Preloading Method
Preloading method is a structural preloading that is usually used in foundation engineering projects. It is one of the efficient means of minimizing settlement under statically and dynamically loaded structures, such as bridges, quay walls and retaining walls. The idea of preloading method is that one applies the anticipated magnitude of loading (binding force, an external force or a combination of these) to the soils, in order to induce a state of stress that is sufficiently close to the ultimate stress due to the anticipated loads. This induced stress is called preloading.
A general preloading scheme involves the application of an external force that roughly corresponds to the anticipated magnitude of future loading. This is followed by a period of applying time that allows the soils to exhibit some degree of non-recoverable deformations that reduce the anticipated magnitude of future loading. The preloading period usually consists of two distinct periods, namely the groundwater reduction period and the loading period.
The groundwater reduction period is the period of time during which the water is removed via vertical drains by draining the soil and then raising the groundwater table and regulating the amount of drainage around the structure. This period is necessary in order to facilitate the consolidation of the soil. The duration of the groundwater reduction period varies according to the soil type and method of preloading, and it may take up to a few months in some cases.
The loading period is the period of time during which the external force is applied to the soils, creating a state of stress that is close to the ultimate state of stress due to future loading. This period also varies according to soil type, and can last up to several months in some cases. During this period, the structure is closely monitored, and preloading may be stopped and adjusted depending on observations and findings.
Once the preloading is complete, the anticipated loads are applied to the structure with closely monitored post-loading monitoring. This monitoring ensures that variations in the preloading state of stress do not cause the structure to become unstable or suffer any unexpected subsidence.
Preloading method can be a very effective and cost-efficient method of improving the performance of foundations, providing that the soil of the project site has the adequate characteristics that allow successful preloading. It is best suited for projects where the soils have already exhibited some degree of compaction due to loading or have the potential to compact when subject to preloading, as well as those sites where the predicted future loading is known or can be realistically estimated. However, preloading method is not suitable for all projects, and there may be cases when alternative solutions should be applied in order to minimize the risk of unexpected failure or damage to the structure.
