Strength grading of saturated softwood foundation piles

In the next generation of Eurocode 5 (EC5), verification rules for the design of timber foundation piles will be included, together with provisions to determine the required values of material properties. The values given in the current Dutch national Annex to EC5 [20] are based on historical data, without providing strength parameters along the length of the wooden pile. In order to ensure the reliability of material property values for effective design, a large testing campaign was conducted on spruce piles from the Netherlands and pine piles from Germany, where the compressive strength and stiffness were determined on saturated segments extracted from head, middle-part and tip of the piles. The relationships between the compressive strength and parameters characterized with visual grading were investigated. A correlation analysis was conducted including the historical data from literature of other wood species, to investigate the most influencing visually-determined material properties on the compressive strength. Three possible strength classes for the characteristic compressive strength of spruce and pine piles were proposed in relation to visual grading, along with design values calculated according to the new draft of EC5 (2023). The results of this study contribute to the engineering design of European softwood foundation piles, supporting the integration of reliable design values into future versions of Eurocode 5.