The Statistics of Monopile Foundation Capacity in Spatially Variable Soil: Is Prediction of Axial Capacity Based on a Single CPT Reliable?-Paper

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The study evaluates whether designing a large-diameter monopile using only a central cone penetration test (CPT) provides reliable estimates of axial uplift capacity in spatially variable sand. This common approach assumes soil strength is laterally uniform, although conditions around the pile may differ from those measured at its center.<br /><br />Researchers combined the UWA-05 pile-capacity method with random-field modelling and Monte Carlo simulation. For each of 500 soil realizations at each variability level, they sized an 8 m diameter, open-ended pile using the central CPT and standard load and resistance factors. They then calculated its “real” capacity from soil-strength values around the pile perimeter. Cone resistance variability (COV) ranged from 0 to 0.6; the assumed horizontal and vertical fluctuation scales were 15.5 m and 0.9 m.<br /><br />As soil variability increased, the calculated pile lengths became more variable, as did the actual capacities. The mean capacity remained close to the design resistance, but the widening capacity distribution increased the annual probability of failure. It rose from 0.78 × 10⁻⁴ in homogeneous soil to 1.77 × 10⁻⁴ at a cone-resistance COV of 0.6. Thus, even with LRFD factors, a design based on one central CPT could fall below the intended reliability in spatially variable soil.<br /><br />The findings indicate that a single CPT may not adequately characterize the soil influencing a large monopile. However, the analysis covers a specific pile diameter and spatial correlation scales, and excludes uncertainty in the capacity-prediction method. Further study is needed to establish how pile dimensions and soil variability affect design reliability.

Keywords

monopile

axial uplift capacity

central CPT

spatial variability

sand

random-field modelling

Monte Carlo simulation

UWA-05 method

load and resistance factors

probability of failure