Current industry practice requires applying with the ”no-gapping’’ criterion for piled foundations. Practically this implies no tensile loads should occur on piles under normal operational conditions at the S3 load level. While this conservative approach aims to ensure foundation integrity, it may result in oversized concrete foundation blocks that increase material use, construction costs, and environmental impact without adequately addressing underlying cyclic degradation risks.
A Paradigm Shift in Risk Assessment
WindBase’s proposed methodology shifts focus from avoiding tensile loads in piles at the S3 level to evaluating and limiting geotechnical capacity degradation under cyclic loading. This approach utilizes cyclic interaction diagrams from EA-Pfähle guidelines, already established as common practice for offshore foundation design evaluation.
The methodology assesses long-term geotechnical pile capacity degradation under cyclic loading in the serviceability limit state. Markov matrices for overturning moments relate to individual pile load amplitudes for each load case. For each load case, resulting damage on geotechnical bearing capacity is calculated using interaction diagrams. A logarithmic damage rule then determines post-cyclic pile bearing capacity across all load cases.
Controlled Degradation Approach
A geotechnical degradation limit of 25% for post-cyclic bearing capacity is proposed to prevent follow-up failure mechanisms. This controlled approach enables more precise risk evaluation from cyclic loading, leading to safer and more informed design decisions compared to conventional implicit limits.
Foundation stiffness, excessive pile deflection (SLS), reinforcement fatigue (FAT), and ultimate limit states (ULS STR & GEO) checks continue using industry standard design codes and numerical models, ensuring full compliance with IEC61400-6.
Quantified Benefits and Implementation
Preliminary design results for a Netherlands project demonstrate significant improvements. The proposed method reduces foundation block dimensions while accepting slight pile length increases. Estimated impacts per foundation include 20% reduced concrete volume (equivalent to 12 fewer truck loads), approximately €60,000 lower costs (10% overall reduction), and 50 tonnes CO2eq reduced environmental impact (15% overall reduction).
Technical Considerations
When allowing gapping in piled foundation design, foundation diameter decreases while cyclic load amplitude on piles increases. This requires additional analyses including pile-soil interface degradation assessment, pile stiffness degradation evaluation, and pile-foundation connection verification. Prefabricated prestressed concrete piles or vibro-combination piles with prestressed cores prove most suitable for gapping-allowed onshore foundation designs.
This methodology demonstrates that challenging conventional assumptions enables significant optimizations while enhancing both sustainability and economic viability for larger, more efficient wind turbine foundations.
Want to know more? Read all about it by checking out the poster and watching the presentation by Thomas Lankreijer: