Foundation Bearing Capacity: Methods, Tests, and Results

The bearing capacity of foundations, along with their investigation and quality control stages, are decisive pillars for the safety and economic viability of any civil engineering project. In this context, geotechnical tests applied to foundations consolidate as instruments to validate the performance of structural elements in situ.

More than a simple check, these tests allow comparing the assumptions adopted during the design phase with the actual response of the foundation-soil system. This way, it is possible to ensure that project specifications are fully met. Furthermore, through these verifications, precise parameters regarding the bearing capacity of foundations and the deformational behavior of piles when subjected to service loads are obtained.

In this regard, the interpretation of these data is fundamental to mitigate risks of excessive settlements or structural failures. Therefore, according to the nature of the request and the objectives of the analysis, the methods for evaluating the bearing capacity of foundations are divided into two main categories: static tests and dynamic tests.

Firstly, static tests determine the bearing capacity of foundations through the application of known loads, gradually increasing and applied at equal time intervals. Simultaneously, the corresponding displacements are recorded, monitored until the pile reaches the programmed maximum load. Subsequently, the process requires performing the necessary unloadings.

On the other hand, dynamic tests aim to verify the behavior of the interaction between the foundation element and the soil through the application of an impact force at the top of the pile. Consequently, this technique allows for the agile evaluation of foundation bearing capacity and pile integrity.

1. Static Tests to Measure Foundation Bearing Capacity

Static Load Test (PCE)

The Static Load Test (PCE) has consolidated as the most reliable method for determining the bearing capacity of foundations and the stress-strain behavior. Furthermore, its central objective is to establish the correlation between the applied load and the vertical displacement (settlement) of the element, allowing the construction of the load-settlement curve.

The execution of the test consists of applying known and increasing loads on the top of the pile. In this sense, hydraulic jacks generate the force, which a stable reaction system must absorb. Such a system can use tie-downs on nearby elements or, likewise, a helical reaction system.

The load application occurs in stages, with time intervals defined by standards, such as NBR 16903/2020. These intervals must be monitored until the pile reaches the programmed maximum load, which typically corresponds to twice the expected working load, or until geotechnical failure and loss of lateral friction of the element is characterized.

Simultaneously, displacements (settlements) are recorded, followed by unloading of the pile. The main product of the PCE is the load versus displacement graph, an analytical tool where the curve is born from plotting the stabilized points at the end of each stage. Therefore, the analysis of this curve allows determining the bearing capacity of foundations and predicting how the structure will behave under actual loads.

This graph represents the response of the foundation and the soil to loading. Furthermore, the descending curve serves as a parameter for visualizing and understanding the results, facilitating comparison. The upper curve represents loading, while the lower one details unloading.

Plate Load Test (Static Load Test on Plate)

The plate load test is performed when there is an interest in defining the bearing capacity of shallow foundations, the vertical reaction coefficient, and the estimation of settlements. In this methodology, a static load must be applied on a rigid steel plate, generally with diameters of 30 cm, 45 cm, or 80 cm, positioned directly on the investigated soil.

Hydraulic jacks perform the loading. However, to enable the application of force, the test requires a robust reaction structure. It is common to use the operational weight of heavy machinery, such as hydraulic excavators or loaded trucks. However, the mass of this equipment must significantly exceed the maximum load to prevent lifting of the system.

The main result obtained is the graph of the relationship between applied load and vertical displacement of the plate. Consequently, the load versus displacement curve is obtained, similar to the Static Load Test (PCE). To ensure technical compliance, we follow the guidelines of NBR 6489.

2. Dynamic Tests to Evaluate Foundation Bearing Capacity

Pile Integrity Test (PIT), Low Strain

The PIT (Pile Integrity Test) is a non-destructive test aimed at verifying structural integrity. As it is a quick and practical method, PIT allows evaluating many piles in a short period of time. This provides a clear advantage for verifying structural integrity, which indirectly ensures the bearing capacity of foundations in mass.

The test consists of applying a low-energy impact to the top of the pile using a hand hammer. An accelerometer fixed to the pile head records the propagation of the generated stress wave. As the wave propagates along the shaft, reflections occur whenever there are variations in the geometric or mechanical properties of the pile, such as changes in cross-section or material defects.

The result is presented in the form of a velocity versus depth graph. In this context, positive reflections indicate reductions in cross-section, while negative reflections are associated with increases in cross-section. Although PIT does not directly provide the bearing capacity of foundations, it acts as a vital quality control tool.

Pile Driving Analyzer (PDA)

The PDA (Pile Driving Analyzer) is a dynamic test whose central objective is the evaluation of foundation bearing capacity and pile integrity. This procedure functions as a tool for verifying the performance of the structural element, allowing diagnosis of whether the pile maintains its physical continuity and has the necessary resistance according to the design.

The PDA is performed through a cycle of successive impacts on the pile head, generating an impact force that measures the interaction between the element and the soil. This process is interrupted when the pile block fails or when two times the element's design load is reached.

During execution, strain gauges and accelerometers record force and velocity values. From these records, information such as total and driven length, penetration resistance, compression force and displacement values, as well as dynamic material properties, such as wave propagation velocity, are obtained.

In this sense, interpretation uses Wave Analysis Theory, through the numerical method CAPWAP. This analysis allows estimating the bearing capacity of foundations in an equivalent static manner, in addition to decomposing the resistance into toe resistance and lateral friction components. Furthermore, the analysis provides data for verifying integrity along the entire length. For more technical details, consult international standards ASTM D4945.

Geotechnical Tests: Conclusion and Importance

Geotechnical tests applied to foundations play a fundamental role in verifying structural performance and reducing risks. The application of static and dynamic tests enables the evaluation of foundation bearing capacity, providing technical support for both project validation and execution quality control.

While static tests provide a direct representation of the load-displacement relationship, dynamic tests enable integrity verification and load estimation. Thus, the appropriate choice of methodology decisively contributes to ensuring the safety, performance, and durability of engineering works.

Geoteste Tests: Characteristics and Differentials

Secure

Allows verification of the load x deformation curve for the design load.

Agile

Optimized methodologies that ensure technical agility for your schedule.

Reliable

With the results, it is possible to ensure the safety of the project's foundation.