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LEANING TOWER OF PISA
PRESERVATION PLANS AND THEIR EVALUATION
The Leaning Tower of Pisa, an 800-year-old stone structure, was
on the verge of collapse in 1990. Since then, the tower has been fenced
off from tourists and construction equipment has surrounded the tower.
The combined efforts of Engineers, Scientists, and Architects have reduced
the danger of collapse. Today those put in charge of the tower are trying
to decide on a plan that would keep the tower preserved for years to come.
This report will evaluate the preservation plans to find which is most
reasonable based on appearance, cost, feasibility, and long-term results.
I. Introduction
VI. Appendix VII. GlossaryVIII. References I. Introduction Every year, millions of people from around the world travel to Pisa, Italy, to see the famous Leaning Tower. The tower is an old stone structure built in the 12th centur. Over the years the tall, thin tower has leaned more and more to the south. The further it leaned, the more recognition it received. This report tells of the history of this tower?s lean in Cause of the Lean. Today the tower is considered by many to be one of the Seven Wonders of the World [?The Seven,? 90]. It is one of the few structures standing with Pisan-Romanesque architecture, an artistic way of building that made gorgeous the cities of northern Italy. The entire Piazza dei Miracoli* (Miracle Square), where the tower stands, was built with the unique architecture. Also in the square is a Cathedral and Baptistery. The leaning tower was actually built to be a campanile, or bell-tower, to the great Cathedral, though its bells no longer ring. Legend has it that Galileo learned heavy and light objects fall at the same rate by dropping both simultaneously from atop the bell tower [Nova, 99]. From all this, the importance of the bell tower is apparent. In 1989, a similarly constructed bell tower at Pivia collapsed, killing three people. This caused fear about the safety of the leaning tower in Pisa. That year the Italian Prime Minister brought together a special commission to assess the condition of the tower, and prevent it from collapsing. In 1990, the commission fenced off the tower from tourists and began their study. The dangerous condition of the tower is explained in the sub-section in this report labeled Stress Due to the Lean. The commission is commonly referred to as the Save-the-Tower Committee, or the Pisa Commission (as it will be called in this report). It is composed of Italian and foreign experts in the fields of structural engineering, geotechnical engineering, and history restoration of monuments [Heiniger, 95]. In the last ten years the Pisa Commission has saved the tower from its dangerous condition by implementing several stabilizing techniques. In order to understand the condition the tower is in, it is necessary to know what the Pisa Commission has done to the tower since 1990. The sub-section of this report called Work Done on the Tower covers this topic. Currently the tower is considered stable, but if the commission were to leave the tower alone, it would inevitably lean until it is once again in danger of collapse. Therefore a number of preservation plans have been proposed. Although such plans may be costly to the Italian people, they would guarantee the landmark to be safe for the many tourists that will come in the future. The Pisa Commission guarantees to open the tower in April of 2001, so a plan should be decided on soon [Virgone, 00]. This report explains to you each of the five preservation plans being looked at by the commission. The explanations given in this report will be based on information gathered from the cited sources. You are encouraged to view the sources for a more complete understanding of the plans. After the explanation is an evaluation of the five plans, which was done based on the gathered information. The criteria for the evaluation are appearance, cost, feasibility, and long-term results. The plans that pass all four criteria are compared and the best is chosen in the Conclusion section.
II. Background and History
of the Tower
The condition of the tower today is a result of the work that has been done on the tower in the past. Before trying to predict the reaction of the tower to today?s preservation plans, it is crucial to know how the tower has reacted in the past. Therefore this section will discuss the cause of the tower?s lean and efforts made to save the tower. The entire Miracle Square is gradually sinking, but some
areas subside faster than others do. It so happens that the bell tower
was built on one of these unfortunate locations. In the first stage of
construction (1170?s), the tower began sinking into the earth due to its
weight. As it sunk, the tower tilted to the north. The soil below the north
side of the tower compressed faster than the soil below the south side,
hence the northward tilt. After the ground froze and thawed many times, the ground below the south side of the tower (the southern soil) began compressing at a much faster rate than the ground on the north side. Therefore by 1272 the tower had sunk so that it tipped to the south [Heiniger, 95]. As the tower was tipping slowly to the south, more of the weight came to rest on the southern soil, so that the southern soil was compacted even more (see figure 1). And this cycle continued: the tower tipped, the soil compacted, the tower tipped more, the soil compacted more, etc. For the past 700 years the tower has also been tilting due to the fluctuations in the water levels. In the last century the rate of the tilting was carefully monitored, and scientists found that the tower tipped more during the rainy season. And in the early 1970?s, when water was pumped from deep layers of sand near Miracle Square, the tower reacted by leaning considerably [?La Torre?, 99]. In the last decade, scientists have discovered that the entire plaza has a slow flow of water moving from north to south under the ground. And a study of the lean of the tower showed that the tower is not only sinking, but also rotating, suggesting that the soils below are shifting with the water.
Contrary to what is commonly believed, the tower was never on the verge of tipping over. But it was on the verge of collapsing, in that some of the stonework was under too much stress. The explanation below tells how the stonework becomes more stressed as the tower leans further. The most stressed stones are those on the first few floors,
because they must support the 14,700-ton tower above. Also the stones on
the south side of the building are under a lot of stress, because of the
tow An inspection of the materials making up the tower has shown that the outsides of the walls are made of strong marble, but the insides of the walls are merely rubble [NOVA, 99]. Therefore the walls cannot support as much stress as it appears. This inspection, along with the finite element model, helped urge the commission to invest in techniques for stabilizing the tower.
The first efforts to straighten the tower were taken 800 years ago by the architects who built the bell tower (as explained in the Appendix). In 1935, it was thought that excess water under the tower weakened the foundation (later proven false), so Italian workers attempted to seal off the base of the tower to prevent water from seeping in from below. They drilled into the bottom of the tower at an angle, and then filled in the holes with a cement mixture. The foundation ended up being damaged from the drilling, and the effect was that the tower inclined at an even faster rate. [Heiniger, 95] The precious tower was left alone until 1990, when the seriousness of the lean could no longer be ignored. The tower?s 5.5-degree lean meant that the top stuck out from the base by 4.5 m [Heiniger, 95]. As indicated by the finite element model and material inspection, this tilt put too much stress on the old stone walls. At this point, the designated Pisa Commission declared the tower in danger of collapse. A fence was placed around the tower while scientists, engineers, and architects scrambled for a solution. In 1992, plastic coated steel wires were wrapped around the south side of the second floor to prevent a type of failure called buckling [NOVA, 99]. Buckling is when an overly stressed wall suddenly bursts outward. It is agreed that if the tower were to fail, it would most likely be due to buckling. The steel wires have been given the credit of preventing such a failure, and are to remain on the tower until 2001 [NOVA, 99].
In 1995, work on a new underground foundation began. The committee was optimistic about the foundation, but the project ended up nearly collapsing the tower. The commission first began freezing the ground in preparation for adding the new foundation of cables and weights [NOVA, 99]. As the ground unthawed some unseen plates cracked. The plates were mysteriously attached as part of the foundation in 1838, and today help support the tower. After the plates cracked, the tower began slowly falling south, so a crane stacked a few more lead ingots on the north side. At one point even the weight of the crane itself was used to halt the tipping tower. The final method, called soil extraction, began in 1998 and is expected to end in the summer of 2000. Contractors are removing soil from under the north side of the tower with drilling equipment. In order to move the tower back to a less stressful position, the north side of the tower needs to sink. To help sink the north side, drills are extracting about a shovel full of dirt a day [?Less Silt,? 98]. Just in case the tower is to react unexpectedly as the dirt is removed, suspending cables have been loosely fixed to the tower. These cables could pull back on the tower if it started leaning. So far the soil extraction has been so successful that the cables have not been needed. ?The soil extraction fix should last about 300 years, even if we did nothing after putting [the tower] where we want it,? said John Burland, a member of the Pisa Commission and the soil mechanic in charge of the excavation [NOVA, 99]. Burland and Michele Jamiolkowski, who is head of the Pisa Commission, took much of the credit for saving the tower by acting on the soil removal plan. Vittorio Novelli, who first proposed soil removal in 1990, also takes some credit [Novelli, 90]. The opinions of Burland, Jamiolkowski, and Novelli will be accounted during the evaluation of the future preservation plans.
Now that the tower has been straightened to a safe position, the question becomes, how can we preserve the position of the tower? Many preservation plans have been proposed to the Pisa Commission. In this section the five most widely looked at plans will be explained.
Soil extraction was begun as just a one-time method for
straightening the tower (see Efforts to Save the Tower, page 4), but it
was so successful that it is now being proposed as a method for preserving
the tower. The same two-year process that is now being completed would
be repeated whenever the tower reaches a dangerous lean.
This solution, as explained by Burland in 1998, would be to dig a slot around the tower about 35 feet from the base and pack it with impermeable clay called Bentonite [Hundley, 98]. In 1999, Burland explained the shield as being a diaphragm*: ?We would put what's called a diaphragm in the ground around the tower, some distance away, which would isolate the ground immediately beneath the tower from the water outside the diaphragm [NOVA, 99].? The argument is that shielding the soil below the tower from surrounding soils would keep fluctuations in the water table from affecting the tower. Due to the changing water table, all of the soils around the tower are moving from north to south (as explained in Cause of the Lean, page 2). The Shielding plan would protect the tower from this flow, which possibly would keep the tower from tilting.
This preservation method is shown in figure 3. It consists of a concrete layer surrounding the tower. Attached to the north side of this concrete are ten cables held tight by anchored weights. The downward pull of the tight cables would replace the downward push of the counterweights.
In 1990, Vittorio Novelli proposed to the commission a Concrete Base plan in detail, which included an above ground cables system, soil removal, and a concrete base. The cable system and the soil removal were eventually implemented in 1998. Although the committee and Novelli are opposing one another, it may still be possible for the committee to go ahead with his original proposal. Figure 6. Around and inside the tower is the proposed
The completed Concrete Base is shown above in figure 6. First a trench would need to be dug around the tower. Then a concrete reinforced structure would surround the tower and be fastened to it [Novelli, 90]. Such a foundation would not slip or sink into the soil.
Since the tower is now in such a stable position, one of the options is to simply stop working on the tower. There is public support for ending the work because of failed attempts, wasted time, and wasted money. When the tower was first closed in 1990, it was promised to be returned to the public within a year. Now, ten years later, the commission is still working. The Italian people are clearly upset with the commission for keeping the tower closed for so long. Many articles have been published in Italy about how the commission has wasted public money. Some titles include ?Tower: Pisans or Tuscans Would Have Spent Less,? ?The Tower has cost 50 billion lire ($ 28.6 million USD)?, and ?The Tower is Straighter, but the Money is Finished? [?Leaning Tower,? 00]. An association called ?Tower Tomorrow? is dedicated to getting the commission to end work and resign [Crescentini, 00].
IV. Evaluation of Preservation
Plans
In this evaluation, the ideal preservation plan will do four things:
Three of the plans include permanently changing the foundation of the tower: Shielding, Anchored Foundation, and Concrete Base. None of these three plans would affect the look of the tower since the changes would be completely underground. Periodic Soil Removal would decrease the tilt of the tower by about 10%, but according to the committee, this reduction in tilt cannot be noticed by the naked eye [NOVA, 99]. Thus none of the plans would affect the appearance of the tower.
Prof. Jamiolkowski said that, by next year, almost $23-million (in US currency) will have been spent on the monument [Johnston, 00]. Adding to this amount is an 11-year loss in tourism. The Italian government never planned on spending millions of dollars to halt and straighten the lean, and now they will not invest in a costly preservation plan. Shielding would be the most expensive method of preservation. Even Burland himself said about shielding, ?It's not hazardous, but it's quite expensive and a bit time-consuming [NOVA, 99].? It does not seem reasonable for the Italian government to invest in this plan. Periodic Soil Removal would also be expensive. The Soil Removal now being completed has cost about 4 million (in US currency) [Virgone, 00]. This cost would be much lower the second time around and the bill would not have to be paid until the work is needed, which likely will be decades or even centuries from now. Therefore Periodic Soil Removal may not be unreasonably costly. The Anchored Foundation and Concrete Base plans would not be as costly as Shielding and Periodic Soil Removal. Equipment, supplies, and workers are now available to build a new foundation. But since these are proven to be risky endeavors, the Italian government would have to spend money on safety precautions. Yet even then the costs look to be reasonable.
Soil removal has proved itself as being a feasible technique in preserving the tower, so the Periodic Soil Removal plan would also be feasible as long as the operation is carried out in the same manner. The three foundation plans, Shielding, Anchored Foundation, and Concrete Base, have to be examined to find out if they are feasible or not. The plan for an Anchored Foundation was once underway in 1995 (see Efforts to Save the Tower, page 4), but digging close to the tower proved to be risky. All three foundation plans involve digging close to the tower, so there is a question as to whether they are feasible or not. The concrete foundation would be the most risky. According to Vittorio Novelli, the digging would be four meters around the tower and three meters deep [Novelli, 92]. He proposed that a cable system hold the tower during the digging because of the danger of the tower falling. Since it is currently not feasible to spend the time and money to build a cable system, the digging would be too dangerous. There are two kinds of digging needed for Shielding: digging a ring around the tower and packing it with clay, and digging under the tower for the diaphragm. Since the ring would be 35 meters from the tower it would be safe. The diaphragm would be much more complicated. Burland said, ?It (the water) would be trapped inside the diaphragm, and its level would be controlled by pumps and wells and so on. That's not an easy operation [NOVA 99].? The Anchored Foundation plan would not have as extreme digging, yet at least ten holes would have to be dug 40 feet down into the ground for the ten cables and weights [Heiniger, 95]. In interviews, some committee members sound a little reluctant to make a second attempt at attaching the new foundation [NOVA, 99].
Periodic Soil Removal would give the best long-term results. Each time soil needs to be removed, the lean of the tower could be altered to the north, south, east, or west with millimeter accuracy [NOVA, 99]. Therefore the ideal position for the tower can always be achieved for as long as the tower stands. The Anchored Foundation and Concrete Base plans would hold the tower firmly in place for a long time. Yet in time, as the soil moves and the water table fluctuates, the tower may end up tipping again (especially with the Anchored Foundation). This is a chance the committee may have to take. One added bonus for these two foundation plans is that they would help in the case of an earthquake. A type VI earthquake may take down the tower, while a type VII would definitely take down the tower. Odds are that a type VI earthquake will occur in Pisa within the next 165 years, and a type VII within the next 520 years [?La Torre,? 99]. Therefore, the tower will likely end up falling due to an earthquake. This fact gives evidence that the Concrete Base and Anchored Foundation plans will give good long-term results. Shielding, the method sought by Burland, may or may not give the expected long-term results. There is not enough evidence available to back up his claim that shielding with a diaphragm would prevent the tower from moving. Lastly, the End Work plan would not keep the tower safe for tourists. Although the tower is safe now, all agree the tower will eventually lean back to a dangerous position. So an End Work plan today would result in a closed tower in the future and a new evaluation of preservation plans at that time.
There is at least one reasonable preservation plan for
each criterion. The overall most reasonable plan can only be found by combining
the results for each criterion, which is done in Table 1. In the table
you will find the five preservation plans in rows and the four criteria
in columns. For each criterion, a plan is given a rank of very good, good,
average, poor, or very poor (based on Evaluation of Preservation Plans,
page 10).
Table 1: Comparison of Preservation Plans
In order to meet the needs of a criterion, a plan needs to have a rank of average, good, or very good. Therefore Periodic Soil Removal and Anchored Foundation are the only plans that pass the requirements. Of these two plans, Periodic Soil Removal has the better overall rank. Therefore this evaluation concludes that Periodic Soil Removal is the best preservation plan for the Leaning Tower of Pisa.
The first stones of the bell tower were laid in 1173. As the first story was being built, the tower had a noticeable lean to the north. To keep the stories level, workers made the columns and arches on the north side taller than those on the south side. Political turmoil halted construction in 1178, in the middle of the fourth floor work. [Heiniger, 95] A new team of architects resumed the construction in 1272. By that time the tower was no longer leaning to the north, but was leaning to the south. The new workers hoped to correct the lean, just as the original architects had hoped, by making the south side taller than the north side. They built the bell tower seven stories high, but were not able to complete the work. By the end of the 13th century, the tower had tilted so far to the south that a group of masons was asked to investigate the problem. [Heiniger, 95] The eighth and final story, the bell chamber, was built between 1360 and 1370 [NOVA 99]. Once again, architects tried to correct for the lean by angling the bell chamber to the north. With all the corrections made to the tower, it ended up being shaped like a banana. Yet the campanile served its purpose for many years by ringing loudly through the streets of Pisa.
Bentonite: a variation of volcanic ash that can hold back liquids. Buckling: when an overly stressed wall suddenly bursts outward. Campanile: a bell tower for a Cathedral (especially in Europe). Counterweight: an object whose weight helps balance another object. Lead ingots acted as counterweights for the leaning tower of Pisa. Diaphragm: flexible underground partition impenetrable by liquids. Finite element model: a computer-based model of a structure including
many 2-D or 3-D elements. The computer can calculate stresses, strains,
forces, etc. in the
Lead ingots: heavy blocks of lead, which helped balance the leaning tower of Pisa. Miracle Square: a square in Pisa with a Baptistery, Cathedral, Cemetery, monuments, and bell tower. Piazza dei Miracoli: Italian for Miracle Square. A square in Pisa with a Baptistery, Cathedral, Cemetery, monuments, and bell tower. Pisa Commission: commonly called the Save-the-Tower Commission.
Started in 1990 by the Italian Prime Minister to insure the tower?s safety.
Michele
Stress: a measure of how much force is being applied to an area.
Crescentini, George, ?EXPOSED,? http://www.torredipisa.com/news/MAGISTRATURA (Pisa, Italy: Tower Tomorrow, April 13, 1999). Heiniger, Paulo, ?The Leaning Tower of Pisa,? Scientific American, vol. 273, no. 12 (Dec. 1995), pp. 62-67. Hundley, Tom, ?Taking a Little Lean Out of the Tower of Pisa,? Chicago Tribune, Aug 20, 1998. Johnston, Bruce, ?Pisa Tower?s Leaning is stopped by professor,? The Daily Telegraph, is. 1740, no.1, (Feb 29, 2000), pp. 2. ?La Torre di Pisa,? http://torre.duomo.Pisa.It (Pisa, Italy: Opera Primaziale Pisana, Sept. 1999). ?Leaning Tower of Pisa News? http://www.endex.com/gf/buildings/ltpisa/ltpnews/ltpnews.htm (Pisa, Italy: Gary Feuerstein, May 2000). ?Less silt, less tilt at Pisa,? Civil Engineering, vol. 68, no. 3 (Mar. 1998), p. 14. Novelli, Vittorio, ?A concrete proposal for the Leaning Tower,? http://www.torredipisa.com (Pisa, Italy: Tower Tomorrow, Jan. 26, 1990). ?The Seven Wonders of the World,? http://www.nationalgeographic.com/resources/ngo/infocentral/phenom/wonders.html
(New York: National Geographic, Sept.
Virgone, Candida, ?The Tower has cost 50 billion lire,? Il Tirreno Giornale, (Jan 12, 2000), pp 1.
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er?s southern incline.
Putting these together, we can conclude that the one section of the tower
under the most stress is on the south side of the first and second floors.
A finite element model of the tower has revealed the exact location of
most stress: the walls surrounding the southern stairway on the second
floor (shown in figure 2) [?La Torre,? 99]. If the tower were allowed to
lean further, stress would increase until the wall fractures.
Although
the steel wires assisted the weak walls, the tower continued to incline
to the south. In 1993, this incline was halted by the stacking of lead
ingots on the north side of the tower [Heiniger, 95]. The ingots acted
as counterweights on the off-balance tower (see figure 3). With the 300
metric tons of lead ingots, the weight of the north side of the tower was
almost equal to the weight of the south side of the tower. Hence the ground
below the south side of the tower was no longer in favor of compressing
faster than the ground below the south side. The monitoring equipment indicated
that the tower had stopped its incline due to the added counterweights.
The Anchored Foundation was one of the first permanent preservation proposals
for the tower [Heiniger, 95]. In 1995, work for this foundation began,
but after a few days ended in the near-collapse of the tower (see Efforts
to Save the Tower, page 4). The accident provoked many of the committee
members to rethink the
