The Tower of Pisa, like normal buildings, was originally built straight or plumbed when it was first constructed in the 12th century. However, as construction progressed, the tower gradually leaned. Unknown to its builders, underneath its shallow foundation, is an unstable layer of clay and silt (Burland, Jamiolkowski and Squeglia). The tower continued to lean gradually until the 20th century which resulted to several corrective measures in order to prevent its collapse.
Figure 1 (Geotech).
The Tower of Pisa is just one of the many structures that were built on poor soil. The Transcona Grain Elevator in Canada also experienced the same failure. In this case, the engineers assumed that the soil is homogenous, relying only on their shallow soil samples. Although these buildings were saved from collapse after several costly rectification attempts, others, however, were not as lucky. The Ocean Tower in Texas, for instance, would have to be demolished after its construction as large cracks on its structure appeared after significant settlement of its foundation.
Figure 2. Transcona Grain Elevator
Figure 3. Ocean Tower
Ideally, the soil in which the structure’s foundation should be laid must be well-drained, compact and strong enough to carry the weight of the building. However, it is not always the case as geological features are naturally inconsistent. So, what happens to a building if it is built in an unstable soil?
1. Sinking & Tilting. Just like the Tower of Pisa and the Transcona Grain Elevator, buildings that are built above unstable soils would experience significant settlement. The settlement would be more likely unbalanced as some portions of the building are heavier causing the building to tilt towards the heavier side. It is also possible that changes in the soil profile may cause the building to tilt towards the side where the soil is softer or more unstable.
2. Structural Damage. Unstable soil will cause unbalanced load distribution causing damage to columns and beams. As load is distributed unevenly, some columns and beams would carry stresses that are beyond their capacity causing them to crack, deform and may even collapse.
It should be noted that unstable soils such as clayey and silty soils may not always be visually apparent. As soil layers changes, it is possible that unstable soils may lay hidden underneath layers of seemingly good top soil for several meter depth. For the same reason, prior to construction, it is important to determine the soil profile where the structure is to be built through soil investigation. This investigation is usually performed by conducting boring tests or soil penetration tests under the supervision of a geodetic engineer.
As determined by a geodetic engineer, boring tests are conducted by drilling several bore holes to a specific depth and taking soil samples for laboratory testing. The bearing capacity of the soil is then determined including its other characteristics such as composition, plasticity and draining capacity among many others. It is only then, after these valuable data are gathered, that a suitable foundation can be designed.
Soil investigation, therefore, could not be undermined or set aside as a repeat of structural failures in the past is most likely to happen, causing unnecessary losses to property and human lives.
Works Cited
Burland, John Boscawen, et al. "The Leaning Tower of Pisa." Geotechnics and Heritage. London: Taylor & Francis Group, 2013. 207-227.
Geotech. Why the Tower of Pisa commence to lean? And how the inclination was stabilised? Ciottina 21, HR 51000 Rijeka, n.d. <https://www.geotech.hr/en/why-the-tower-of-pisa-commence-to-lean/>.
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