Characterization of the mechanical properties of saturated spruce (Picea abies) and pine (Pinus sylvestris) foundation piles

The lack of strength values for wooden foundation piles in the design standards for timber (Eurocode 5) hinders their proper engineering design and assessment. In order to fill this gap, an extensive experimental campaign was conducted to characterize the mechanical properties of large-scale, water-submerged spruce ( Picea abies L. ) and pine ( Pinus sylvestris L. ) piles. This was achieved through the execution of axial compression tests on 253 full-scale pile segments. Wet compressive strength and stiffness values were derived for both spruce and pine piles, applicable to the whole pile and/or its parts: head, middle-part, and tip. The quality variables that most influenced the wet compressive strength of the piles were density, knot ratio (KR), number of annual rings (age), and growth rate. Based on this, characteristic strength values were derived for piles with the following grading limits: KR < 0.5, age between 20 and 100 years, and a growth rate < 5 mm/year. These variables were used as key parameters to develop prediction models for the wet compressive strength of spruce and pine piles. The saturated compressive strength values and grading boundaries presented in this study contribute to the engineering design of timber piles and support the integration of reliable design values into future versions of Eurocode 5. • The lack of strength values for wooden foundation piles in the current Eurocode 5 hinders the appropriate engineering assessment of these elements. • Mean and characteristic wet values for compressive strength and modulus of elasticity of spruce ( Picea abies ) and pine ( Pinus sylvestris ) piles were determined with and extensive experimental campaign involving 253 pile segments. • The mechanical properties determined were studied in relation to quality variables of timber that could possibly affect the wet compressive strength. • The results of this study The results provide a basis for the engineering assessment of wooden foundation piles and they can be used together with the provisions given in the draft prEN 1995-1-1 (2021). • Two regression models were developed, for both spruce and pine piles, respectively. The wet compressive strength could be predicted based on the density ρ 12 and visually graded parameters such as knot ratio (KR), rate of growth (RoG) and age (annual rings) of the pile. For each wood species the models can be used for visual or machine grading.