Deep foundations: types, design, and construction

When the superficial layers of the soil are not suitable to support the loads of the construction to be built, it is necessary to transfer these efforts to the deeper layers, through deep foundations.

Let’s see which types of deep foundations are usually used and which structural calculation tools can support the design.

Direct and deep foundations: what changes

By foundation, we mean the technological unit that acts as a static connection between the building and the ground and that is responsible for transmitting the loads imposed on the construction to the soil. Therefore, the structures in elevation, together with the foundations and the ground, constitute a single functional unit.

To design the foundations, data concerning the entire functional unit are essential and include:

• the characteristics of the soil (derived from an appropriate plan of geological, geophysical, and geotechnical surveys conducted on-site and laboratory analyses);
• the loads transmitted from the building to the ground (evaluated based on the type of structure to be built);
• the morphology of the intervention area (identified through field surveys conducted with various data acquisition techniques);
• the depth of the foundation base level.

Based on this data, it is important for the designer to choose the type of foundation, based on the morphology (extent of the bearing surface, etc.), the static behavior, and the characteristics of the soil on which they rest.

Macroscopically, foundations can be classified as:

• direct, when they transmit the loads of the construction to the most superficial layers of the soil (raft foundations, pile caps, etc.);
• deep, when they transfer the efforts to the deeper layers of the soil (pile foundations, micropiles, etc.).

In turn, these macro categories are divided into further subcategories, based on their morphology and the method of load transfer.

Configurations of foundations on plinths

Below we delve into types and characteristics related to deep foundations.

To delve into shallow foundations instead, read this article Shallow foundations: types and BIM design methods.

What are deep foundations

Deep foundations, or deep footings, are structural elements used to transfer the load of a building structure to the underlying soil through layers of soil that are more stable and capable of supporting the weight. This type of foundation is essential when the surface soil is unsuitable for supporting the weight of the construction or when it is necessary to reach more compact and resistant layers.

Foundations on piles

Types of deep foundations

Deep foundations can be divided into different types, including:

• Driven or drilled piles: vertical structural elements reach the required depth by driving or drilling, with or without soil removal. They are made using different techniques and technologies and in different materials including:
  • Wood, mainly used in historical buildings, generally have a diameter of around 20 cm and a maximum length between 8 and 10 meters;
  • Reinforced concrete, usually with a diameter of about 30 cm, can reach depths of up to 30 m, ensuring strength and durability over time;
  • Bentonite, a clay material derived from the alteration of vitreous effusive rocks. When in contact with water, it becomes an impermeable and water-repellent gel, characterized by a significant volume increase (15-20 times the initial volume) and by an absorption that makes the mass weigh 5 times more than an equal amount of dry material. These types of piles are used in the presence of particularly incoherent soils or very shallow water tables;
    
  • Franki, the metal tube-shaped pile is characterized by a concrete tip. Once driven into the ground, the concrete tip will form the base of the pile, settled on the most resistant soil layer, at the chosen level;
  • Simplex, consisting of a tube-shaped pile with a removable tip. The metal tube-shaped pile is driven into the ground, preventing the excavated soil from being invaded by debris during pouring. Once driven into the ground, the tip is opened to allow the pouring of concrete. At the end of the operation, the tube is withdrawn;
    
  • Benoto, a particular type of drilled pile, characterized by significant diameters (up to 2 m), suitable for supporting very heavy loads (in the order of several tons).
• Micropiles (or small-diameter piles): similar to drilled foundation piles but of smaller dimensions (generally with a diameter between 10 and 30 cm), micropiles are used when space is limited or when a lesser impact on the surrounding soil is required, as well as for soil consolidation interventions in addition to being a foundation element for elevated structures. They can reach depths between 2 and 50 m.

Types of deep foundations

Executive Technology of Deep Foundation Piles

The realization of deep foundation piles is a complex process that requires the use of specialized equipment. This process allows the construction of “columns” in reinforced concrete within the ground, with the base placed at significant depths, corresponding to soil layers with adequate mechanical load-bearing characteristics. Thanks to this system, it is possible to transfer surface loading forces to deeper and generally more resistant soil layers, allowing the construction of stable foundations even in the presence of soils with poor mechanical characteristics. There are several techniques for the construction of deep foundation piles, including Continuous Flight Auger (CFA) piles or continuous helical piles, Full Displacement Piles (FDP), etc.

Here are the main phases involved in the execution process of deep foundation piles:

• rotary drilling excavation: the first phase involves excavating the soil using a hydraulic drill equipped with various tools suitable for the type of soil. Different tools are used such as the centrifugal auger for low and medium consistency or loose soils, spiral drills with different types of teeth to penetrate rocky layers, and core barrels with picks to tackle rocks or harder soils;
• placement of reinforcement: after removing the excavation material, a pre-assembled metal cage is inserted into the drilled hole, formed by longitudinal bars wrapped in a welded spiral. This metal cage constitutes the reinforcement of the pile, increasing structural resistance and providing support during the concrete pouring;
• concrete pouring: the final phase involves pouring concrete into the drilled hole. The concrete, after the necessary curing times, will form a single body with the previously placed steel reinforcement. The concrete is usually poured using specific pumps or through the use of concrete mixer trucks and then compacted to remove any air bubbles and ensure uniform distribution.

It is important to note that, depending on the type of soil, the possible presence of a water table, or specific executive and design requirements, two main technologies for drilling small diameter piles are distinguished:

• rotation or roto-percussion with fluid circulation used as a means for disintegration and transport of excavation material. Generally used in cohesive and incoherent soils under a water table. The operation involves the rotation of a chisel, placed at the base of a battery, which crushes the soil by rotating (hence the technique is also called “core destruction drilling”). The circulation of the drilling fluid (composed of bentonite mud or polymers) pumped into the hole is intended to cool and lubricate the chisel, remove debris, and stabilize the hole walls during drilling operations. The components of the drilling system must be sized in relation to the depth to be reached, the diameter of the pile to be constructed, and the type of soil to be drilled;
• roto-percussion with down-the-hole hammer and extraction of excavation material with compressed air without the use of drilling fluids; generally used in incoherent soils in the absence of a water table.

and two methods of concrete pouring:

• in a single low-pressure pour, through the reinforcement tube, achieving a sufficiently uniform steel element cover (usual diameters 13-30 cm);
• in multiple high-pressure pours, using non-return valves, in order to increase the flow even in poor soils (usual diameters 10-20 cm).

Self-Drilling Micropiles: What Are They

Self-drilling micropiles represent an innovative solution in the field of foundations, offering a combination of speed, efficiency, and strength. This type of micropile is created by rotary percussion drilling a threaded hollow steel bar into the ground, equipped with a sacrificial drilling point. During the process, a cement grout is used as the drilling and washing fluid.

The process of creating self-drilling micropiles involves:

• drilling and inserting the bar: drilling begins with the insertion of the bar into the ground, using specialized machinery. The bar is designed to intercept the resistant soil layer, ensuring a stable base for the transfer of structural loads;
• creation of the cementitious bulb: during drilling, a cementitious bulb forms around the bar, acting as an anchoring element, allowing the transfer of loads to the surrounding soil. This process is essential to ensure the stability and strength of the foundation;
• corrosion protection: cementing the bulb not only ensures structural stability but also protects the steel bar from corrosion, prolonging its lifespan and ensuring the long-term safety of the structure.

The construction process takes place in a single phase, which includes drilling, bar insertion, and cementation. This makes the technique particularly fast and efficient, reducing construction times and overall project costs.

Unlike other techniques, there is no need to remove soil during drilling, making self-drilling micropiles ideal for sites where soil contamination and disposal of excavated material would be problematic and costly.

This technique has various applications, including:

• consolidation or integration of existing or new building foundations;
• consolidation of various types of infrastructures;
• consolidation of slopes and small landslides;
• consolidation of industrial pavements to allow load increases.

Moreover, the versatility of self-drilling micropiles also allows for the installation of inclined piles to optimize load transfer at depth or drilling into an existing foundation for consolidation or integration. Thanks to their numerous advantages and various possible applications, self-drilling micropiles represent a reliable and efficient solution for geotechnical and structural challenges in various construction and civil engineering contexts.

Design Tools for Indirect Foundations

Before the construction phase and execution of deep foundation piles and micropiles, all challenges related to the design must be addressed.

Using BIM software for structural calculation offers significant benefits in the design of foundation piles and indirect foundations in general. Firstly, BIM software allows for the creation of detailed and accurate digital models of the building structure and the surrounding soil, integrating geometric, structural, and geotechnical information into a single model. This approach allows for a more comprehensive and precise assessment of site conditions and structural requirements, enabling informed decision-making during the design of foundations.

Furthermore, BIM software provides advanced tools for structural analysis and simulation of the behavior of foundation piles under different loads and loading conditions. These tools allow for evaluating the suitability of the designed piles to withstand the expected load and identifying any critical or overloaded areas. Additionally, the ability to perform dynamic analyses and simulate the seismic behavior of piles helps improve the safety and stability of the structure in case of seismic events.

Another significant advantage of using BIM software for structural calculation is the ability to optimize the design of foundation piles based on the specific project requirements. Designers can explore a wide range of foundation solutions and quickly assess the impact of different options on structural performance and costs. This enables the identification of the optimal solution that maximizes structural efficiency and minimizes construction costs.

Finally, integrating BIM models with other software and BIM management platforms allows for greater collaboration and coordination among the various design team members and stakeholders. This promotes more effective communication, reduces the risk of errors and conflicts during the construction phase, and improves the overall project delivery.

Here is a video showing how to insert foundation piles in a BIM model using structural calculation software.