How is Soil Resistance to Driving determined for a Drivability Prediction?

Soil Resistance to Driving (SRD) is determined by translating geotechnical site investigation data, primarily CPT or SPT results, into a resistance profile that reflects the forces a pile must overcome during installation. Unlike static bearing capacity, SRD accounts for the dynamic conditions of driving: shaft friction degradation, toe resistance, and the effects of repeated blows on surrounding soil. The sections below unpack the methods, parameters, and practical limitations that define a reliable SRD calculation.

What methods are used to calculate soil resistance to driving?

SRD is calculated by applying empirical or semi-empirical methods that convert CPT or SPT data into predicted shaft friction and toe resistance values during pile installation. The most widely used approaches in offshore and onshore practice are methods such as the Alm & Hamre model, which derives SRD from CPT cone resistance and undrained shear strength, combined with friction fatigue relationships that describe how resistance changes as the pile penetrates deeper.

The Alm & Hamre model builds total static resistance from two components: side friction and toe resistance. For clay, residual shaft friction is linked to undrained shear strength and Overconsolidation Ratio (OCR). At OCR = 1, residual friction is approximately 0.35 times the undrained shear strength; at OCR above 10, this drops toward 0.2 times the undrained shear strength. For sand, an exponential transition from initial to residual shaft friction is applied, with residual values typically reached after 20 to 30 metres of further penetration. Toe resistance in clay is estimated as approximately 11.5 times the undrained shear strength, while in sand the best-fit relationship uses 0.4 times the CPT cone resistance over the full pile tip area.

It is important to note that not all soil types respond to standard SRD methods equally. Low-to-medium density chalk, for example, requires a dedicated approach. Research by Buckley, Kontoe, Jardine and co-authors introduced the Chalk Resistance to Driving (CRD) method, which incorporates the relationship between cone penetration resistance and driving resistance, the reduction of local stresses as driving progresses, and effective-stress-dependent interface shear failure. Standard sand or clay SRD methods applied to chalk can produce results that diverge significantly from observed installation behaviour.

How does SRD differ from static bearing capacity?

SRD is the resistance a pile encounters during installation, while static bearing capacity is the resistance a pile provides under service loading after installation: these are related quantities but not the same. The dynamic conditions of driving, including repeated impacts, soil remoulding, and pore pressure generation, mean that the resistance acting on a pile during driving can be substantially different from the long-term load-carrying capacity measured in a static load test.

Randolph and Deeks make this distinction explicit in their work on dynamic pile-soil interaction. Parameters such as quake, stiffness, damping, and ultimate resistance all influence the dynamic response during driving in ways that do not directly translate to static capacity. Soil around the pile tip and shaft is disturbed, remoulded, and in some cases partially liquefied during installation. Over time, as excess pore pressures dissipate and soil reconsolidates, capacity recovers, a process known as set-up.

For drivability analysis, the quantity of interest is specifically the resistance during installation. Using long-term static capacity directly as SRD input would overestimate resistance in many soils and lead to incorrect hammer selection or refusal predictions. This is why SRD methods are calibrated against back-calculated driving records rather than static load test results.

What soil parameters are needed as input for an SRD calculation?

An SRD calculation requires geotechnical parameters that describe both the strength and stiffness of the soil along the pile shaft and at the toe, including CPT cone resistance, undrained shear strength, OCR, and dynamic soil model parameters such as quake and damping coefficients. These inputs feed both the resistance profile and the dynamic soil model used in stress-wave analysis.

The minimum geotechnical data typically needed include:

CPT data: cone resistance and sleeve friction along the full depth profile
Undrained shear strength: for cohesive layers, derived from laboratory testing or CPT interpretation
Overconsolidation Ratio: influences friction fatigue rates in clay
Soil layering and boundaries: accurate identification of layer transitions affects where resistance changes occur
Dynamic parameters: quake values (shaft and toe), damping coefficients, and stiffness values for the chosen soil model
Pile geometry and material properties: cross-section, wall thickness, material impedance, and structural limits

Bielefeld and Middendorp emphasise that soil parameter inputs must be directly linked to site investigation results. Selecting dynamic parameters independently of the geotechnical data introduces inconsistency between the resistance profile and the model behaviour. Where CPT data is unavailable, SPT results or borehole data can be used, but with greater uncertainty in the derived SRD profile.

How is SRD used inside a drivability analysis?

SRD forms the soil resistance component of a coupled hammer-pile-soil model, where stress-wave calculations are performed at successive penetration depths to predict blow count, pile stresses, energy transfer, and penetration rate throughout the full installation process. The SRD profile is not a single value: it is a depth-dependent input that drives the simulation from the first blow to final penetration.

The conceptual structure of a drivability model links four components: the hammer and driving mechanism, the helmet, cap, cushion, and any follower, the pile itself, and the soil providing time-, velocity-, and displacement-dependent shaft and toe resistance. These components interact at every simulated blow. The ground resistance does not just determine how much force the pile must overcome. It also influences the dynamic behaviour of the entire driving system. A stiffer soil-pile combination can alter impact characteristics, particularly for diesel hammers where system response affects combustion and drop height.

At each penetration level, the simulation produces a set of predicted quantities, including blow count or penetration rate, impact energy, energy transferred to the pile, hammer efficiency, maximum compressive and tensile stresses, total resistance split between shaft and toe, pile displacement, and forces and velocities at key points along the pile. For concrete piles, stresses in the concrete and reinforcement and the development of cracking may also be relevant outputs. Tools such as AllWave-PDP and AllWave-VDP perform this type of stress-wave simulation for both impact and vibratory driving scenarios.

For vibratory driving, soil fatigue plays a particularly important role. As the pile penetrates under thousands of load cycles, soil along the upper shaft has experienced far more cycles than soil near the current pile tip. A single uniform beta factor, describing the ratio of residual to initial soil strength, does not capture this variation realistically. Advanced models treat fatigue as a function of local loading history at each point along the shaft.

Why does SRD vary during and after pile driving?

SRD varies because repeated blows remould and disturb the soil around the pile, reducing shaft friction progressively as penetration increases, a process called friction fatigue, while pore pressures, soil stiffness, and interface conditions also change dynamically during and after installation.

Friction fatigue is the primary reason why SRD at any given depth is lower than the initial resistance measured at first contact. As the pile passes through a soil layer, the shaft friction in that layer degrades from an initial peak toward a residual value. The rate of degradation depends on soil type and OCR. In normally consolidated clay, residual friction may be reached after approximately 30 metres of additional penetration; in heavily overconsolidated clay with OCR above 16, residual conditions can develop within 10 metres.

Laboratory research by Middendorp and Van Brederode on shaft friction in sand demonstrates that resistance dependent on acceleration, velocity, and displacement behaves differently under repeated impacts. As a series of blows is applied, ground resistance changes even when other model parameters remain constant. This confirms that a single viscous damping parameter cannot fully represent all aspects of dynamic soil response.

After driving stops, resistance typically recovers over time as excess pore pressures dissipate and soil reconsolidates around the pile. This recovery, set-up, means that the SRD at the end of driving is often lower than the static capacity measured days or weeks later. Conversely, in some sands, relaxation can occur where capacity decreases after driving. Both phenomena reinforce why SRD and long-term bearing capacity must be treated as distinct quantities.

How accurate are SRD-based drivability predictions in practice?

SRD-based drivability predictions are engineering estimates, not deterministic guarantees: their accuracy depends on the quality of the geotechnical input, the appropriateness of the chosen SRD method for the soil conditions, and whether the prediction accounts for the inherent variability of ground resistance through scenario analysis.

Alm and Hamre did not only develop average back-calculated SRD profiles. They also investigated the statistical spread around those profiles. Their work supports combining a best-estimate resistance profile with bounds that capture inherent ground variability. Running only a single nominal prediction understates the real range of possible installation outcomes. A robust drivability study therefore includes lower-bound, best-estimate, and upper-bound SRD scenarios to examine:

Whether the selected hammer has sufficient capacity reserve
Where refusal might occur under unfavourable conditions
What maximum tensile and compressive stresses could develop
How sensitive installation time and blow count are to ground uncertainty
Whether a heavier hammer or alternative installation strategy is needed as a contingency

Several sources of uncertainty consistently affect prediction quality. These include dynamic shaft and toe resistance values, accuracy of layer boundary identification, remoulding behaviour under repeated blows, velocity effects, actual hammer efficiency, cushion behaviour, pile material properties, structural stress limits, and local impedance changes along the pile. A sensitivity analysis that tests how conclusions change under realistic parameter variation is more informative than a single best-estimate run.

The quality of the prediction is ultimately bounded by the quality of the inputs and by how systematically previous predictions have been compared against measured installation data. Where driving records from similar piles in similar ground conditions are available, back-calibration significantly improves confidence in the SRD model selected.

How We Support Your Drivability Study

We combine decades of experience in pile driving analysis with in-house developed software and a deep understanding of soil-pile interaction to deliver drivability predictions that go beyond a simple blow-count graph. Our approach addresses the full installation process, from hammer selection and SRD profiling to scenario analysis and risk assessment.

When you work with us on a drivability study, we provide:

SRD profile development: translating your CPT, SPT, or borehole data into resistance profiles using appropriate methods for your soil conditions, including specialist approaches for challenging soils such as chalk
Lower, best-estimate, and upper-bound scenarios: so your team understands the full range of possible installation outcomes, not just the nominal case
Stress-wave simulation using our AllWave software package, covering both impact driving (AllWave-PDP) and vibratory driving (AllWave-VDP), with outputs including blow count, pile stresses, energy transfer, and penetration rate at every depth
Sensitivity analysis: identifying which parameters most influence your installation risk and where contingency planning is warranted
Hammer and equipment review: assessing whether your planned installation equipment can achieve target penetration within acceptable stress limits
Specialist support for offshore and onshore projects: including wind energy foundations, marine structures, and infrastructure piling

A reliable drivability prediction reduces the risk of early refusal, unexpected pile damage, and costly remedial actions on site. Contact us to discuss your project and find out how we can support your foundation installation planning.