Tower of Pisa Issues - Essay Example

 Tower of Pisa Introduction The leaning tower of Pisa was constructed to show the wealth of the city of Pisa to the world. The people of Pisa were good sailors and they managed to conquer many lands including; Jerusalem, Norway and Morocco among others. They also had enemies who were the people from Florence. In order to show their strength to the enemy they decided to build the useless bell tower (the tower of Pisa).This was to go in line with the Cathedral, Cemetery and the Baptistery. As suggested by Mitchel (2001), the leaning tower of Pisa is located in Campo dei Miracoli commonly known as ‘field of miracle’ in Italy, behind the cathedral. It has a height of 55.86m above the ground on its low side and 56.70 m on its high side. At the base, the width of the walls is 4.09m and 2.48m at the top. The tower is estimated to weigh around 14,500 tones. It has 296 or 294 steps with the seventh floor having two fewer steps on the north facing staircase. Before its restoration done between 1990 and 2001, the tower leaned at angle of 5.5 degrees. Today, the tower leans at an angle of about 3.99 degrees. This implies that its top is 3.9 m horizontally displaced from where it ought to be if the tower was vertically perfect. Its construction was done in three steps for a period of 177years.The first floor building began in year 1173 which was a period marked by military success as well as prosperity. The tower of Pisa was built to be a bell tower for Pisa’s cathedral in 1173. According to Khan (2005), around five years later, the tower began to tilt downward, just after the completion of the third floor. This was as a result of three meter foundation which was set on a weak and dense clay mixture unstable to support it. This design had been set from the start. Due to this, the construction was stopped for almost a century. Architects had thought that giving the soil time to settle would stabilize the tower. It also happened that the city was frequently engaged in battles at that time. The construction of the tower continued in 1272 with Giovanni di Simone acting as the architect. In compensation for the leaning, the engineers constructed the upper floors with one side being taller than the lower side. As a result of this kind of construction the tower is curved. Its construction was halted for the second time in 1284 when Genoans defeated the Pisans during the battle of Meloria. The second floor built by Tommaso di Andrea Pisano was finished in 1319. The last storey was added in 1360 and the tower was completed in 1370. By the time of completion the tilt of the tower could actually be noticed. The Causes for the Leaning of the Tower of Pisa As suggested by Puzrin, Alonso and Pinyol (2010), experts have not been agreeing on the problem leading to the failure of the project. Some argue that it is static while others argue that it was the ground sinking or the effects of the design used by the particular architect. The main cause of the leaning is attributed to the reaction of composite clay, sand and shells on which the tower is built on. The tower was prone to two major risks one being failure in the structure of the fragile masonry and toppling as a result of the breaking up the of the foundation’s subsoil. One of the solutions put in place to counter this problem was the installation of a counter weight on the northern side of the base of the tower so as to stop the tilting. This solution did not succeed and therefore another solution was initiated in 1995. This involved inserting compressed steel cables and the same compressing was done to the subsoil. This instead increased the leaning of the tower. After the period of structural restoration, the tower is now undergoing surface restoration so as to repair visual damage especially corrosion and darkening. In 1964 the Italian government requested for assistance in saving the tower from collapsing, however it was considered to leave the tilt as it was vital for promoting tourism in the city of Pisa, as suggested by D’Alfonso (2005) Owing to the failure of the solutions used for restoring the tilting of the tower, the Italian commission embarked on a subsoil study program in 1965. This program comprised of three vertical borings. Some tests were also done to the clay layer. The tower’s subsoil consisted of three horizons with the first horizon comprising of surface material above the sand, clays and silts. The second horizon consists of four strata; upper clay, intermediate clay, intermediate sand and lower clay. The third horizon contains dense silt sand. It is noted that, the differences between each horizon are mostly horizontal across the tower’s site apart from the depression resulting from the tower’s settlement. Therefore the leaning position of the tower cannot be attributed to the clay layer in the second horizon. Though the clay found in the second horizon forms the base for the most parts of the tower’s settlement, factor in the first horizon seem to be of the great significance in as far as the lean is concerned. Grey sand is put on top of the upper clay in the second horizon. In most cases sandy soil covers the grey sand towards the northern side of the tower. On the opposite side that is the southern side, there are found yellow silt soils and yellow clayey silts. The resistance measured in horizon B the grayish upper clay is subdivided according to the differences in water retention as well as plasticity. The bearing pressure of the tower’s foundation on the sands in horizon A is estimated to be 50.7 ton/meter squared. When analyses of the bearing capability of the first horizon are done, it shows a safety factor ranging from around 3m to 4m against average pressure and of 1.5m to 2m against maximum pressure. This is based on bearing capacity relations applied and the assumed water content of the sand soil. If there developed instability caused by the bearing capacity it would have occurred due to the presence of weak clay at second horizon. According to Mitchel (2001), analysis done by Terzaghi reduced the stress distribution on the second horizon. Thus it is important to analyze the ability of the clay to support vertical stress of around 59tons/meter squared. After the clay is consolidated under the tower’s induced strength, the measured strength ought to be greater compared to the one at the foundation. A number of analyses on bearing capability have been put in place for various conditions and stages in the process of constructing the tower. In every case a condition of safety is obtained against un-drained failure. This condition assumes a certain percentage of uniform stress above the clay. Lessons learnt the case study on the leaning tower of Pisa From the failure of the designing of the tower of Pisa, civil engineers are bound to learn various lessons. For one, footings form the major and the most significant part of any construction of a building as it may lead to success or total failure. For instance when dealing with soft soil, it is important to know that in concrete footings, bigger footing reinforced with additional steel should be put in place. Secondly one should dig down beyond the soft soil and set a deep footing. Thirdly, the soft soil should be replaced with adequate soil that produces the bearing capacity required for the design. Fourth it is wise to flood the surface after digging trenches and then compact gently, this allows the soil to be more stable for a building. Also make use of Geogrids as they give an effective average which lowers pressure below the surface. Finally inject slurry made of soil and cement, this needs four important pieces of equipments. These include: a drill rig for advancing the slurry, a tank for mixing cement, a pump and a special tool for mixing soil and cement as suggested by Delate (2005) Conclusion In conclusion, there is another way of determining horizontal pressure and tension reinforcement which provides the base for internal design with reinforced walls. The main idea here is that compaction pressure, reinforcement and the stiffness of the soil properties are considered. Some analysis imply that the stiffer the reinforcement and the higher the pressure injected during the compaction time, the high the tensile pressure applied by the reinforcements. Parametric analysis indicates that the soil resistance, soil weight, its depth, its relative reinforcement and compaction are main factors which determine tension reinforcement. Influence as a result of compaction is greater for walls reinforced with relatively reinforcement soil stiffness index. The horizontal earth’s pressure coefficient can be higher than the pressure at the top of the wall. It can also be greater than the earth’s pressure to depth of as high as 6.1m or more according to relative index stiffness of the soil reinforcement. References D'Alfonso, A. (2005). Gambling with Failure. Toronto: Exile Editions, Ltd. Delatte, N. (2005). Beyond failure: forensic case studies for civil engineer. Florida: ASCE Publications. Khan, I. (2005). Textbook Of Geotechnical Engineering. Connaught circus, Delhi: PHI Learning Pvt. Ltd. Mitchel, J. (2001). Selected Geotechnical Papers of James K. Mitchell: Civil Engineering Classics. Florida: ASCE Publications. Puzrin, A, Alonso, E. & Pinyol, N. (2010). Geomechanics of Failures. New York NY: Springer. Read More