工程地质 report.docx

Reasons Behind the Standing Leaning Tower Of PisaAbstract This paper explores the reasons behind the standing leaning Tower Of Pisa. The study the foundation of the tower shows that the root cause of the lean was a perched water table upon the upper silt layer below the north side of the tower and a new drainage system was the exact way to ease the leaning. So engineering geology is the basic knowledge engineers should obtain to prevent hazards resulting from geological factors.1. Introduction Engineering geology is the application of the geological sciences to engineering study for the purpose of assuring that the geological factors regarding the location, design, construction, operation and maintenance of engineering works are recognized and accounted for. Engineering geologists are educated and obtain experience related to the recognition and interpretation of natural processes, the understanding of how these processes impact human made structures and knowledge of methods by which to mitigate against hazards resulting from adverse natural or human made conditions. During the planning, environmental impact analysis, civil or structural engineering design, value engineering and construction phases of public and private works projects， and during post-construction and forensic phases of projects, engineering geology studies may be performed. Then engineering geologists provide geological and geotechnical recommendations, analysis, and design associated with human development and various types of structures. 2. BackgroundFigure 1: Leaning Tower of Pisa The Leaning Tower of Pisa, see figure 1, is located in the Pisas Cathedral Square. It is known worldwide for its unintended tilt. Its construction started in August 1173 but the work was interrupted for some political reasons rather for its leaning and the construction was resumed in 1272. The towers tilt began during construction, caused by an inadequate foundation on ground too soft on one side to properly support the structures weight. The tilt increased in the decades before the structure was completed, and gradually increased until the structure was stabilized by efforts in the late 20th and early 21st centuries. However, the doubt why the tower of Pisa leaned puzzled engineers all over the world. The answer was not known until 2001, when a serious stabilization effort was completed. The following passage will give the answer.3. Specific issues It was known prior that the tower had been built atop an inadequate foundation which was only 3 meters thick and was constructed on very soft silty soil. But the primary cause of the tilt, which was finally discovered by Pro. John Burland, was a fluctuating water table which would perch higher on the towers north side, causing the towers characteristic slant to the south. Subsoil profile and soil properties see figure 2.Figure 2: subsoil profile and soil properties under Leaning Tower of Pisa Professor John Burland stated that if the tower was erected completely for the first time, it would crush. Exactly the same Tower of Pisa, exactly the same foundation, just the difference of construction speed makes a significant distinction of effective stress path in foundation, then greatly affects the development of general shearing strain which is stress path dependent; and finally determines whether or not the foundation is destroyed and whether or not the leaning Tower of Pisa is fallen over. The viewpoint from Burland that the tower would have fallen over if had work continued is explained by stress path dependency; it is pointed out that if the leaning Tower of Pisa is built more slowly, the effective stress path will be more farther to the critical state stress line; and then, the lean of the Leaning Tower of Pisa today maybe much little. Beside finding the root cause of the lean, people did much efforts to prevent the tower from falling over.i. The first modern attempt at stabilization of the tower occurred in 1935, when engineers attempted to seal the base of the tower by drilling a network of holes into the foundation and then filling them with a cement grout mixture. However, it just slightly increased the lean. ii. The second team first braced with steel tendons, to relieve the strain on the vulnerable masonry; and then 600 tons of lead ingots were stacked around the base of the north side of the tower to counterweight the lean. Later on, they opted for 10 underground steel anchors, to invisibly yank the tower northwards. However, this made the tower even closer to collapse. So they planned to install anchors, 40 meters deep, from tensioned cables connected to the towers base. The team, in view of Pisas high water-table, froze the underlying ground with liquid nitrogen before any anchors were installed, to protect their excavations from flooding. However, it was not taken into account that water expands when it freezes. The freezing groundwater pushed up beneath the tower and created gaps for further settlement of the tower. The plan failed. Then the team added another 300 tons of lead ingots in a desperate attempt to prevent the loss of the tower.iii. The team eventually agreed that soil extraction was the only viable solution that would be acceptable to all concerned parties, as it had the advantage of not touching the tower itself, thereby placating the art historians.The process involved the installation of helical drills surrounded by hollow steel casings to remove soil from below the high north side of the tower.While more soil could have been removed, the soil extraction program reduced the stress on the vulnerable first story enough to be safe, yet also maintained the distinctive lean of the landmark.The team estimated that it would take approximately 200 years for the tower to return to its pre-stabilization inclination, and the tower was reopened to the public in 2001.iv. The problem was finally solved by a team led by professor Burland. Mr. Burland introduced a new drainage system beneath the piazzas north side; upon discovering that the root cause of the lean was a perched water table upon the upper silt layer below the north side of the tower, which fluctuated during the rainy season, sometimes coming within 12 inches of the surface. The new drainage system addressed this condition, and is hoped to permanently alleviate additional movement. The inclination continues to be monitored daily and revealed that the tower did not move for the first time! 4. Conclusion From this paper, it can be known that the lean of Leaning Tower of Pisa is due to the a fluctuating water table and its standing is due to the discontinuous construction and the property of subsoil profile. Though Leaning Tower of Pisa become a world famous landmark for its lean, we should avoid the lean of buildings, bridges and towers as the existence of lean would make the erections