Finite Element Analysis of Laterally Loaded Open-Ended Steel Piles Driven in Chalk

Open-ended steel piles are commonly driven to support offshore wind energy structures. Their design poses significant challenges in chalk, a very weak brittle limestone found in several regions worldwide. Impact driving causes chalk de-structuration and fracturing around the piles, greatly affecting their lateral load-bearing performance. This was observed in recent field tests undertaken in the UK on piles with different lengths, diameters and thicknesses, exhibiting both geotechnical and structural failures. Most of these lateral loading tests, including those conducted on larger and longer monopiles, were completed recently and were never analysed numerically. This paper presents results of 3D Finite Element analyses conducted on open-ended steel piles with different diameters (up to 1.22 m), embedded lengths (up to 10.16 m) and wall thicknesses (up to 44.5 mm), allowing to explore the marked scale effects observed on site. The newly available field tests also showed that steel yielding can occur before geotechnical failure is reached in chalk when testing piles with practical dimensions. However, steel yielding is usually neglected when modelling soil-pile interaction in geotechnical applications. The paper also aims at covering this gap by introducing a simplified modelling approach to account for elasto-plastic pile behaviour. The analyses delivered generally good matches with field behaviour and allowed to explore the main geotechnical uncertainties affecting accurate pile-chalk interaction predictions, mainly including the extent of the chalk fracturing induced by pile driving and its impact on chalk mechanical properties. The studies provide new and vital guidance for those involved in designing large driven piles for chalk sites.