The Leaning Tower of Pisa has experienced many phases of construction and attempted remedies throughout its history. Its construction began in 1173 AD, and by the end of this initial phase, a tilt was already noticeable.
Due to a foundation of soft soil composed of mud, salt, and clay, the tower’s foundation began to settle unevenly, and by the time builders had reached the third story of the planned eight, the tower leaned slightly to the north. The initial attempted remedy was to increase the height of the columns and arches on the side that was sinking, but progress was halted before construction could begin on the fourth story due to political unrest. The tower remained unfinished for nearly 100 years, and construction began again in 1272. During this gap, the soil under the tower had continued to subside unevenly, and the tower had become south-leaning, the direction it still leans today. Engineers made another adjustment to the fifth story, but construction was interrupted again in 1278 with only seven stories completed. Construction was finally completed between 1360 and 1370, but the tower had continued to settle at an alarming rate. Engineers again attempted to correct the lean by angling the eight story northward.
The tower’s progress was not carefully monitored until 1911, when measurements revealed that the top of the tower was moving at an alarming rate of around 1.2 millimeters per year. The breaks in construction that prolonged the completion of the tower had allowed the underlying soil to settle, and although the gaps were no doubt causes for frustration, the tower may have likely fallen had construction gone according to schedule.
Another remedy was attempted in 1935, when engineers aimed to seal the base of the tower to prevent excess water under the foundation from accelerating the tower’s decline. A series of angled holes were drilled into the foundation and then filled with a cement grouting mixture. This attempt, however, only proved to worsen the problem when the tower began to lean even more precipitously. Further efforts to stabilize the tower with various types of bracing and reinforcement were carried out, but none of these measures succeeded. Years passed, and the structure reached an incline of 5.5 degrees. Then in 1989, a similarly constructed bell tower in Pavia, Italy suddenly collapsed. The tower of Pisa was closed to the public due to the concern of officials, and an international team was formed a year later to finally eliminate the danger the tower posed.
John Burland, a soil mechanics specialist and a key member of the team proposed that extracting soil from below the tower’s northern foundation could potentially pull the tower to a safe incline. After analyizing data pulled from simulations and computer models, the team decided to move forward with the plan. Steel bands were wrapped around the tower’s first level to prevent stone fracturing. The team placed 750 metric tons of lead weights on the tower’s northern side and poured a concrete ring around the tower’s base. A series of cables were connected to the concrete ring and anchored the tower far below its foundation. The team commenced to drill underneath the foundation, and around 20 liters of soil were removed with the drill during each run. Slowly, the ground above began to settle. Combined with the pressure from the cables, the tower finally began to pull vertically. This was repeated over several years in 41 different locations.
The tower’s lean was decreased by 44 centimeters by 2001, at which point officials opened the tower to the public. Even without further drilling, the tower continued to straighten another 4 centimeters until no motion was detected by 2008. The masonry of the tower is now the primary concern, specifically in the lower stories where the forces of centuries of leaning have been largely directed. The tower could potentially fall if any of this material broke down. Even with these concerns, engineers expect the tower to remain stable for at least another 200 years. By that time, technological improvements will likely be capable of preserving the tower for much longer.
Have you had the opportunity to visit the Leaning Tower of Pisa? Do you have any insight into its tumultuous history? Feel free to share your thoughts with us below.
A cartoon in one of the trade magazines years ago showed these two men dressed as in medieval times with a straight tower of Pisa in the background. One says to the other, “I skimped on the foundation but no one will ever know.”
Having been involved in the early years of Washington Metro when major buildings next to cut and cover excavations were underpinned, I do not understand why such methods were not used here. To be clear I was not in any way shape form or fashion involved in these underpinning operations. At least the work as described does not sound similar to what was done in DC. What was done there appears to have been very successful, with in some cases 80+ feet deep holes all but to the face of the building.
When I was in the Navy we went by bus from La Spezia where we were ported to Florence and spent most of the early part of the day there and then went over to the Leaning Tower of Pisa. We had just visited Athens, Greece a couple of weeks before to see the Parthenon and the Market Place below , as well as, Mars Hill. The Leaning Tower and the Cathedral adjoining it were very cool. It was in 1980 and it was open to climb the stairs. Since most of it, if not all was made of marble, and the stairs wore on the end in the direction of where it was leaning. It was quite small on the stairway and climbing it was tiring. I got some good photos of it from the ground and several while we climbed to the top where the bells were located. Seeing the works of art and the cathedrals was something I’ll never forget.
In May 2004, my wife and I visited Firenze, Italy. One day, we caught the train to Pisa to visit the Leaning Tower of Pisa. We did a lot of walking that day. A kind Italian medical doctor that we met on the train showed us a good restaurant for lunch. He also told us some of the history of the Leaning Tower. We did not go up the Tower that day, but we wee impressed by the leaning building, and I have a small model of the structure on my desk. We also enjoyed seeing the Duomo and Baptistery.
Did climb the tower ages ago.. it was 1982 when it was still open and most likely to the most leaning angle. Remember the sensation of climbing the stairs at that awkward angle making the steps very uneven.. on one side you almost moved horizontally while at the opposite the steps were twice as tall.