Tower of Pisa: The Leaning Tower of Pisa
The leaning Tower of Pisa is one of the ancient Seven Wonders of the World, even if it were straight up. According to history facts the design is somewhat similar to the design of the Tower of Babel. The bell tower has 7 bells, which are tuned to a musical scale and counts 294 steps. The Tower of Pisa raises 55 mts from the ground and the subsoil had to bear his enormous weight of around 14.453, tons and the center of gravity is about 22.6 high. The exterior foundation itself spans stand about 19.5 mts with a central aperture of 4.5mts.
Tower of Pisa: Construction StageIt all started in 1173. The original two levels of the Tower of Pisa didn’t lean, but the structure began to lean when construction moved to the third level and beyond in 1178. Various solutions were tried out once the architect took notice of the lean in 1185. The soil at the chosen site was much too unstable to support such a large structure. The construction of the Tower of Pisa stopped for nearly a century, because of Pisa's wars with the neighboring city Florence. Work started again in 1272 and four floors were built in an altered angle to the previous levels, but the Leaning Tower of Pisa started to lean in the direction of the taller side. In 1284 construction stopped again, because Pisa was conquered by Genoa in another war. In 1370 the tower, eight stories and 200 feet (60 meters) high, was officially completed.
Tower of Pisa: The Problem
Experts have been divided whether the problem was of static or was a sinking of the ground problem or even an effect designed by the architects. The foundation for the Tower of Pisa were laid in 1173, constructed mainly of marble and lime, the tower was built in a circular ditch, about five feet deep, over ground consisting of clay, fine sand, and shells. The cause of the lean is the reaction of the composite of clay, fine sand, and shells that the tower is built on. This soil mix is more compressible on the south side, but over the years as the tilt increased, the Tower of Pisa stopped sinking and began to rotate, causing the north side moving up toward the surface. Tests during the 20th century have conclusively proven that the inclination began after the construction. The study of the , subsoil has revealed an inter-layered clayish material washed by subterranean waters.
Tower of Pisa: The Solution
The Tower of Pisa's structure was subject to two main risk: structural failure of the fragile masonry and collapse due to the breaking up of the subsoil around the foundations. Recent possible solution were leading by installing a counterweight of about 660 tons on the north side of the tower's base in order to stop rotation. It failed. Then, during 1995 freezing of the inserting steel cables and freezing of the subsoil was tried, but this cause the lean to increase
Later on, scientist and engineers detected that soil extraction was the key to bring the tilt back to stable conditions. Soil was extracted from two layers of earth: the top layer of sandy soil and the second of marine clay. While soil is being removed, the ground compression surges and clay consolidates, giving a stronger foundation.
The drills extract soil from inside a casing without acting upon other elements or outside of it. The drill cavity closes smoothly when the drill is retracted, then soil settles, forming a cradle that cushions the tower as it shifts slightly to the north.
By using this method, engineers have reduced the lean back toward the center by 20 inches (50 centimeters), where it was in 1838. The top of the tower now leans just over 13 feet (4 meters) off center.
Tower of Pisa: Lesson Learned
Footings are the most principal and important part of any building. It may guarantee your success or a total failure of the project. We have to learn from experiences and the Leaning Tower of Pisa, is one of the most beneficial experiences that we can have in the construction industry. Although, the problem of tilting is solved, you don’t want to have that kind of problem in your project. Here are some tips to get you started when dealing with soft soils.
1- When Building over soft soils you may have to excavate down past the soft spot and place a deeper footing.
2- Replace the soft soil with adequate soil that will produce the bearing capacity specified in the design.
3- Build a bigger footing and reinforced it with additional steel (in concrete footings).
4- Use friction piling or end load bearing piling if the soil type below is suitable.
5- It is a common practice to flood the ground once the trenches have been dug and then to compact thoroughly. This improves cohesion and makes the soil considerably more stable to build on.
6- Injecting soil/cement slurry. This type of process requires four key pieces of equipment: a drill rig to advance the slurry to design depth, a batch plant or tank to mix the cement slurry, a pump to push the slurry to the drill rig, and specialized tooling to blend the cement slurry with soil in situ.
7- Using Geogrids will provide an effective mean for reducing pressure below traffic surface.
There are different combinations and techniques for each particular project. It will all depend on the type of materials being used, the type of structure, and the specific soil conditions in each case. Furthermore, have in mind all the required regulations and codes that must be met in every condition.