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Foundation Piles: Design Guidelines

Foundation piles are structural elements that allow reaching the most resistant soil layers located at depth. Discover how to design them

There are situations where it is not possible to find sufficiently resistant soil on the surface to safely and stably support a structure. However, thanks to deep foundations and in particular to foundation piles, it is possible to effectively solve this problem.

In the following article, we will explore the different types of foundation piles, each with specific characteristics. We will see how these elements can be used according to the project’s needs and the soil characteristics. We will also provide some useful tips for their design.

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What are foundation piles?

Foundation piles are structural elements that fall into the category of “deep or indirect foundations”, as they have the ability to transfer the vertical load of the structure to more resistant and stable soil layers located at greater depths.

Unlike shallow foundations, which transfer the load to the ground only through the bearing plane, pile foundations transmit the load both by friction along the lateral surface of the foundation and by point load, depending on the ratio between the pile diameter and its height.

They can be made of different materials, such as reinforced concrete, steel, or wood. They are cast in place or driven into the ground to reach the bearing layers and provide the necessary resistance against external forces that would not be manageable by a shallow foundation.

The main objective of foundation piles is to reduce long-term settlements, exploiting the mechanical characteristics of deep substrates and avoiding issues related to the presence, in the superficial layers, of soft soils, water tables, or voids.

When to use foundation piles?

Among the different types of foundations that can be used to ensure the safety and stability of a structure, it is generally advisable to rely on foundation piles when:

  • the surface soil has insufficient bearing capacity to support the structure’s load. Foundation piles allow reaching deeper and more stable soil layers, distributing the load effectively to avoid settlements and excessive deformations;
  • the structure needs to be isolated from surface soils that are subject to harmful phenomena such as erosion, washout, or the action of external agents. Foundation piles allow placing the foundations below these problematic soils, thus protecting the structure from harmful effects;
  • it is technically impossible to build shallow foundations, as in the case of offshore structures or in the presence of soils characterized by high filtration gradients, incoherent soils, or situations that make the use of other types of foundations impractical;
  • the expected settlements are incompatible with the type of structure to be built, as they exceed the maximum limit that the structure can withstand. Foundation piles, by reaching deep depths, allow for controlled management of absolute and differential settlements, thus ensuring long-term stability and safety for the structure.

Types of foundation piles

Foundation piles can be classified based on different criteria that take into account the material, dimensions, or construction technologies used for these structural elements.

With reference to dimensions, they can be divided into:

  • small-diameter piles with a diameter less than 25 cm and lengths mainly ranging from 5 to 20 m;
  • medium-diameter piles with a diameter between 30 and 60 cm, with lengths typically ranging from 5 to 25 m;
  • large-diameter piles with a diameter exceeding 80 cm and lengths often in the range of 20 to 40 m.

Depending on the material used, there can be:

  • wooden piles: made from tree trunks and characterized by a trunk with straight fibers, free of impurities and knots. They are driven into the ground by pounding. The higher the resin content in the wood, the higher the strength and durability of the pile. The availability of large wooden piles may be limited, and their durability over time may be influenced by soil moisture and xylophagous organisms;
  • metal piles: made of steel, these piles offer greater strength and durability and are suitable for situations requiring high load-bearing capacity. They are often used in the construction of skyscrapers or industrial structures. During the installation process, metal piles are driven or drilled, depending on the construction technique adopted. Compared to other types of piles, they are more resistant and durable, making them particularly suitable in projects requiring special robustness and structural strength;
  • concrete piles: the majority of pile foundations are made using reinforced concrete piles. This type can be divided into two main categories:
    • precast piles: this category includes reinforced concrete piles that are prefabricated outside the construction site. After a suitable curing and hardening period, they are transported to the installation site. Here, they are driven into the ground using techniques such as pounding or drilling, leveraging their structural strength;
    • cast-in-place piles: this category includes reinforced concrete piles that are built directly on-site, unlike prefabricated ones. The distinctive feature of this type is the method of inserting the metal casing, used as a formwork. This casing can be driven into the ground by pounding or drilling and can be of the “disposable” type or recovered once the concrete has completed the hardening process.

Finally, based on the construction technology adopted for the pile, a distinction can be made between:

  • driven piles: made by inserting the pile into the ground through the pounding or percussion process. This process involves driving the pile by pushing it, without removing material, but only by moving or compacting it;
  • drilled piles: involve the removal and replacement of soil using drilling equipment. Typically, a drill or auger is used to bore the ground, removing the material during the operation. Subsequently, the pile is inserted into the created hole. This method is particularly useful in the presence of more compact or rocky soils, where simple pounding may be ineffective. The removal and replacement of soil allow for the creation of a more resistant and stable foundation.
Types of foundation piles

Types of foundation piles

Foundation piles calculation: Factors and Steps

The calculation of foundation piles is a complex process that requires an accurate evaluation of various geotechnical, structural, and environmental factors. The main phases involved in this process are:

  • Soil characterization: before starting the design, it is essential to conduct a proper geological and geotechnical survey to understand the characteristics of the soil on which the foundations will be built. This study may include various tests (such as boreholes, static penetration tests, pressuremeter tests, etc.). The collected data will provide crucial information about the soil composition, its strength, the presence of water tables, and other factors that can influence the foundation design;
  • Load definitions: the next step is to determine the actions that the foundations will have to withstand. Based on the soil characteristics and the expected loads, the most suitable type of pile is selected;
  • Pile sizing: the sizing of the piles is carried out taking into account the soil characteristics and the applied loads. Parameters such as the pile tip resistance, lateral resistance, etc., are considered;
  • Verification of bearing capacity: it is necessary to verify that the piles are able to support the expected loads without exceeding the allowed limits. This is done by calculating the bearing capacity of the piles and comparing it with the applied loads. The bearing capacity can be determined using analytical methods or by performing load tests on piles.
Calculation of pile foundations - EdiLus

Calculation of foundation piles – EdiLus

To simplify the calculation and design of a foundation pile, you can rely on specialized tools such as a structural engineering analysis software. To delve deeper into the design and calculation process, we recommend watching the example videos below on how to “Insert foundation piles” and “Define soil material layers for foundations and set options for calculating the ultimate load”.

How to insert foundation piles

How to define soil material layers for foundations and set options for calculating the ultimate load

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