Assessment of in-situ stress distribution and mechanical properties of wooden foundation piles instrumented with distributed fiber optic sensors (DFOS)

This case study explores the utilization of distributed fiber optic sensors (DFOS) in wooden foundation piles, for assessing and monitoring the stress distribution along their length. Three spruce and three pine foundation piles instrumented with DFOS were driven into the soil in a testing field in Amsterdam and axially loaded in compression. Since DFOS provided strain information, calculating the stress distribution in the piles required knowledge of their stiffness properties, which inherently vary from the head to the tip. Consequently, the piles were extracted and their overall wet dynamic elastic modulus (Ec,0,dyn,wet) was determined through frequency response measurements. Subsequently, the piles were segmented, transported to the TU Delft Laboratory and subjected to mechanical testing. For each segment, the mechanical properties were determined and their variability along the pile was studied, in particular for the static modulus of elasticity (Ec,0,stat,wet). This enabled a comprehensive assessment of the actual in-situ stress distribution (Δσactual,stat and Δσactual,dyn) along the length of the piles, calculated with DFOS strains and the pile stiffness (Ec,0,stat,wet and Ec,0,stat,dyn). Given the novelty of the DFOS application to timber piles, a validation of the accuracy was conducted on 3 pile segments equipped with DFOS. These segments underwent laboratory compression testing, allowing for a direct comparison between DFOS strain readings and strains measured with linear potentiometers attached to the pile segments. The results revealed good accuracy of DFOS in controlled lab conditions, with a maximum stress deviation of 0.65 MPa. Since the testing field featured a 6-meter-deep predrilled layer, where negligible shaft friction was mobilized, the no-friction stress (Δσno-friction) approximately aligned with Δσactual,stat on the piles. At pile tips, the maximum applied 300-350 kN compressive load (i.e. Δσno-friction = 20-26 MPa), resulted in Δσactual,stat = 4-7 MPa, highlighting shaft friction effect. The calculated Δσactual,dyn with a single Ec,0,stat,dyn for the whole pile, led to 3 MPa stress overestimation at pile tip. Although this calculation is conservative, the detailed knowledge of the variation of stiffness properties along the pile would result in a more efficient structural use.