Introduction to dynamic loading test and the concept of set
The high-strain dynamic loading test, globally known as PDA due to the English acronym for Pile Driving Analyzer, constitutes a widely employed method in geotechnical engineering for evaluating the behavior of deep foundations. The execution of the procedure complies with the requirements established by the Brazilian standard ABNT NBR 13208 and international standards such as ASTM D4945. During the application of impact to the pile head by a hammer, instrumented sensors record strains and accelerations as a function of time, generating fundamental data for signal analysis.
Within this context of instrumentation and data recording, the term set plays a central role in interpreting the results obtained in the field. It is a fundamental quantity for correlating the energy applied by the driving equipment or the test hammer with the resistance mobilized by the foundation element at each blow. Understanding the exact definition of this parameter and the correct way to obtain it prevents misconceptions in the evaluation of bearing capacity.
What is set and how it relates to permanent displacement
Set can be technically defined as the permanent penetration that the pile undergoes with each hammer blow. In practical field terms, it represents the irrecoverable displacement of the foundation element after the occurrence of dynamic impact and the subsequent relief of elastic stresses. Set should not be confused with the total elastic displacement that the pile experiences during the passage of the compression wave.
While the stress wave travels along the pile shaft, the material compresses elastically, and the surrounding soil undergoes deformations that contain elastic and plastic components. After the dynamic event ends, the pile partially returns to its original position due to elastic recovery, but retains a portion of accumulated penetration. This residual penetration is the set, typically measured in millimeters per blow.
Difference between set and rebound
For the correct interpretation of the PDA test and driving operations in deep foundation works, it is imperative to distinguish set from another related concept, which is elastic rebound:
- Set: corresponds to the permanent displacement or definitive penetration of the pile during a blow.
- Elastic rebound: represents the recoverable elastic movement portion that the pile head performs immediately after reaching maximum displacement under the action of impact.
The sum of set and elastic rebound results in the maximum total displacement suffered by the element during impact. Geotechnical engineers use these quantities together with the force and velocity signals obtained by the PDA transducers to feed the signal analysis algorithms and estimate the mobilized static resistance.
The importance of set in PDA interpretation
The precise recording of set in the field provides indispensable calibration for wave equation-based analysis methods, modeled by specific software associated with the PDA equipment. The energy transferred to the pile depends directly on the hammer mass, drop height, and efficiency of the load application system. When the field-measured set is compared with the set calculated by the numerical wave equation model, the engineer obtains a highly reliable control parameter.
If the observed set is significantly smaller than predicted by the model for a given energy, this may indicate a static resistance greater than expected at that moment of the test. On the other hand, an excessively high set for the same applied energy may point to insufficient mobilization of toe resistance or problems associated with the shaft integrity. Therefore, set acts as a direct indicator of the immediate geotechnical response of the soil-pile system under dynamic loading.
Measurement procedure and operational care
Correct set acquisition requires technical rigor during the execution of the PDA test in the field. Responsible professionals must use appropriate measuring devices to record permanent displacement with millimeter accuracy at each instrumented blow. For precast concrete or metallic piles, references are marked on the shaft so that visual reading or by optical and electronic instruments captures the exact penetration value.
Among the main operational precautions required by technical standards, highlights include:
- Ensuring that the impact system is perfectly aligned with the longitudinal axis of the pile, avoiding eccentricities that generate bending and false penetration readings.
- Verifying that the pile head has adequate preparation conditions, with an intact cushion to ensure efficient transfer of dynamic energy.
- Recording the number of blows and the hammer drop height simultaneously with the electronic acquisition of force and velocity signals.
- Discarding blows where anomalous eccentricities or failures in the coupling of piezoelectric sensors and accelerometers occur.
Limitations in set interpretation
Although set is an indispensable parameter in dynamic analysis, its isolated interpretation does not replace a complete engineering model. The relationship between permanent penetration and the pile's bearing capacity depends on complex variables, such as the type of soil encountered, the geotechnical system's stiffness, dynamic soil damping, and temporal phenomena of resistance gain or loss, such as pile setup and relaxation.
Furthermore, the set value obtained during the test reflects the behavior of the pile under the high-strain dynamic loading applied at that specific moment. Extrapolating this result to predict the long-term behavior of the foundation or the performance of the entire pile group requires complementary analyses, considering the general guidelines of standard ABNT NBR 6122 for foundation design and execution.
Specialized technical support in dynamic testing
The correct interpretation of set and other parameters generated by the dynamic loading test requires deep knowledge of soil mechanics, wave propagation in continuous media, and current regulatory criteria. Errors in obtaining permanent displacement or in calibrating the wave equation can compromise the evaluation of the foundation's bearing capacity.
Geoteste has a highly qualified technical team and state-of-the-art instrumentation for the execution and interpretation of PDA tests in accordance with ABNT NBR 13208 and ASTM D4945. We operate in all stages of quality control for deep foundations, providing accurate diagnoses to ensure the safety and efficiency of your project. Contact our engineers to discuss your project needs and learn about our solutions in static, dynamic, and integrity tests.
