How is pile fatigue calculated in AllWave-PDP?

AllWave-PDP calculates pile fatigue by tracking the stress cycles generated at each depth increment during a simulated driving sequence, then comparing the accumulated damage against material-specific S-N curves using Miner’s rule. The calculation runs automatically as part of the driveability prediction, so fatigue assessment is integrated into the same simulation that predicts blow count, penetration, and pile stresses. The sections below unpack each step of that process in detail.

What inputs does AllWave-PDP use to calculate pile fatigue?

AllWave-PDP requires three categories of input to calculate pile fatigue: a complete description of the pile geometry and material, a soil resistance profile derived from site investigation data, and a defined hammer configuration. Together, these inputs drive the stress-wave simulation from which fatigue-relevant stress histories are extracted at every depth increment throughout the installation.

On the pile side, the program needs cross-sectional dimensions, wall thickness (for tubular steel piles), elastic modulus, yield strength, and the material’s fatigue classification. These properties determine both the stress magnitudes the pile experiences and which S-N curve applies to the fatigue assessment.

For the soil, AllWave-PDP accepts CPT, SPT, and laboratory data as the basis for building a Soil Resistance to Driving (SRD) profile. The soil model accounts for friction fatigue, the progressive degradation of local shaft friction as the pile tip advances past a given soil layer and that layer accumulates more loading cycles. Without this effect, SRD profiles for long piles would be significantly overestimated, and the resulting stress cycle amplitudes would be unreliable.

The hammer input includes the hammer type selected from the built-in hammer library, stroke or energy settings, cushion properties, and helmet mass. These parameters control the energy delivered per blow and therefore the peak stress amplitude at each depth, which is the primary driver of fatigue damage accumulation.

How does AllWave-PDP count stress cycles during pile driving?

AllWave-PDP counts stress cycles by applying the rainflow counting method to the stress-time history computed at each node along the pile for every hammer blow. The one-dimensional stress-wave simulation, based on the Method of Characteristics, calculates how compressive and tensile stress waves travel through the pile, interact with shaft resistance and toe resistance, and reflect back. Each blow produces a stress-time trace at every depth, and rainflow counting extracts the individual stress ranges and their cycle counts from that trace.

The process repeats for every blow across the full penetration sequence. Because blow count varies with depth, the number of stress cycles accumulated at a given pile section depends on how many blows were needed to drive through the overlying soil. Sections near the pile head experience every blow from the start of driving to final penetration, while sections near the toe only accumulate cycles from the blows delivered after the tip reaches that depth.

This depth-dependent accumulation is physically realistic and important. A pile driven through 60 metres of soil will have its upper shaft sections exposed to far more stress cycles than its lower sections, and the fatigue damage distribution along the pile length reflects that asymmetry directly in the AllWave-PDP output.

Which S-N curves does AllWave-PDP apply to pile fatigue?

AllWave-PDP applies S-N curves consistent with the fatigue design standards relevant to the pile material and application, most commonly the curves defined in DNV-RP-C203 for steel piles in offshore and marine environments. The appropriate curve depends on the weld classification or structural detail category assigned to the pile section being assessed, since welds and geometric discontinuities reduce fatigue resistance relative to the parent material.

For open-ended tubular steel piles, which dominate offshore wind and oil and gas foundation work, the relevant S-N curves distinguish between air and seawater environments, with and without cathodic protection. Seawater without cathodic protection significantly reduces fatigue life compared to the in-air reference curves, and AllWave-PDP allows the user to specify the exposure condition so the correct curve is applied.

The S-N curve input is user-defined in the sense that the engineer selects the appropriate curve for the pile’s structural detail category and environment. The software then uses that curve to convert the stress ranges extracted by rainflow counting into partial damage values. This means the quality of the fatigue result depends directly on the engineer’s correct identification of the governing weld class and environmental condition, not just on the accuracy of the driving simulation.

How is cumulative fatigue damage calculated from driving data?

AllWave-PDP calculates cumulative fatigue damage using Miner’s rule, also called the Palmgren-Miner linear damage hypothesis. For each stress range extracted by rainflow counting, the program divides the number of cycles at that stress range by the number of cycles to failure at the same stress range as read from the S-N curve. Summing these ratios across all stress ranges and all blows gives the cumulative damage index D at each pile section.

A damage index of D = 1.0 represents theoretical fatigue failure. Design standards typically require D to remain well below 1.0 after applying a safety factor, often 0.1 to 0.5 depending on the consequence class and inspectability of the pile. AllWave-PDP reports D as a function of depth, so your team can immediately identify which sections of the pile accumulate the most damage during installation.

It is worth noting that Miner’s rule is a linear model and does not capture sequence effects, meaning it treats high-amplitude cycles and low-amplitude cycles as independent regardless of the order in which they occur. For most pile driving scenarios this is an accepted engineering approximation, but it is one of the explicit assumptions that should be reviewed when interpreting results for piles with unusual driving sequences or highly variable soil profiles. This reflects the broader principle embedded in AllWave-PDP’s approach: the output is a well-founded prediction, not a deterministic answer, and engineering judgement remains part of the assessment.

What does the fatigue output in AllWave-PDP tell you about pile integrity?

The fatigue output in AllWave-PDP tells you the cumulative damage index D at every depth along the pile after the full simulated installation, giving you a direct picture of where fatigue risk concentrates and whether the pile remains within acceptable limits before it even enters service. Sections where D approaches or exceeds the design threshold signal that the current hammer selection, driving sequence, or pile wall thickness may need to be revised.

Beyond the headline damage index, the output connects fatigue to the other integrity checks AllWave-PDP performs simultaneously. A section that shows high compressive or tensile stress peaks in the stress envelope is typically also the section that accumulates the most fatigue damage, because both effects stem from the same stress-wave dynamics. Reviewing fatigue output alongside maximum tension and compression stresses gives a more complete picture of pile integrity than either check alone.

The fatigue results also have a practical planning function. If the damage index is acceptable for the primary hammer but marginal for a backup hammer with higher energy per blow, that information directly informs contingency planning before mobilisation. Similarly, if the model shows that a specific depth interval, often where soil resistance peaks sharply, drives most of the cumulative damage, your team can evaluate whether pre-drilling, soil improvement, or a reduced stroke at that interval would bring the damage index to an acceptable level without compromising penetration.

Because AllWave-PDP generates fatigue output as part of the same simulation that predicts blow count, installation time, and refusal risk, the results support a genuinely integrated assessment. Fatigue is not a separate post-processing step but a parallel output of the AllWave software package driveability simulation, which means any change to hammer, soil model, or pile geometry updates the fatigue picture automatically.

How We Support Pile Fatigue Assessment and Driveability Prediction

We work with foundation contractors, offshore developers, and engineering consultancies at every stage where pile fatigue and driveability matter, from early design through installation monitoring and post-installation review. Our support covers both the technical execution of AllWave-PDP simulations and the engineering interpretation of results.

Driveability studies with integrated fatigue assessment: We run full AllWave-PDP simulations that combine blow count prediction, stress envelope analysis, and cumulative fatigue damage in a single workflow, giving your team a complete picture of installation risk before mobilisation.
Soil model development from CPT and laboratory data: We build site-specific SRD profiles that incorporate friction fatigue and depth-dependent soil behaviour, so the stress cycles used in the fatigue calculation reflect realistic driving conditions rather than simplified assumptions.
Hammer selection and optimisation: We evaluate multiple hammer configurations and stroke settings to identify the combination that achieves target penetration while keeping fatigue damage and peak stresses within design limits.
Installation monitoring and back-analysis: During driving, we use Dynamic Load Testing and Pile Driving Analysis to measure actual stress histories and compare them against the predicted values, refining the model and validating the fatigue assessment in real time.
Independent technical review: For projects where an independent check of driveability assumptions or fatigue calculations is required, we provide expert review and written assessment against applicable standards.

If your project involves complex soil conditions, long steel piles, or tight fatigue budgets, contact us to discuss how we can support your driveability and fatigue assessment work. Reach out to our team to start the conversation.