Dynamic load testing on piles can be a powerful ally for the project, or merely a decorative piece of paper, depending on the quality of the report. For the client, the central point is knowing exactly what to demand to obtain traceable results, compatible with NBR 6122 and technically usable in the project and on the construction site.
Below, I present a practical guide, structured so that engineers of any level can critically evaluate dynamic load test reports, focusing on methodology, signals, processing, parameters, and traceability.
1. Normative Role of Dynamic Load Testing
NBR 6122:2022 explicitly recognizes dynamic loading tests as one of the procedures for estimating the load capacity of piles, complementing static tests and semi-empirical methods. Furthermore, the standard refers to NBR 13208 for the execution and interpretation of dynamic tests, based on the one-dimensional wave equation theory.
Brazilian and international studies show good correlation between dynamic load tests interpreted with CAPWAP and static tests, when the test is well planned and conducted. Thus, the dynamic load test report ceases to be merely an “annex” and becomes a fundamental piece of geotechnical validation for the deep foundations project.
2. What is Dynamic Load Testing?
In high-strain dynamic load testing, a controlled impact is applied to the pile head, simultaneously recording acceleration and strain, and interpreting these signals using the wave equation. From these records, the software reconstructs the distribution of stresses along the shaft and tip, estimating shaft and tip resistances, and generating a load versus displacement curve equivalent to the static one.
This procedure is standardized by specific norms, such as ABNT NBR 13208 in Brazil and ASTM D4945 internationally, which define requirements for instrumentation, minimum energy, number of blows, and validity criteria for signals. Therefore, the report the client receives needs to explicitly demonstrate that these requirements were met or, if not, what limitations arise therefrom.
3. Test Methodology that Must Appear in the Report
The first section the client should check is the methodological description. It must allow any engineer, reading the report, to understand how the test was conceived, executed, and interpreted.
3.1 Normative References and Objective
The report must clearly state the adopted standards and technical documents, for example:
- NBR 6122 for design criteria and the use of dynamic tests in capacity verification.
- NBR 13208 for the dynamic loading test procedure on piles.
- Any supplementary references, such as PDA and CAPWAP technical guides or relevant scientific literature on the correlation between dynamic and static tests.
In addition, the objective of the test must be explicit. For example, verification of geotechnical capacity of test piles, production control, correlation with static testing, or calibration of semi-empirical methods.
3.2 Planning the Testing Program
The report must present the testing program in a summarized and traceable manner:
- Total number of piles on the construction site and number of dynamically tested piles, by typology.
- Criterion for pile selection: test piles, representative production piles by geotechnical region, piles with higher design load.
- Dates of pile execution and test dates, allowing evaluation of setup or soil relaxation effects.
NBR 6122 allows for the replacement of part of static load tests with dynamic tests, in a proportion of five dynamic tests for each static test, provided that representativeness criteria are met. Therefore, the report needs to make it clear what sampling was adopted and how it relates to the entire set of piles on the construction site.
4. Instrumentation and Signal Quality
Without adequate instrumentation, there is no reliable dynamic load testing. The report must document how the pile was instrumented and how the signals were verified.
4.1 Type and Position of Sensors
The report must provide, for each tested pile:
- Number of strain and acceleration sensor pairs installed.
- Distance of sensors from the pile cut-off elevation and the pile head at the time of testing.
- Side of the pile where the sensors were affixed and type of fixation used, ensuring adequate coupling.
Instrumentation studies demonstrate that the sensor position influences the quality of interpretation, especially in separating shaft and tip components. Therefore, this information cannot be missing.
4.2 Equipment, Calibration, and Acquisition Configuration
The client must require that the report presents, at least:
- Model of the acquisition system (e.g., PDA or equivalent), firmware or software version, and company responsible for the operation.
- Sampling frequency used and duration of the recording window per blow, demonstrating compliance with NBR 13208 and manufacturer recommendations.
- Sensor calibration information, with date and verification procedure, even if in summarized form.
These elements allow evaluating whether the measurement chain is compatible with the quality required for wave equation analysis.
5. Signals that the Dynamic Load Test Report Must Present
Signals are the “heart” of the test. Reports that only present a table with calculated capacities, without raw signals or at least representative graphs, compromise technical traceability.
5.1 Time × Force and Time × Velocity Records
It is recommended that the report includes, for at least one representative blow on each pile:
- Force × time graph at the pile top, in units consistent with the analysis.
- Velocity or displacement × time graph corresponding to the same blow.
- Clear indication of the impact instant and main reflections, with a legible time scale.
These graphs allow the engineer to qualitatively check the quality of coupling, the absence of spurious noise, and the coherence between force and velocity.
5.2 Separated Downward and Upward Wave Signals
In interpretation using the Case method and CAPWAP, the measured force is decomposed into a downward wave and a reflected wave, associating resistance mobilization over time and depth. Therefore, the report must, at a minimum, present a clear description of how this decomposition was performed and, ideally, illustrative graphs.
When the report does not show this decomposition, the client loses the ability to evaluate whether the adopted soil model is consistent with the observed behavior.
6. Processing, CASE, CAPWAP, and Interpretation Criteria
The numerical results of dynamic load testing depend strongly on the analysis model used. Therefore, the processing section of the report is critical.
6.1 Description of Analysis Methods
The report must state, for each pile:
- Method used for initial capacity estimation: for example, Case method with specific factors.
- Signal matching refinement software, such as CAPWAP or equivalent, with version indication.
- Criterion adopted for selecting the “test” blow, including drop height, energy, and achieved set.
Research comparing dynamic tests with static tests shows that consistency in energy criteria and blow selection is crucial for the approximation between capacities obtained by HSDT (High-Strain Dynamic Testing) and SLT (Static Load Testing).
6.2 Soil Model and Adjustment Parameters
The client must require that the report presents, even if in summarized form:
- Initial local stiffness (quake) parameters adopted for shaft and tip.
- Dynamic damping coefficients assumed along the shaft and at the base.
- Final distribution of shaft friction and tip resistance obtained in the adjustment, with indication of whether tip or shaft resistance was fully mobilized.
Such parameters are not “internal details” of the software. On the contrary, technical documents emphasize that the interpretation of dynamic tests should always explicitly state stiffness and damping assumptions so that other professionals can re-evaluate the result, if necessary.
7. Essential Geotechnical Parameters in the Report
Once the methodology, signals, and processing are understood, the client needs to look at the numbers that truly matter for the project.
7.1 Ultimate and Allowable Capacities
For each tested pile, the report must clearly present:
- Estimated geotechnical ultimate load from the dynamic test, separating shaft friction and tip resistance.
- Criterion used to transition from ultimate capacity to allowable load or design resistance force, explicitly stating safety factors or weighting coefficients.
NBR 6122 indicates typical global safety factors between 1.6 and 2.5 for the transition between ultimate load and allowable load, depending on whether the basis is testing or calculation. Thus, the report must clearly state which values were adopted and why.
7.2 Shaft × Tip Distribution
It is important that the report presents the decomposition of total resistance into:
- Portion mobilized by lateral friction along the shaft.
- Portion mobilized by tip resistance.
Well-interpreted dynamic tests allow reconstructing this distribution with good accuracy, which is fundamental for discussing group effects, negative skin friction, and settlement behavior. Without this decomposition, the use of the result for calibrating semi-empirical methods (Aoki-Velloso, Décourt-Quaresma, average coefficients, etc.) is limited.
8. Structural Verification of the Pile from the Dynamic Test
Dynamic load testing not only evaluates geotechnical capacity. It also allows verifying whether structural stresses during impact remain within the limits of NBR 6122 and structural design standards.
The report must present, for each pile:
- Maximum compressive stress measured or calculated during the test blows, compared with the design strength of concrete or steel.
- Maximum tensile stress, when relevant, especially for metallic, composite, or tension piles.
- Explicit comment on pile integrity, indicating whether signals suggest discontinuities, damage, or sudden changes in stiffness.
This is consistent with the approach of NBR 6122, which treats the allowable load of a pile as the lesser of the geotechnical capacity and the structural capacity.
9. Traceability: Connecting the Test to the Project
Traceability is the point that usually separates robust reports from fragile documents. The client needs to ensure that, years later, it is possible to understand exactly what was done and compare results with the actual performance of the structure.
9.1 Complete Pile Identification
Each tested pile must be uniquely identified in the report:
- Pile number or code, according to the foundation design.
- Axis, grid, or plan coordinates of the structure.
- Pile type, diameter, section, total length, and designed and as-built tip elevation.
- Concrete, steel, or grout used, with fck or fyk and dates of concreting or driving.
This data allows comparing the test result with the original design, whether in geotechnical or structural terms.
9.2 Link to the Geotechnical Project
The report must explicitly state how the obtained capacities compare with:
- Project working loads for each pile and for the corresponding group.
- Capacity predictions obtained by semi-empirical methods from SPT or CPT, when available.
- Any static load tests performed on piles from the same construction site or geotechnical region.
NBR 6122 expressly recommends that the interpretation of load tests, static or dynamic, consider the local stratigraphy, calculation methods used in the project, and the ultimate and serviceability limit states. Thus, simply presenting a table of capacities is not enough. The global coherence of the set must be discussed.
9.3 Storage of Raw Files and Possibility of Reanalysis
Finally, the report must record that:
- Raw test files (force and acceleration records per blow) have been stored and can be provided to the contractor upon request.
- The version of the interpretation software is documented, allowing for eventual future reprocessing with updated parameters.
Practices reported in the literature show that reanalyses of dynamic tests with new soil models are common and useful in long-term performance studies. Without raw files and clear metadata, this possibility is lost.
10. How the Client Can Use This Checklist in Practice
In practice, the client can adopt the following flow when analyzing a dynamic load test report:
- Check if the report states adopted standards and test objective.
- Verify if the pile sampling meets the minimum requirements of NBR 6122, given the total number of piles and working stresses.
- Check for documentation of instrumentation, equipment, and acquisition configuration.
- Confirm the presence of representative and coherent force × time and velocity × time graphs.
- Verify if the analysis clearly presents: ultimate capacity, allowable load, shaft × tip distribution, and structural verification.
- Assess if traceability is complete, linking pile, test, and project.
If any of these sections are absent or poorly documented, the client has a technical and normative basis to request complements or discuss the validity of the test as an instrument for foundation acceptance.
Content structured in this way helps consolidate a culture of deep foundations based on standards, well-documented testing, and traceable decisions, reducing uncertainties and conflicts throughout the structure's lifespan.
Geoteste performs dynamic load testing (PDA test) throughout Brazil. Consult service regions and request your quote.
