What are the different soil fatigue models in AllWave-PDP?

AllWave-PDP offers several soil fatigue models that simulate how soil resistance changes during pile driving as a result of repeated loading cycles. The available models include the Beta Factor model, the Alm and Hamre model, and CPT-based friction fatigue approaches, each representing a different level of complexity in capturing how soil strength degrades along the pile shaft. Understanding which model to apply, and how its settings shape your results, is what separates a reliable drivability study from one that carries hidden risk.

How does soil fatigue affect pile driving predictions in AllWave-PDP?

Soil fatigue affects pile driving predictions by reducing the effective shaft resistance along the pile as the number of loading cycles increases. In AllWave-PDP, this means that the soil resistance to driving (SRD) is not a fixed value but changes with penetration depth and the cumulative load history experienced by each soil layer. Ignoring fatigue leads to overpredicting resistance, which can cause a driveability study to underestimate blow counts or miss refusal risk entirely.

When a pile is driven into the ground, the soil surrounding the upper part of the shaft experiences far more loading cycles than the soil near the pile toe. A layer at the top of the profile may have been loaded thousands of times by the time the pile reaches its target penetration, while the soil just above the toe has only recently been engaged. This means that applying a single, uniform residual strength across the entire shaft is physically unrealistic.

In AllWave-PDP, the friction fatigue mechanism is built into the soil model so that each layer carries its own loading history. The software tracks how many cycles each soil element has experienced and adjusts its resistance accordingly. This produces a depth-dependent SRD profile that reflects the actual degradation pattern rather than a simplified average. The result is a more accurate blow count prediction, a better estimate of maximum achievable penetration, and a more realistic picture of where refusal risk may develop.

What soil fatigue models are available in AllWave-PDP?

AllWave-PDP provides three main soil fatigue modelling approaches: the Beta Factor model, the Alm and Hamre model, and CPT-based friction fatigue formulations. Each model describes the relationship between initial soil strength and residual driving resistance differently, and each requires a different set of input parameters derived from ground investigation data.

Beta Factor model

The Beta Factor model is the simplest approach. It defines a single ratio, Beta, between the residual soil strength and the initial static capacity. A Beta value of 1.0 means no degradation; lower values represent increasing fatigue. The model is straightforward to apply and works well for preliminary assessments or when detailed CPT data is limited. Its main limitation is that it applies the same reduction uniformly across the entire shaft, which does not reflect the depth-dependent nature of real fatigue behaviour.

Alm and Hamre model

The Alm and Hamre model introduces a more physically grounded description of friction fatigue. It relates the reduction in unit shaft friction to the number of cycles experienced at each depth, using parameters calibrated against field observations. This model captures the variation in degradation along the shaft more realistically than a uniform Beta value, making it better suited for longer piles or profiles where the contrast between upper and lower shaft resistance is significant.

CPT-based friction fatigue formulations

CPT-based approaches derive the initial SRD profile directly from cone penetration test data and then apply fatigue reduction functions layer by layer. These formulations link the cone resistance and sleeve friction measurements to both the initial shaft capacity and the rate at which that capacity degrades under cycling. Because the input comes directly from measured soil behaviour, this approach tends to produce the most site-specific and defensible predictions, particularly for offshore projects where CPT data quality is high.

How do the AllWave-PDP soil fatigue models differ from each other?

The three soil fatigue models in AllWave-PDP differ primarily in how they represent the spatial variation of fatigue along the pile shaft, the number and type of input parameters they require, and how closely they reflect the physical mechanism of resistance degradation under repeated loading.

Spatial resolution: The Beta Factor model applies a single reduction ratio to the entire shaft. The Alm and Hamre model and CPT-based formulations both allow resistance to vary layer by layer, reflecting the fact that upper soil layers accumulate more cycles than deeper ones.
Input requirements: Beta requires only one parameter per soil unit. The Alm and Hamre model requires cycle-dependent degradation parameters. CPT-based models require a full CPT profile with interpreted layer properties.
Physical realism: Beta is a simplification. Alm and Hamre captures the cycle-count dependency more explicitly. CPT-based models tie the fatigue behaviour directly to measured soil properties, making them the most site-specific option.
Calibration basis: Beta values are often taken from published ranges or back-analysis of comparable projects. Alm and Hamre parameters are calibrated against instrumented pile installations. CPT-based parameters are derived from the cone data itself, supplemented by empirical correlations.

In practice, the choice between models is not purely technical. It also depends on the data available, the stage of the project, and the level of confidence required in the prediction. A Beta Factor model may be entirely appropriate for a feasibility study, while a CPT-based approach is expected for a detailed engineering phase on an offshore wind foundation project.

Which soil fatigue model should be used for a given soil condition?

The right soil fatigue model depends on the quality of your ground investigation data, the soil type, the pile geometry, and the consequences of an inaccurate prediction. As a general rule, use the most detailed model that your available data can reliably support.

Sandy soils with high-quality CPT data: CPT-based friction fatigue formulations are the preferred choice. Sands show pronounced fatigue behaviour, and CPT data provides the layer resolution needed to capture it accurately.
Clayey or mixed profiles with limited CPT coverage: The Alm and Hamre model offers a good balance between physical realism and data requirements. It handles the cycle-dependent degradation in cohesive soils without requiring a full CPT-derived profile.
Preliminary studies or data-sparse situations: The Beta Factor model is appropriate when detailed soil data is not yet available or when a rapid sensitivity check is needed. Use it with a range of Beta values to bracket the prediction.
Chalk or weak rock: Chalk presents specific challenges because its fatigue behaviour differs from silica sands. Dedicated research, including work published on pile driveability in low-to-medium density chalk, has shown that standard fatigue models may need adjustment for these materials.
Long offshore monopiles: The depth-dependent variation in loading history is most pronounced for long piles. CPT-based or Alm and Hamre models are strongly preferred over a uniform Beta approach in these cases.

Sensitivity analyses across model types and parameter ranges are always useful, regardless of which model you select as your primary approach. Comparing results from two models helps you understand the uncertainty band in your prediction and identify the depths where the choice of fatigue model has the greatest influence on blow count.

How do soil fatigue model settings influence drivability study outcomes?

Soil fatigue model settings directly control the predicted SRD profile at each penetration step, which in turn drives blow count, maximum achievable penetration, pile stress levels, and refusal risk. Small changes in fatigue parameters can produce large differences in predicted installation behaviour, particularly in the upper portion of the shaft where cumulative degradation is greatest.

When you reduce the Beta value or increase the fatigue rate in the Alm and Hamre model, the software calculates a lower residual shaft resistance. This typically reduces the predicted blow count in the upper shaft layers, which may seem conservative in terms of driving effort but can mask the true resistance at the toe. Conversely, if fatigue is underestimated, the model predicts higher shaft resistance throughout driving, which inflates blow counts and may incorrectly suggest refusal at depths that are actually achievable.

The fatigue model settings also influence the tension and compression stress predictions. Lower residual shaft resistance changes how the stress wave interacts with the pile-soil system, which affects the magnitude and location of peak stresses. This matters for pile integrity assessments and for evaluating whether the selected hammer can operate within its design limits throughout the full installation sequence.

A well-structured drivability study treats fatigue model settings as variables to be tested, not fixed inputs. Running the analysis with upper-bound and lower-bound fatigue parameters gives your team a realistic range of outcomes and helps identify which settings are most sensitive to uncertainty in the ground data. This approach aligns with the broader principle that a drivability prediction is a structured forecast with explicit assumptions, not a single deterministic calculation.

How We Support Your Drivability Studies with AllWave-PDP

We use AllWave-PDP as part of a complete drivability engineering workflow that connects ground investigation data, soil modelling, wave equation analysis, and post-installation validation. Our approach covers the full prediction-to-measurement cycle, so your team gets results that are both technically defensible and practically useful.

When we carry out a drivability study for your project, we:

Interpret your CPT, SPT, and laboratory data to build a site-specific soil model with appropriate fatigue parameters
Select and justify the most suitable soil fatigue model for your soil conditions and pile type
Run sensitivity analyses across fatigue parameter ranges to quantify prediction uncertainty
Predict blow count, pile stresses, energy transfer, and refusal risk at each penetration step
Monitor the actual installation and perform signal matching to back-analyse the real SRD
Compare measured behaviour against the prediction and update the soil model for future use

This closed learning loop means that every installation we monitor improves the quality of the next prediction. Our AllWave software package underpins this workflow, combining impact and vibratory driving simulation with dynamic load testing analysis in a single integrated environment.

If you are planning a foundation installation and want a drivability study that goes beyond a single wave equation run, contact our team to discuss your project requirements.