The majority of bridges and quay walls in the inner city of Amsterdam rely on wooden foundation piles. Most of these were constructed 100–300 years ago, implying several challenges for the assessment of the current residual load-carrying capacity and their reliability. In Amsterdam, the wooden piles supporting bridges and quay walls remain entirely under the water table, which means that only bacterial decay can occur. Bacterial degradation proceeds at a slow rate, allowing the piles to perform their function for many years, although causing a reduction of the load-carrying capacity over time. To this end, the municipality of Amsterdam started a large project where non-destructive micro-drilling measurements were employed, with the goal of capturing the in-situ level of decay and the remaining strength of wooden foundation piles. The applicability of micro-drilling was studied on 60 wooden piles with various decay levels, driven between 1727 and 1922, and retrieved from two bridges in Amsterdam. An algorithm was developed for analysing the micro-drilling signals, aimed at determining the decayed outer layer of the pile (soft shell). The micro-drilling approach was validated with the results of mechanical testing on the piles. This study contributes to reliably assessing the decay and remaining load carrying-capacity of wooden foundation piles utilizing in-situ micro-drilling measurements.@en

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 (E_c,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 (E_c,0,stat,wet). This enabled a comprehensive assessment of the actual in-situ stress distribution (Δσ_actual,stat and Δσ_actual,dyn) along the length of the piles, calculated with DFOS strains and the pile stiffness (E_c,0,stat,wet and E_c,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 (Δσ_no-friction) approximately aligned with Δσ_actual,stat on the piles. At pile tips, the maximum applied 300–350 kN compressive load (i.e. Δσ_no-friction = 20–26 MPa), resulted in Δσ_actual,stat = 4–7 MPa, highlighting shaft friction effect. The calculated Δσ_actual,dyn with a single E_c,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.

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In the historic city centre of Amsterdam (NL), the predominant foundation system is comprised of wooden piles. Due to their placement below the water table, these foundations are susceptible to bacterial decay. This study aims to investigate and compare various methods for characterizing decay patterns within the cross sections of piles retrieved from two bridges in Amsterdam. The examined piles span different construction years: three originate from 1727, four from 1886, and two from 1922. Following extraction, the piles were transported to TU Delft Stevin II Laboratory, where they underwent further subdivision into three segments, each representing the head, middle, and tip, resulting in a total of 27 segments. The effects of bacterial decay were characterised by performing micro-drilling measurements, small-scale material and compressive tests on prismatic samples extracted from the segments' cross sections, computed tomography scans, and light microscopy observations. Microscopic examination revealed severe degradation in all segments dating back to 1727, extending 20–50 mm from their surface. This outcome was also confirmed by the other adopted methods: the corresponding prisms had large moisture contents and poor mechanical properties, while low basic densities and drilling amplitudes were obtained from CT scans and micro-drilling measurements, respectively. On the contrary, the internal sections of the 1727 segments exhibited no evidence of decay and demonstrated properties consistent with those observed in sound segments from 1886 and 1922. Finally, the observed gradients of density, strength, and stiffness were well correlated with micro-drilling measurements, which can therefore be reliably used as on-site assessment method to reconstruct the properties of the piles.

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The compressive strength properties of timber foundation piles in Amsterdam were characterised by small-scale compression tests on saturated round wooden discs. The discs were extracted from the head, middle, and tip of five spruce (Picea abies) piles dating back to 1727, 1886, 1922, and 2021. Several piles were subjected to underwater bacterial decay, causing a reduction of their load-carrying capacity over time. The amount of decay was determined with micro-drilling measurements. The results of small-scale tests were compared to large-scale axial compression tests to assess the feasibility of retrieving equivalent strength properties, considering the influence of diameters, decay, and wood knots.

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The majority of the bridges in the historic city centre of Amsterdam are supported by wooden foundation piles. Most of these were constructed 100–300 years ago, currently raising concerns about potential safety issues. The wooden piles under the bridges remain entirely under the water table, potentially subjected to bacterial decay in anaerobic conditions. Bacterial degradation proceeds at a slow rate, allowing the piles to perform their function for many years, although causing a reduction of their load-carrying capacity over time. To this end, a large experimental campaign was conducted to characterize the material and mechanical properties in relation to biological decay of 60 spruce and fir piles, dated back to 1727, 1886 and 1922, retrieved from two bridges in Amsterdam. Large-scale compression tests were carried out on 201 pile-segments extracted from head, middle-part and tip of the piles to determine their remaining short-term compressive strength. Micro-drilling measurements were conducted on each pile and analysed with a TU-Delft-developed algorithm, aimed at determining the soft shell – the width of the decayed outer layer of the piles’ cross section. Micro-drilling allowed to accurately assess the remaining sound cross section of the pile, which resulted to be well correlated to its mechanical properties. The extent of decay throughout the cross-section of the piles was assessed within sapwood and heartwood through experimental models from literature and validated with Computed Tomography (CT) scanning. This allowed to identify that bacterial decay was only present in the non-durable sapwood, even in very degraded piles. Moreover, the soft shell resulted to be rather uniform along the piles’ length. The analysis of decay, supported by micro-drilling and CT scans, allowed to develop experimental equations to predict the remaining short-term compressive strength along the pile length. The micro-drilling technique is now used on a large scale in Amsterdam, supporting the assessment of the remaining load carrying-capacity of wooden foundation piles in the city, aiding in the planning of conservation, maintenance and preservation strategies.@en

The majority of bridges and quay walls in the centre of Amsterdam are supported by 100–300 years-old wooden foundation piles subjected to bacterial decay. Bacterial degradation proceeds at a slow rate, allowing the piles to perform their function for many years, although causing a reduction of their load-carrying capacity over time. In this study, micro-drilling measurements were employed to capture the amount of decay and remaining short-term compressive strength of the historic wooden piles. The applicability of micro-drilling was studied on 60 wooden piles with various decay levels, retrieved after 100–295 years of service life. An algorithm was developed for analysing the micro-drilling signals, aimed at determining the decayed outer layer of the piles’ cross section, and validated with the results of mechanical testing on the piles. The micro-drilling technique is now used on a large scale in Amsterdam, supporting the assessment of the wooden foundation piles in the city.

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This work investigated the influence of knots on the compression strength of wooden foundation piles. The study involved 110 pile segments sawn from 18 spruce and 9 pine piles with a mean diameter of approximately 200 mm, and moisture contents above fiber saturation. The mechanical properties were determined performing both full-scale compression tests on pile segments, and small-scale experiments on discs sawn from selected segments, considering samples with and without knots. A knot ratio (KR) was defined analysing the knots layout of each wooden pile, and evaluating how the compressive strength was influenced by size, number and layout of knots. As final step, a prediction model was implemented based on the dry density and KR of wooden piles, to estimate the influence of knots on their compressive strength.

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In the historic city centre of Amsterdam (NL), the most widespread foundation system consists of wooden piles. With the aim of modelling and predicting remaining service life of these foundations and the piles in particular, one of the possible methods for collecting data and monitoring their condition consists of micro-drilling (MD) measurements. This work evaluates the reliability of MD measurements in identifying decayed portions and specific features of wooden foundation piles, considering different moisture content (MC) values. To this end, 24 segments were selected, sawn from wooden piles extracted from site, and having time in service (TS) of 2 to 294 years (with reference to 2021, the year of extraction). 240 MD measurements were conducted at varying MC values of 7% to 212%. The obtained MD profiles showed for all TS a slight decrease in drilling resistance when increasing MC. However, from the MD signals it is possible to reliably detect the areas affected by biodegradation phenomena (e.g. bacterial decay) along the drilling depth, regardless the MC of the segment or its gradient along the drilling depth. The present study contributes to research aiming at utilizing (in-situ) MD techniques for reliably assessing and quantifying decay and to be used in remaining service life planning of wooden foundation piles.

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Timber pile foundations are widespread in many areas around Europe and North-America. Especially in areas with weak soils, timber pile foundations have been a very good and economic solution. However, aging of the foundations can become a problem, as physical, biological and/or chemical degradation may occur over time. Now, that foundations can be up to 500 years in cities like Venice, Amsterdam, Hamburg, Boston and many others, questions arise about the reliability and which assessment methods can be used in order to estimate the current load carrying capacity and their residual service life. Residual service life is depending both on the time-to-failure behaviour of wood, as well as, the dead and live loads on the piles below buildings, quay walls and bridges. The approach taken integrates degradation models with a reaction kinetics based damage model. For wood that remains consistently below the waterline, the combination of bacterial degradation and long term loading is considered the most important, but degradation by fungi also may occur depending on soil and groundwater conditions. The integral assessment model will function as a tool for repair and maintenance strategies for asset managers and structural engineers.

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