I. Introduction
On rare occasions, a project may be known for its failures rather than its successes. One notable example of this occurrence can be found in the Leaning Tower of Pisa, located in Pisa, Italy. When construction began in 1173, geotechnical engineering methods were either not well established or nonexistent, which could most likely explain the poor choice of having the tower located above soft subsoil. A shallow foundation depth of roughly 3 meters could likely be accounted to poor engineering methods as well. Construction on the tower continued until 1178, when work on the third floor of the tower was still in progress. At this point the tower began to sink into the foundation. Since the Republic of Pisa was engaged in battles with neighboring regions at this point, construction was halted which would hopefully allow the soil beneath the foundation to settle.
After approximately 94 years, construction on the tower resumed. In order to rectify the tilt of the structure, one side of each floor was built taller than the other, giving the tower a slight curve to help compensate. The seventh floor of the tower was finally completed in 1319, with the bell chamber being finished in 1372. In 1990, the tower was closed to the public to commence renovation efforts. Various methods of stabilizing the soil were employed, some successful and others not. After eleven long years of arduous work, the Leaning Tower of Pisa finally reopened to the public in 2001. Though the tower now only tilts at less than 0.2 arc-seconds (less than 1/10000th of a degree) per year, the tower’s lean will always be a reminder of how crucial proper site work is to any engineering project.
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| Fig.1. Leaning Tower of Pisa |
As mentioned in the previous paragraph, the stabilization of the
tower’s subsoil was one of the major projects for the government of Italy.
Starting in the early 1990s, a group of professional geotechnical engineers
were hired to remedy the tilting problem of the tower. Their major task was to
find out the issues with the underlying soil and possible solutions to fix it.
The first step in stabilizing the tower’s subsoil was the soil
investigation. As a result of this, the three main layers of subsoil were
found. The deepest layer was made up of dense marine sand with the next layer
being soft marine clay and the very top layer consisting of soft sandy and
clayey silts. When the tower was originally built the foundation was too
shallow, meaning the foundation is sitting on soft clay. This soft clay did not
have the necessary bearing capacity which is why the tower began to settle
after the third level. Also, a water table lying only 1 to 2 meters below
the surface was found under the top layer of soil. The piezometric levels of
the groundwater for the two sides of the tower were at different heights. This
fluctuation of the water table was mainly caused by the two major rainy seasons
encountered in Pisa (Autumn and Winter). This difference in water table levels
for both the north and south sides was found to be the main cause of the
tower’s rotating problem. Because the south side has a higher water table
during the rainy season, the clay on this side absorbed more water which caused
the clay to shrink more than the north side when the water table went down
during the dry months. This means that there was more movement and settling on
the south side than the north side causing the inclination of the tower.
III. Stabilization Methods
After the discovery of the main problem, various solutions were
proposed. These solutions included the addition of lead weights on the top
northside of the tower, installation of a drainage system on the north side,
loading the top surface ground of the north side with pressing slabs loaded by
ground anchors, and quite a few other solutions. Some of these solutions failed
and a few of them worked. As an example, in 1993 a temporary stabilization
method was improvised by adding 600 tons of lead to the north side of the
tower’s foundation which reduced the tower inclination by a small amount. In
1995 the attempt to install underground steel anchors almost caused the tower
to collapse. Before the anchors could be installed the geotechnical crew had to
find some way to prevent water from getting into their excavations. Their
solution was to freeze the ground with liquid nitrogen. Unfortunately, because
water expands when it freezes, gaps were created after the freezing was over
causing the tower to lean farther southward. In order to stop the tower from
collapsing, the stabilizing load of lead was increased to 900 tons. After the
attempts of these temporary solutions, the geotechnical crew decided to come up
with a permanent solution that would ensure the stability of the structure.
Three main solutions were found to be reliable. For their first attempted
solution the geotechnicians
decided to use an underexcavation method. To do this, geotechnical engineers
used boreholes, also known as the auger boring method. A series of holes were
drilled towards the north side of the foundation. The maximum number of holes
to be drilled, as well as the interval between them, was determined. After the
entire process of soil extraction, the tower gradually started to rotate
towards the north side. In other words, this underexcavation method was a
success for the hired crew. Another approach used to stop or reduce the tilting
problem was to attach the shallow foundation of the tower to the nearby
concrete ring. This ring was called the Catino wall. To do that, the
geotechnical crew used tensioned steel cables and steel reinforcements which
were both embedded in a concrete slab. This concrete slab was a liaison between
the north side of the tower’s foundation and Catino wall. This approach become
a success as well because the tilting problem was significantly reduced.
Lastly, the geotechnical crew decided to install a drainage system under the
north side of the foundation. The main purpose of this drainage system was to
control the level of the groundwater table of the subsoil. To achieve this, the
crew connected a well at the end of the drainage system and used this well to
help them pump water out from under the foundation at the desired rate. This
approach become a success too because, after the installation of the drainage
system, the piezometric surface for both the north and south side of the tower
was maintained at a constant level.
IV. Conclusion
Though the geotechnical crew that worked on stabilizing the Leaning Tower of Pisa from 1990-2001 faced quite a challenge, they were successfully able to find a combination of solutions that will keep the tower standing for a long time. Even though geotechnical engineering methods have vastly improved since 1173, the Tower of Pisa has been a long-standing reminder of the fact that sitework and subsoil exploration is an extremely important step in the foundation design process. The surface and subsurface of the earth vary greatly from location to location, so it is often imperative to be creative (as the geotechnical crew working on the Tower was) with ground improvement solutions. Even though it is uncertain just how long the tower will maintain its stability, it is estimated that the structure will take at least 200 years to return to the same angle as it was at in its 1993 restoration phase.
V. References
Bartusiewicz, Mike, Dong-Hyun Chung, and et al. "The Leaning Tower of Pisa." CE 203 - Engineering Synthesis I. Iowa State University, 16 Apr 2007. Web. 20 Jun 2012. <http://home.eng.iastate.edu/~tge/ce203/group3ppt.pdf>.
Burland, John, Michele Jamiolkowski, and Carlo Viggiani. "Leaning Tower of Pisa: Behaviour after Stabilization Operations." International Journal of Geoengineering Case Histories. 1.3 n. page. Web. 20 Jun. 2012. <http://casehistories.geoengineer.org/volume/volume1/issue3/IJGCH_1_3_2.pdf>.
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