When designing the foundation for a structure, provided that the safety factors and maximum allowable settlement limits are met, a shallow foundation will be chosen because of its lower cost and ease of construction compared to other types.
However, in civil engineering or building projects where we encounter soil with low bearing capacity or high deformability in the surface layers, it will be necessary to use another type of foundation capable of transferring the structure’s loads to greater depths, where the soil has sufficient strength to support them.
Next, we'll look at the different types of deep foundations that exist, when these types of foundations are necessary, and how they work:
Types of Deep Foundations
Although there are other types of deep foundations, such as caissons, Indian wells, and retaining walls, in this article we will focus on the most commonly used type: piles.
Its main characteristic is the significant difference in length between one of its dimensions and the other two. In this case, we can define a pile as a foundation whose length is between 6 and 8 times its minimum length.
As for the different types of piles that exist, they can be classified according to the following criteria:
Based on the method of installation, there are two types of piles: displacement piles and extraction piles. The former, also known as driven piles, are those that are driven into the ground, causing lateral displacement of the soil.
In contrast, for extraction piles, a hole is first excavated in the ground and then filled with concrete. It should be noted that the construction method chosen for these structures—whether buildings or other types—will determine the calculation of the settlement load.
With regard to pile size, and using diameter as a reference, we will consider three categories: those larger than one meter, known as large-diameter piles; the most commonly used piles, ranging from 45 to 100 cm; and small-diameter piles, ranging from 30 to 45 cm.
Piles with a diameter of less than 30 cm—known as micropiles—are constructed using a different technique and have different procedures for calculating ultimate limit states (ULS) than the ones described above.
In terms of shape, there are three main types: cylindrical structures cast in place, prefabricated prismatic concrete structures, and, finally, annular structures, which have a hollow interior.
Another way to classify piles is based on the material from which they are constructed. In addition to concrete piles (precast or cast-in-place) and steel piles, there are composite piles made from a combination of these two materials, as well as wooden piles.
- Method of Load Transmission
Finally, piles can be classified based on how they transmit the loads they receive from the structure. Thus, if a pile rests on soil with high bearing capacity, the load is transmitted primarily through the tip, and these are known as column piles. Conversely, if a large portion of the load is transmitted through friction between the shaft and the soil, they are known as floating piles.
Situations in Which Deep Foundations Are Used
Now that we have reviewed the different classifications of piles based on various criteria, the question we must ask ourselves is: When are deep foundations necessary?
Below, we'll outline some of the most common situations in which this solution is chosen:
- When the load transmitted through the structure exceeds the settlement load, which occurs with high loads or low-strength soil.
- In cases where the foundation footings are very limited in size, such as in the foundations for industrial equipment.
- For heterogeneous soils where the stable subgrade is found at different elevations, thereby reducing differential settlement.
- In areas where surface water is present, preventing the construction of shallow foundations, such as along a riverbed.
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