having the following characteristics: - plan of support 12.00 m x 20.00 m opportunely consolidated with micropiling in reinforced concrete; - general base in reinforced concrete the same dimensions and with thickness of 2.00 m; - vertical walls and horizontal base of reinforced concrete, 80 cm thick, formed together, six large hollow cases 3.80x5.60x5.10 m., filled, as required, with inert material (ballast); - external buttress to anchors in reinforced concrete, 80 cm thick (31 inches), corresponding to the four vertical walls; - embedments at the top, extended from the vertical walls, will provide a place of support for the attachments for the tensioning of the cables. 2) Cables: 31 cables of braided steel will anchor to this plinth, type "Seale Warringon", 49 threads, 24 millimeters in diameter, that will wrap around the Tower at the five "orders" of rings, between the 2nd and the 6th, distributed as follows: 1st band (6th order): 11 cables 2nd band (5th order): 5 cables 3rd band (4th order): 5 cables 4th band (3rd order): 5 cables 5th band (2nd order): 5 cables For proper tension, a progressive reduction will be made in the number of the cables reaching up from the plinth to the Tower. The attachments, one for each cable termination, will be adequately anchored, in depth, to the ribs of the plinth.
Plan with location of the Plinth
3) Protective Shield: To reduce the combined effect on the Tower's masonry, caused by the pressures transmitted from the cables, appropriate metallic shields will be built at each order of rings, duly strengthened, and able to distribute as uniformly as possible the compressive action of each connecting rod over a broad area of the masonry of the particuar ring. They are constituted from double T steel beams (IPE type,10 cm) vertically disposed, each 30 cm, periodically opposed with edges in layers each 25 cm, all contained between two layers of adequate thickness. For the safeguard of the masonry and for the best distribution of the load transmitted from the cables, a pad of wood or synthetic material is scheduled between the layer of the shield and this masonry. 4) Amount of tension in the cables: The amount of tension in the cables is mediated by electronic control following calculations of absolute precision and adapability, to the instant, based on the variation of the inclination of the Tower. In the delicate first phase, however, during application of the tension, a gradual, progressive traction of the cables will be implemented, in order to guarantee, that the load on the material of the Tower does not incur danger for any order. Proceeding therefore from the tall lower level, the shield will be placed on he work, and the cables of the first level, and traction of about 10% of the final value will be imposed at this time. The work of laying the shield and then the cables of the next upper level will follow with the same formality and with an increase of the tension of the underlying cables. Only with the completion of the structure (shield and cables) will the cables be placed under full tension. Beginning from this moment and up to the finish of the work of settlement of the Tower the braced structure will act provisionally like a work absolutely autonomous and of sure security.
Protective Shield
2nd PHASE: Control of the Tower Corrections
1) Side Excavations: The organization of the work of gradual removal of soil includes the construction, in opposing excavations, of two caissons in reinforced concrete, about 30 m. long between their parallel sides, within which lodge the equipment and the necessary motors to remove the soil. The excavations will be performed in positions diametrically opposite relative to the circle of the foundation of the Tower, distant about 10 m. from its foundation, and parallel to the line of maximum inclination of the Monument. The caissons will be reinforced concrete, cast in place or prefabricated, and will contain a horizontal opening for the passage of the abrasive wire. In this casison, will set the platform on which the pulley for the slide of the wire will move . In the headings of the two caissons will rest the motors that will control the speed, and the positioning of the abrasive wire. 2) Removal of the soil: In order to satisfy the hypothesis of a reduction of inclination of the Tower of 2°, would require removal of a wedge of soil underlying the Tower's foundation of a maximum dimension in height of 67 cm (26 inches). This will happen by means of a cutter on a helical wire (or diamond wire) of the type used in the excavation of marble. Since the wire will always act at the same rates for to form "to wedge" of the removed earth, the time of contact of the abrasive wire will be regulated in the affected zone: the correct thickness will correspond, proportionally, to a correct time of contact. The removal of the wedge (in a gradual way and with constant control of the movements of the Tower) accompanied by the deliberate variations of intensity of the tension of the cables create a systematic, safe, straightening of the Tower up to the new preset inclination.
Scheme of Execution of the Cut - Section
The abrasive action will begin on the surface, at a due distance from the Tower, and will be such to immediately isolate a parallelopiped of soil, containing the foundation of the Tower, separated from the aforesaid initial cut and from the two parallel excavations of the caissons. This prism of earth support sinks softly from the continuous, gradual and regulated removal of the material and in the corresponding measure to the soil removed. It is, in fact, a mechanical, slow and inexorable action, controlled and regulated by the speed and position of the abrasive wire - action not great, but that can cause the desired effects. The excavated soil will be deposited on the sides of the caissons from where it is periodically removed. The job will cycle almost continuously, to reach, in the end, the desired result in the smallest possible time.
Scheme of Execution of the Cut- Transverse Section
3rd PHASE: Completion of the job
1) Amplification of the foundations: When the desired inclination is achieved, provisions will be made to widen and support the existing foundation. While maintaining the cables according to the scheduled traction plan based on calculations, an excavation of the soil around the foundations will be performed (circular in shape and with rectangular section) for a width of 4 m and a depth of 3 m, a precautionary measure to expose the existing stable masonries in order to create a perfect adhesion, and within which to build the new plinth of foundation. The work of consolidation consists of a trapeze structure in reinforced concrete, constituted from a circular shaped section, firmly anchored to the existing one (with which it will form a uniform body) by means of holes placed in the existing foundation that, armed opportunely with reinforcing, hook the new body to the existing. The new plinth will have a base of 4 m and a varying height from 3 m (corresponding to the plane of the existing foundation) to 1.5 m at the other extremity. To create the maximum adherence on the existing structure the new circular crown in reinforced concrete will be posttensioned by means of looped cables. For a better guarantee (and also for reasons of equilibrium for the effort of compression transmitted from the new structure to the existing circular crown of foundation) the central nucleus of the existing foundation will also be filled with reinforced concrete. 2) Work Completion: The dismantlement of the metallic structures (cables and protective shield), the demolition of the plinth of anchorage, the burial or the demolition of the two caissons and the restoration of the Piazza dei Miracoli in the part affected by the work, will complete the job.
Tower Section of Completed Work
Translation by Gary Feuerstein, 8 March 1998
Personal e-mail to:
gary@endex.com
