to SAVE the TOWER of PISA
1996
by Vittorio Novelli
INTRODUCTION
In these pages, in extreme simplification, is contained the "problem" of the Tower of Pisa.
In the 13th Century, when the Tower of Pisa was not yet completed, a lean was detected in the construction caused by unstable soil underlying the foundations. Today (1996) the Tower leans about 470 cm., still attributable to the slow, but unrelenting, differential yielding of the ground through the centuries. The speed with which the monument inclines is increasing, but, most importantly, the actual magnitude of the lean (by the limits of static stability), make an effective and substantial intervention on the work absolutely unavoidable. An intervention, thus, that has three fundamental characteristics:
1) It must be immediately effective, in terms of safeguarding the existing structure;
2) It must reduce the inclination of the Tower in a sensitive way;
3) It must annul, or at least slow down, the speed with which the Tower is increasing its inclination.
These three characteristics correspond, in their respective order, with the three phases that constitute the intervention plan known as "OPERATION TOWER."
RESULT
The final goal of intervention is to return the Tower of Pisa to the city and to the whole world, as it has been up to few years ago: visible and enjoyable from the outside as well as from the inside, without restriction in any way - the nature, the location and the aesthetics; all in conditions of absolute safety for today and, above all, for tomorrow.
In the technical plan, the objective of the intervention is to reduce the lean of the Tower in a progressive and monitored manner, operating from the first criteria under conditions of relative safety, finally realizing the widening of the base of support of the Tower to result in optimal stability.
The succession of the three phases, the exact operational formality, and the criterions of plan, are synthetically described in the following pages.
FIRST PHASE
Consists of the anchorage of the Tower, by means of an adequate number of cables of steel, to a provisional structure, constructed as necessary, between the wall of the Piazza and the L'Opera Primaziale, about 90 m. away from the Tower, opposite to its inclination.
The distribution of the cables and the value of the tension attributed to each (by means of hydraulic cylinders under electronic control) will be of the following from:
- confirm the bearing contact on the ground along the circumference of base of the foundation; and
- minimize the tangential effects between the different foundations.
In other words, the cables will artificially induce in the Tower a static inside condition approximately similar to what the same would have in conditions of perfect verticality. This static condition corresponds, obviously, to the situation of maximum safety for the integrity of the Tower and for the safety of whoever will operate the following interventions.
The provisional structure of anchorage does not constitute a problem to the safety of the Tower. The anchorage is a proposed construction of reinforced concrete partially buried, integrated with pile works of adequate depth, whose distance from the Tower is such to exclude any unwanted interference with the static safety during the action of the cables.
The problem of the contact between the cables and the delicate material of which the Tower is constructed are easily resolvable by means of the use of appropriate protective shields.
SECOND PHASE
Consists of the controlled straightening of the Tower using an impulse press that partially compensates the existing inclination, achieved by the removal, slowly and progressively, of a wedge of soil underlying the Tower foundation.
Among the possible techniques of removal of the ground, one considered most suitable is by means of the abrasive action of a diamond coated wire. The wires run in two parallel and symmetrical trenches in plan and rotate on a vertical axle at an appropriate distance from the Tower. The carts that will drive the wires will travel along the trenches in this abrasive action, performed always on the same horizontal plane, relative to the ground underlying the foundation of the Tower.
Without entering the technical details of the action, it is emphasized that the effect of the wire is based on two actions: proper removal of the bearing soil and transport of the soil to the outside using mechanical adhesion of the soil to the wire.
Though the actions are generally simultaneous, it is possible to tailor the soil removal, by the following parameters:
- the type of the wire; - the methods of slide of the wire as regards the ground; - the placement of the whole system of cut.
At equilibrium between the two actions, the entity of the material locally removed, and therefore of the yielding of the ground, will be directly proportional to the number of passes of the wire in the subsoil: this value will be null corresponding to the central axis of the Tower, and it will increase progressively along the burrowing away from the center, increasing in direction opposite to that of the lean.
During each phase of yielding of the Tower, a rigorous system of control of the movement of the work will be operational and, more in general, of the arrangement of the neighboring ground. There will be, in fact, answers in real time in order to pilot the course of the operations.
The same information will also allow for adjustment of the simultaneous traction applied to the cables of the anchorage installed in the course of the first phase. In this way, despite the progressive evolution, the results will not alter the stresses on the monument, neither, above all, the general safety conditions of the corrective action. There is considerable difficulty in the prediction of the effects of the operations of this second phase: both on the amount and speed of influence on the rotation of the Tower. From evaluations of all theoretical results, a reasonable proposal of speed of rotation is not more than 1 degree in six months, to allow progressive arrangement of the work. In regards to the total amount of rotation, we maintain that it must be contained between 1 and 2 degrees, postponing however to a later time the selection the exact value.
THIRD PHASE
Having reached the desired inclination, before removal of the anchorage cables, the consolidation of the Tower by means of amplification of the existing foundation will be undertaken. This intervention fixes the best position at which the Tower, still firmly anchored, has been brought back, a value of inclination smaller than in any time of many centuries.
The operation consists of filling in concrete of the central nucleus of the actual foundation, following construction of an external circular crown made of post-tensioned concrete, firmly joined to the existing structure through attachment of reinforcement.
In this phase will be effected all the usual technical details to make the new foundation immediately active, insuring in each step that, as the cables of anchorage are taken off, the maximum guarantee of effectiveness has been made as the local effect of the intervention of the partially unloaded zone is taken from the foundation to perform normally once more.
The removal of the same cables will therefore be immediately possible so that ulterior action of the ground, in opposition to the rotation, will resist the speed of future inclination, with consequent probable zero resettling, or however drastic a reduction.
After verification of the effectiveness of the general performed intervention, the provisional structure of anchorage of the cables will be demolished.
OTHER CONSIDERATIONS
Based on other qualified plans, the fundamental characteristic of the Plan of Intervention that we propose is its conceptual simplicity. There is no sophisticated calculation, no futuristic method, but the practical application of elementary notions of Physics and of Construction Techniques.
We certainly do not here consider the absolute and mathematical safety of the result, being impossible to draw such an experience in subject that nobody as yet has undertaken. Of this, however, we are certain: we do not recognize ourselves, neither we will ever, in the position of those who have defined the Perfect Plan. We will objectively continue to collaborate in order that the problem of the rescue of the Tower of Pisa runs the simplest course, even if less pleasant, to the solution.
AUTHOR'S STATEMENT
The Surveyor Vittorio Novelli of Cesena, researcher and inventor of new technical solutions for housebuilding, particularly for reinforced concrete, already titular of patents in material n 1056566, n 1071013, n 1204146; wanting to offer a contribution for the solution of the matter of the Tower of Pisa, after having considered the solution attentively, having made the various inspections necessary to acquire the data for a concrete and valid hypothesis of this definitive setup.
The undersigned has elaborated a plan of work, that to his notice appears fit and effective for a definitive strengthening of the Tower of Pisa, with notable reduction of the inclination.
For illustration of the operational plan there follows technical descriptions which indicate the essential procedures to guarantee definitive stability of the Tower with appreciable economy of costs, and would also allow during the execution, the free and sure access of visitors to the monumental works.
Information provided courtesy of Isaia Caporali at http://www.torredipisa.com/
Translation by Gary Feuerstein, 22 February 1998.
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