MH
M. Hemel
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1
Historic quay walls in many Dutch cities are supported by an array of vertical timber piles which run through soft soil deposits and rest on a sand layer, providing end-bearing support. As these structures experience horizontal loads, the foundation piles are loaded in bending. This is the dominant loading case of pile foundations of dams, lock heads, and sometimes bridge abutments as well. To accurately model and evaluate the timber pile foundations, a proper estimate of their bending properties is essential. Therefore the mechanical properties of existing spruce foundation piles, retrieved from a historic quay wall (1905) at Overamstel in Amsterdam, Netherlands, were studied. Six piles were subjected to a four-point bending experiment. The outer fiber stress was kept constant between the point loads, leading to a failure at the weakest cross section. Measurements of the curvature and force distribution were taken along the pile length during loading. In addition, biological decay in the outer layer of the timber piles, also referred to as the soft shell, was identified with microdrillings. Internal strains were measured successfully by gluing fiber-optic wires inside the soft shell of the timber piles. The experiments indicated significant variations in modulus of elasticity and modulus of rupture across the tested population, but indicated a strong correlation. Modulus of elasticity averaged 16.5 GPa with a variation coefficient of 0.30, whereas the modulus of rupture averaged 23.2 N/mm2 with a variation coefficient of 0.26. Bacterial deterioration was found to be independent of both the outer pile diameter and the location along the timber pile. The soft shell had an average thickness of 21 mm, but it did not contribute significantly to the structural strength of the piles. This study could present a template for assessing the remaining service life not only of historic quay walls but also of other timber pile foundations under bending loads.
...
Historic quay walls in many Dutch cities are supported by an array of vertical timber piles which run through soft soil deposits and rest on a sand layer, providing end-bearing support. As these structures experience horizontal loads, the foundation piles are loaded in bending. This is the dominant loading case of pile foundations of dams, lock heads, and sometimes bridge abutments as well. To accurately model and evaluate the timber pile foundations, a proper estimate of their bending properties is essential. Therefore the mechanical properties of existing spruce foundation piles, retrieved from a historic quay wall (1905) at Overamstel in Amsterdam, Netherlands, were studied. Six piles were subjected to a four-point bending experiment. The outer fiber stress was kept constant between the point loads, leading to a failure at the weakest cross section. Measurements of the curvature and force distribution were taken along the pile length during loading. In addition, biological decay in the outer layer of the timber piles, also referred to as the soft shell, was identified with microdrillings. Internal strains were measured successfully by gluing fiber-optic wires inside the soft shell of the timber piles. The experiments indicated significant variations in modulus of elasticity and modulus of rupture across the tested population, but indicated a strong correlation. Modulus of elasticity averaged 16.5 GPa with a variation coefficient of 0.30, whereas the modulus of rupture averaged 23.2 N/mm2 with a variation coefficient of 0.26. Bacterial deterioration was found to be independent of both the outer pile diameter and the location along the timber pile. The soft shell had an average thickness of 21 mm, but it did not contribute significantly to the structural strength of the piles. This study could present a template for assessing the remaining service life not only of historic quay walls but also of other timber pile foundations under bending loads.
Amsterdam quays under pressure
Modelling and testing of historic canal walls
Amsterdam faces the challenge of maintaining a domain of 200 km historic quay walls, which is a vital part of the city’s historical landscape. Many quays are currently in poor condition and require renovation or replacement in the near future, significantly impacting the city. The quay walls can be up to 300 years old and their structure consists of a masonry cantilever wall on top of a timber floor, which is supported by headstocks founded on multiple vertical timber pile rows. In recent years, quay walls have shown signs of damage, partial collapse, and early warnings of such events. The most recent and severe incident was the collapse of the Grimburgwal in 2020, where approximately 20 meters of quay wall suddenly collapsed, plunging into the canal within a matter of seconds. Consequently, it is important to be able to predict the resistance of these structures and understand their potential failure mechanisms. The most common and severe failure mechanism observed in Amsterdam’s city centre is the lateral failure of the pile foundation. Calculating the resistance against this mechanism with existing models, leads to estimates of insufficient strength and safety. It seems that these models are too conservative, because in reality, the majority of the existing structures that proof unsafe on paper is performing quite well in practice. The discrepancy between the models and reality arises from uncertainties in the working principles of historic quay walls, geometrical unknowns, as well as uncertainties in soil and structural properties.
This thesis provides a comprehensive understanding of the lateral failure of the pile foundation by full-scale quay wall experiments and it proposes a computational model to predict the resistance against this failure mechanism.
To gain a comprehensive understanding of the lateral failure mechanism, an unique and extensive experimental program has been conducted on an existing historic quay wall, founded on timber piles. The quay is located at Amsterdam Overamstel and dates back to 1905. Experiments have been conducted at three different system levels. At level 1, four-point bending experiments have been performed on individual piles to obtain the bending material properties. At level 2, lateral pile group experiments have been conducted on two 3x4 pile groups to study the pile-soil-pile interactions. At level 3, proof load experiments have been carried out on entire full scale quay wall sections, to study the overall behaviour of the quay. As part of the experimental program, an extensive geotechnical site investigation has been performed. The experimental approach chosen enables a stepwise validation and calibration for computational quay wall models.
Through the experimental program, it is demonstrated that among all potential failure mechanisms, the lateral failure mechanism is most likely to occur when a quay wall is subjected to large surface loading at its backside. Examples of such loads in practice are parked or moving cars, heavy vehicles or goods. The mechanism is triggered by an increase in soil stresses at the backside of the quay, which pushes the foundation towards the water. This, in turn, results in the bending of the timber piles, accompanied by the development of bending stresses. State-of-the-art models (ABAQUS, PLAXIS and spring models) were used to predict the failure surface load of the Overamstel quay, with an estimated value of approximately 20kPa. However, in reality, the quay demonstrated significantly greater strength, as failure was not observed even for loads as high as 55kPa. While part of this underprediction can be attributed to experiment-specific effects not considered in the prediction analysis, the substantial underprediction of the failure load still emphasizes the conservatism in current modelling approaches.
Clear indicators of the lateral failure mechanism include the inclined position of the top of the piles, broken piles, settlements at the backside of the quay, and lateral deflection of the foundation. These indicators can effectively be monitored, as demonstrated by the employed monitoring plan in the experiments. Elements of this plan, such as inclination sensors mounted on the pile caps, can be implemented in Amsterdam’s city centre to detect signs of lateral failure. The foundation piles experience fracture when they reach a state of full yielding, which occurs when the bending stresses in the timber surpass the modulus of rupture across the entire cross-section of the pile. Bending experiments conducted on timber piles indicate a substantial variance in both the modulus of rupture (variation coefficient of 0.26) and the modulus of elasticity (variation coefficient of 0.3). Consequently, the piles exhibit a wide range of flexural stiffnesses and bending moment capacities. These discrepancies stem from natural variability and biological degradation of the timber, which lead to the formation of a weakened outer layer or “soft shell” starting at the perimeter of the piles, going inward. The soft shell thickness is approximately 10% of the external pile diameter and it does not contribute to the structural strength of the piles.
The substantial variations in load carrying capacities within a timber pile group can be primarily attributed to the variations in pile stiffness and bending capacity. Surprisingly, typical pile group effects such as in-line, side-by-side, pile free height, and pile diameters do not have a large contribution to the variations in individual lateral pile resistances found. When multiple piles are considered together, significant variations between individual piles compensate each other, leading to a group resistance that was almost identical in the two pile group experiments. This finding is advantageous from a computational modelling and risk assessment standpoint. Within the tested pile groups at the Overamstel site, with 200-300 mm diameter piles, partial yielding starts at approximately 100 mm of group deflection. The first pile breakages are expected to initiate at 140 mm of deformation; however, due to the redistribution of lateral loads among the piles, it does not directly result in group failure. Nevertheless, when deformations exceed 200 mm, a majority of the piles will break, leading to group failure. It is vital to emphasize that the transition from the initial onset of yielding to group failure requires merely a slight additional lateral load of 15%.
An analytical quay wall model has been developed to predict the resistance against lateral failure of historic quay walls. This model comprises a framework of elastic beams embedded in an elastic foundation, which is externally loaded by a linear elastic soil model based on Flamant’s theory. The framework is made up of multiple Euler-Bernoulli beams, connected to each other by boundary and interface conditions. The stiffness of the connection between piles and headstock is described by a pile-headstock interface model. The elastic foundation is represented by a series of independent p-y springs, approximated with a bilinear elastic-perfect-plastic model. A method is developed to include the pile-soil-pile interaction and the influence of a sloping surface by adjusting the plastic branch of the p-y springs. This method has been validated through multiple experiments documented in literature in which steel piles were used, eliminating material property uncertainties. The analytical quay wall model has been validated and calibrated with the Overamstel quay wall experiments, employing the stepwise approach. In the first step, the bending properties of the timber piles were obtained from the level 1 bending experiments. Subsequently, in the second step, the model’s capability to describe laterally loaded pile groups was validated through the level 2 pile group experiments. Finally, the Flamant soil model and the model’s ability to describe a historic quay were validated using the level 3 quay experiments. As a final step, the model was compared with finite element computations, demonstrating a good agreement in displacements and forces. The analytical quay wall model accurately predicts lateral displacement, pile bending moments, and bending stresses at various depths, allowing for the assessment of pile fracture under specific surface loads. Its key advantages over state-of-the-art finite element modelling software include robustness, computational speed, feedback loops (e.g. force and displacement-dependent pile-headstock connection stiffnesses), minimal input requirements, and no numerical stability issues at large deformations. The model is highly suitable for trend analysis, sensitivity studies, and probabilistic analysis due to its short computational time in seconds, compared to complex three-dimensional FEM software that takes minutes to hours. The effectiveness and potential of the validated analytical quay wall model have been demonstrated in two “follow up” studies, described below.
In the first study, the quay model has been employed to investigate the failure of the Grimburgwal. With the model it was demonstrated that bending stresses in the timber piles exceeded the modulus of rupture as a consequence of local deepening of the canal in front of the quay. It therefore provides valuable insights for Amsterdam’s historical centre. The analyses have served as an additional validation step for the analytical quay wall model developed in this thesis, specifically for applications to the quay walls of Amsterdam’s historical centre.
In the second study the quay model has been used to effectively showcase the potential of Bayesian updating by incorporating evidence of survived loading situations and corresponding deformations. This approach enables refinement of the reliability predictions and parameter distribution uncertainties, leading to a more accurate prediction of the resistance against the lateral failure mechanism of quay wall foundation piles. Depending on the type of evidence, an a-priori reliability prediction for a quay wall that fails to meet safety standards can be updated to any of the three consequence classes (CC3, CC2, and CC1b) outlined in NEN8700. In a fictive case study, a quay wall with an a-priori reliability of β = 1.5 has been increased to β = 3.2 by including evidence of an extreme survived load of 10 kN/m2 that resulted in displacements of less than 4mm. This is a decrease in failure probability by two orders of magnitude, showing the potential impact of using observational information in combination with Bayesian updating
The main practical implication of this thesis has been the improvement in modelling accuracy, as a result of the Overamstel experiments. The revised “gain” in modelling accuracy for bending moments and deflection was 43% and 37% respectively. This improvement can be attributed to advancements in modelling techniques, such as accurately simulating pile-soil-pile interaction and modelling the pile-headstock connection, as well as utilizing precise location-specific geotechnical and structural material properties as model input. The improved modelling accuracy results in a less conservative evaluation of the quay walls, leading to a reduction in the number of unnecessarily rejected quay walls for the Amsterdam quay wall domain.
The most practical recommendations for Amsterdam are: a) to develop accurate techniques for mapping quay wall configurations, b) to implement comprehensive quay wall monitoring systems in the city centre, c) to utilize the analytical model in future studies and assessments, d) prioritize geotechnical site investigations before making model predictions, and e) perform non-destructive tests in the city centre and incorporate this information in the assessment.
The methods and insights developed in this dissertation enhance the understanding of the lateral failure of historic quay walls and enable more precise predictions of their resistance against such failures. As such, the model can be effectively used to support decisions on their safe use, remaining service life, and the need for their replacement.
...
Amsterdam faces the challenge of maintaining a domain of 200 km historic quay walls, which is a vital part of the city’s historical landscape. Many quays are currently in poor condition and require renovation or replacement in the near future, significantly impacting the city. The quay walls can be up to 300 years old and their structure consists of a masonry cantilever wall on top of a timber floor, which is supported by headstocks founded on multiple vertical timber pile rows. In recent years, quay walls have shown signs of damage, partial collapse, and early warnings of such events. The most recent and severe incident was the collapse of the Grimburgwal in 2020, where approximately 20 meters of quay wall suddenly collapsed, plunging into the canal within a matter of seconds. Consequently, it is important to be able to predict the resistance of these structures and understand their potential failure mechanisms. The most common and severe failure mechanism observed in Amsterdam’s city centre is the lateral failure of the pile foundation. Calculating the resistance against this mechanism with existing models, leads to estimates of insufficient strength and safety. It seems that these models are too conservative, because in reality, the majority of the existing structures that proof unsafe on paper is performing quite well in practice. The discrepancy between the models and reality arises from uncertainties in the working principles of historic quay walls, geometrical unknowns, as well as uncertainties in soil and structural properties.
This thesis provides a comprehensive understanding of the lateral failure of the pile foundation by full-scale quay wall experiments and it proposes a computational model to predict the resistance against this failure mechanism.
To gain a comprehensive understanding of the lateral failure mechanism, an unique and extensive experimental program has been conducted on an existing historic quay wall, founded on timber piles. The quay is located at Amsterdam Overamstel and dates back to 1905. Experiments have been conducted at three different system levels. At level 1, four-point bending experiments have been performed on individual piles to obtain the bending material properties. At level 2, lateral pile group experiments have been conducted on two 3x4 pile groups to study the pile-soil-pile interactions. At level 3, proof load experiments have been carried out on entire full scale quay wall sections, to study the overall behaviour of the quay. As part of the experimental program, an extensive geotechnical site investigation has been performed. The experimental approach chosen enables a stepwise validation and calibration for computational quay wall models.
Through the experimental program, it is demonstrated that among all potential failure mechanisms, the lateral failure mechanism is most likely to occur when a quay wall is subjected to large surface loading at its backside. Examples of such loads in practice are parked or moving cars, heavy vehicles or goods. The mechanism is triggered by an increase in soil stresses at the backside of the quay, which pushes the foundation towards the water. This, in turn, results in the bending of the timber piles, accompanied by the development of bending stresses. State-of-the-art models (ABAQUS, PLAXIS and spring models) were used to predict the failure surface load of the Overamstel quay, with an estimated value of approximately 20kPa. However, in reality, the quay demonstrated significantly greater strength, as failure was not observed even for loads as high as 55kPa. While part of this underprediction can be attributed to experiment-specific effects not considered in the prediction analysis, the substantial underprediction of the failure load still emphasizes the conservatism in current modelling approaches.
Clear indicators of the lateral failure mechanism include the inclined position of the top of the piles, broken piles, settlements at the backside of the quay, and lateral deflection of the foundation. These indicators can effectively be monitored, as demonstrated by the employed monitoring plan in the experiments. Elements of this plan, such as inclination sensors mounted on the pile caps, can be implemented in Amsterdam’s city centre to detect signs of lateral failure. The foundation piles experience fracture when they reach a state of full yielding, which occurs when the bending stresses in the timber surpass the modulus of rupture across the entire cross-section of the pile. Bending experiments conducted on timber piles indicate a substantial variance in both the modulus of rupture (variation coefficient of 0.26) and the modulus of elasticity (variation coefficient of 0.3). Consequently, the piles exhibit a wide range of flexural stiffnesses and bending moment capacities. These discrepancies stem from natural variability and biological degradation of the timber, which lead to the formation of a weakened outer layer or “soft shell” starting at the perimeter of the piles, going inward. The soft shell thickness is approximately 10% of the external pile diameter and it does not contribute to the structural strength of the piles.
The substantial variations in load carrying capacities within a timber pile group can be primarily attributed to the variations in pile stiffness and bending capacity. Surprisingly, typical pile group effects such as in-line, side-by-side, pile free height, and pile diameters do not have a large contribution to the variations in individual lateral pile resistances found. When multiple piles are considered together, significant variations between individual piles compensate each other, leading to a group resistance that was almost identical in the two pile group experiments. This finding is advantageous from a computational modelling and risk assessment standpoint. Within the tested pile groups at the Overamstel site, with 200-300 mm diameter piles, partial yielding starts at approximately 100 mm of group deflection. The first pile breakages are expected to initiate at 140 mm of deformation; however, due to the redistribution of lateral loads among the piles, it does not directly result in group failure. Nevertheless, when deformations exceed 200 mm, a majority of the piles will break, leading to group failure. It is vital to emphasize that the transition from the initial onset of yielding to group failure requires merely a slight additional lateral load of 15%.
An analytical quay wall model has been developed to predict the resistance against lateral failure of historic quay walls. This model comprises a framework of elastic beams embedded in an elastic foundation, which is externally loaded by a linear elastic soil model based on Flamant’s theory. The framework is made up of multiple Euler-Bernoulli beams, connected to each other by boundary and interface conditions. The stiffness of the connection between piles and headstock is described by a pile-headstock interface model. The elastic foundation is represented by a series of independent p-y springs, approximated with a bilinear elastic-perfect-plastic model. A method is developed to include the pile-soil-pile interaction and the influence of a sloping surface by adjusting the plastic branch of the p-y springs. This method has been validated through multiple experiments documented in literature in which steel piles were used, eliminating material property uncertainties. The analytical quay wall model has been validated and calibrated with the Overamstel quay wall experiments, employing the stepwise approach. In the first step, the bending properties of the timber piles were obtained from the level 1 bending experiments. Subsequently, in the second step, the model’s capability to describe laterally loaded pile groups was validated through the level 2 pile group experiments. Finally, the Flamant soil model and the model’s ability to describe a historic quay were validated using the level 3 quay experiments. As a final step, the model was compared with finite element computations, demonstrating a good agreement in displacements and forces. The analytical quay wall model accurately predicts lateral displacement, pile bending moments, and bending stresses at various depths, allowing for the assessment of pile fracture under specific surface loads. Its key advantages over state-of-the-art finite element modelling software include robustness, computational speed, feedback loops (e.g. force and displacement-dependent pile-headstock connection stiffnesses), minimal input requirements, and no numerical stability issues at large deformations. The model is highly suitable for trend analysis, sensitivity studies, and probabilistic analysis due to its short computational time in seconds, compared to complex three-dimensional FEM software that takes minutes to hours. The effectiveness and potential of the validated analytical quay wall model have been demonstrated in two “follow up” studies, described below.
In the first study, the quay model has been employed to investigate the failure of the Grimburgwal. With the model it was demonstrated that bending stresses in the timber piles exceeded the modulus of rupture as a consequence of local deepening of the canal in front of the quay. It therefore provides valuable insights for Amsterdam’s historical centre. The analyses have served as an additional validation step for the analytical quay wall model developed in this thesis, specifically for applications to the quay walls of Amsterdam’s historical centre.
In the second study the quay model has been used to effectively showcase the potential of Bayesian updating by incorporating evidence of survived loading situations and corresponding deformations. This approach enables refinement of the reliability predictions and parameter distribution uncertainties, leading to a more accurate prediction of the resistance against the lateral failure mechanism of quay wall foundation piles. Depending on the type of evidence, an a-priori reliability prediction for a quay wall that fails to meet safety standards can be updated to any of the three consequence classes (CC3, CC2, and CC1b) outlined in NEN8700. In a fictive case study, a quay wall with an a-priori reliability of β = 1.5 has been increased to β = 3.2 by including evidence of an extreme survived load of 10 kN/m2 that resulted in displacements of less than 4mm. This is a decrease in failure probability by two orders of magnitude, showing the potential impact of using observational information in combination with Bayesian updating
The main practical implication of this thesis has been the improvement in modelling accuracy, as a result of the Overamstel experiments. The revised “gain” in modelling accuracy for bending moments and deflection was 43% and 37% respectively. This improvement can be attributed to advancements in modelling techniques, such as accurately simulating pile-soil-pile interaction and modelling the pile-headstock connection, as well as utilizing precise location-specific geotechnical and structural material properties as model input. The improved modelling accuracy results in a less conservative evaluation of the quay walls, leading to a reduction in the number of unnecessarily rejected quay walls for the Amsterdam quay wall domain.
The most practical recommendations for Amsterdam are: a) to develop accurate techniques for mapping quay wall configurations, b) to implement comprehensive quay wall monitoring systems in the city centre, c) to utilize the analytical model in future studies and assessments, d) prioritize geotechnical site investigations before making model predictions, and e) perform non-destructive tests in the city centre and incorporate this information in the assessment.
The methods and insights developed in this dissertation enhance the understanding of the lateral failure of historic quay walls and enable more precise predictions of their resistance against such failures. As such, the model can be effectively used to support decisions on their safe use, remaining service life, and the need for their replacement.
A quay wall directly next to a building, both dating from around 1870, collapsed along the Grimburgwal in Amsterdam, the Netherlands, on 1 September 2020. The historic quay wall consisted of a masonry wall, built of a timber deck supported by several rows of timber piles of about 12 m long. As over 200 km of such quays exists in Amsterdam and streets are usually very busy, the collapse triggered the question of the safety of the remaining quay walls in the city. A forensic investigation was carried out to determine the failure mechanisms and factors that contributed to the collapse. The investigation aimed to learn from this event and to prevent similar failures in the future. The main failure mechanisms and contributing factors were identified and confirmed using an integrated model of the quay, which is both simple and robust. The model was used to perform a sensitivity study taking all relevant uncertain factors into account. This work provided valuable insight into the main collapse mechanisms of the wall. Based on the results of this forensic study, it is possible to assess other historic quays.
...
A quay wall directly next to a building, both dating from around 1870, collapsed along the Grimburgwal in Amsterdam, the Netherlands, on 1 September 2020. The historic quay wall consisted of a masonry wall, built of a timber deck supported by several rows of timber piles of about 12 m long. As over 200 km of such quays exists in Amsterdam and streets are usually very busy, the collapse triggered the question of the safety of the remaining quay walls in the city. A forensic investigation was carried out to determine the failure mechanisms and factors that contributed to the collapse. The investigation aimed to learn from this event and to prevent similar failures in the future. The main failure mechanisms and contributing factors were identified and confirmed using an integrated model of the quay, which is both simple and robust. The model was used to perform a sensitivity study taking all relevant uncertain factors into account. This work provided valuable insight into the main collapse mechanisms of the wall. Based on the results of this forensic study, it is possible to assess other historic quays.
The historic canal wall structures in many Northern European cities have been built as masonry walls on a timber deck founded on timber piles. For analysis of the resistance of those structures and assessment of their remaining service life, suitable and accurate calculation models are needed. Thereto an analytical method was developed for modelling laterally loaded pile groups in layered sloping soil. In the proposed method, the bending of a pile, which is subjected to a lateral load and axial load, is described by a beam on a Winkler elastic foundation in which the soil behaviour is represented by a series of independent p-y springs, idealized with a bilinear elastic-perfect-plastic approximation. The plastic limit was computed with Brinch Hansen ultimate soil resistance and the elastic soil response by the Ménard stiffness. The plastic limit was corrected for each depth, based on the reduction of the passive soil wedge due to pile group effects and the presence of a sloping surface. The analytical model was calibrated and validated with three field experiments, one full-scale lateral load test of a 3 × 5 pile group in soft clays and silts (Snyder, 2004), one full scale lateral load test of a single pile located on a slope in layered soils (Mirzoyan, 2007) and one small scale lateral load test of a single pile located near a slope in sand (Abdelhalim et al., 2020). The proposed method can adequately predict bending moment distributions and pile deflections and in addition, a good consistency between the analytical model and experimental tests was observed. The method is very fast, making it suitable for probabilistic, Monte Carlo type, simulations and reliability updating to determine the probability of failure of quay walls or other structures with horizontally loaded piles.
...
The historic canal wall structures in many Northern European cities have been built as masonry walls on a timber deck founded on timber piles. For analysis of the resistance of those structures and assessment of their remaining service life, suitable and accurate calculation models are needed. Thereto an analytical method was developed for modelling laterally loaded pile groups in layered sloping soil. In the proposed method, the bending of a pile, which is subjected to a lateral load and axial load, is described by a beam on a Winkler elastic foundation in which the soil behaviour is represented by a series of independent p-y springs, idealized with a bilinear elastic-perfect-plastic approximation. The plastic limit was computed with Brinch Hansen ultimate soil resistance and the elastic soil response by the Ménard stiffness. The plastic limit was corrected for each depth, based on the reduction of the passive soil wedge due to pile group effects and the presence of a sloping surface. The analytical model was calibrated and validated with three field experiments, one full-scale lateral load test of a 3 × 5 pile group in soft clays and silts (Snyder, 2004), one full scale lateral load test of a single pile located on a slope in layered soils (Mirzoyan, 2007) and one small scale lateral load test of a single pile located near a slope in sand (Abdelhalim et al., 2020). The proposed method can adequately predict bending moment distributions and pile deflections and in addition, a good consistency between the analytical model and experimental tests was observed. The method is very fast, making it suitable for probabilistic, Monte Carlo type, simulations and reliability updating to determine the probability of failure of quay walls or other structures with horizontally loaded piles.
Bezwijken Grimburgwal
Leerpunten voor het Amsterdamse areaal
In dit rapport is via een forensic engineering aanpak een eerste analyse gegeven (rapid assessment, RA) van het bezwijken van de Grimburgwal (GBW) op 1 september 2020. Het doel van het onderzoek is om te achterhalen welke lessen zijn te leren van het bezwijken van de Grimburgwal voor de overige kades in Amsterdam. De volgende onderzoeksvragen zijn in dit onderzoek beantwoord op basis van de in dit rapport gepresenteerde informatie en analyses. Hierbij dient te worden aangegeven dat veel duidelijk is geworden maar dat ook nog gegevens ontbreken. Verder zijn alleen analyses uitgevoerd die in deze fase van Rapid Assessment konden worden uitgevoerd. Op basis hiervan is de beantwoording van de onderzoeksvragen als volgt:
Beantwoording vraag 1: Welke faalmechanisme(n) hebben significant bijgedragen aan het bezwijken van de Grimburgwal?
De Grimburgwal kade is in september 2020 in verschillende fasen bezweken. Als eerste was er sprake van een horizontale vervorming van de kade en enige dagen voor bezwijken zijn gaten in het straatwerk aangetroffen. De kade is daarna via een verticale vervorming losgeraakt en uit het vlak richting de gracht gevallen. Hierbij is een sectie van circa 25 m geheel onder water verdwenen. Het bezwijken is aan het oostelijke deel begonnen. Het westelijke deel is als gevolg daarvan
meegetrokken en gekanteld.
Als belangrijkste faalmechanisme is opgetreden een horizontale buigvervorming van de houten paalfundering gevolgd door het breken van de houten palen. De vervorming is het gevolg van een lokaal diepere bodemligging van de gracht. Deze verklaring berust op de verwachting dat er in elk geval onder een gedeelte van de kade slechts twee palenrijen achter elkaar aanwezig waren dan wel effectief functioneerden. Dit is conform het bestek, de duikinspectie en is tevens aannemelijk gezien de zeer korte afstand van de kade tot het pand BG2. Door bestaande scheurvorming was de mogelijkheid tot herverdeling van de krachten langs de kade in langsrichting verminderd, waardoor de sterkere delen (met wèl drie palenrijen) niet meer in staat waren de belasting over te nemen. De analyses zijn uitgevoerd met als aanname een ongedegradeerde houtsterkte. De sterkte van het hout was nog niet vastgesteld bij publicatie van dit rapport.
Beantwoording vraag 2: Wat zijn de onderliggende oorzaken van deze mechanismen?
De oorzaak van de diepere bodemligging is in dit onderzoek niet onderzocht. Naar alle waarschijnlijkheid is het draaien van boten een logische oorzaak hiervoor, omdat de instorting precies nabij het draaipunt voor brug 201 heeft plaatsgevonden. Het is waargenomen dat de diepte van de gracht in de richting van de brug toeneemt en daarna weer afneemt. De herhaaldelijke aanvaringen hebben de kade verzwakt, wat mogelijk verklaart waarom deze precies op de plaats van de schade als eerste is gescheurd. Tevens is dit het smalste deel van de kade, met mogelijk slechts twee palenrijen. Er lijkt geen sprake van belangrijke mate van aantasting van het hout (op basis van inspecties). De trigger (het laatste duwtje) voor de instorting is waarschijnlijk het vernieuwen van het straatwerk geweest van (eerst mei en later) augustus 2020. Het straatwerk zorgt voor een extra belasting op de kade. Dit straatwerk was nodig als gevolg van de reeds opgetreden kadevervorming. De bijdrage van de rondwaterstroming en eventuele droogte/regen is niet bekend, doch kan een rol hebben gespeeld in combinatie met de al opgetreden vervorming en ontstane lekweg van achter de kade richting de gracht voor de vorming van de gaten in het straatwerk. De gebroken hemelwaterafvoer is zeer waarschijnlijk een gevolg en geen oorzaak van de instorting.
Beantwoording vraag 3: Op welke manier zijn de resultaten van dit onderzoek in te zetten voor beoordeling van de veiligheid en het nemen van maatregelen voor de overige kademuren in Amsterdam?
Uit deze Rapid Assessment is gebleken dat de geometrie van de kade een belangrijke bijdrage heeft gespeeld in de gevoeligheid van de kade voor ontgronding/verdieping van de gracht. Door de combinatie van een kade die mogelijk op twee palen en zeker deels op drie palen rust is een uitspraak over de representativiteit van deze kade niet met volledige zekerheid te doen. De kade had volgens afdeling monumenten van Amsterdam (MenA) sowieso een wat afwijkende opbouw en een kade(deel) met twee palen is zelfs sterk afwijkend van het gebruikelijke areaal. Op basis van de bij vraag 1 en 2 beschreven mechanismen en oorzaken die ook voor de kade met drie palen gelden, kan desondanks worden gesteld dat maatregelen die voor de Grimburgwal worden aanbevolen ook voor overige kades van toepassing zijn.
Uit deze Rapid Assessment is gebleken dat de geometrie van de kade een belangrijke bijdrage heeft gespeeld in de gevoeligheid van de kade voor ontgronding/verdieping van de gracht. Hieruit volgen de volgende mogelijke
maatregelen/vervolgacties:
- Controle of er (meer) kades met korte kespen / vloerconstructies zijn en slechts twee palenrijen. Deze zijn kwetsbaarder dan de constructies met meer rijen palen. Deze kades kunnen met voorrang worden getoetst en zonodig versterkt.
- Uitvoeren van een controle op de waterdiepte en ontgrondingen, vooral op routes van scheepvaart en bij kades met twee of drie palenrijen. Er kunnen dan maatregelen genomen worden bij een te grote waterdiepte/ontgronding.
- Locaties met aanvaarschades met voorrang te onderzoeken.
- Systematisch aandacht te besteden aan zettingen achter de kades en problemen met leidingen of riolering, omdat dit kan duiden op het horizontaal vervormen van de kade, al dan niet door een diepere bodemligging. Er kan bijvoorbeeld een registratie/meldingssysteem gebruikt worden voor het optreden van verzakkingen aan de kades/het straatwerk. Als er een opdracht tot herstraten wordt gegeven dient altijd EERST een analyse van de oorzaak van de verzakking te worden gemaakt die het gedrag van de kade meeneemt. Dit kan ook voorkomen dat herstraatwerk tot vergroting van de belasting
op de kade leidt.
- Uitvoeren van metingen (liefst horizontaal aan de kades maar ook verticaal) om de relatie van de vervorming met mogelijk falen te onderzoeken (numeriek of experimenteel). Analyse van de zakkingen van de kades met Insarmetingen
kan helpen om andere “hotspots” in kaart te brengen. Door middel van nadere analyses dient te worden vastgesteld welke grenswaarden een indicatie voor falen geven. Het effect van droogte op de zakkingen in de stad dient hierbij mede te worden onderzocht.
In het onderzoek van de GBW is gebleken dat er veel factoren mogelijk een bijdrage hebben geleverd aan het falen. Enkele daarvan konden niet worden uitgesloten maar zijn ook niet bevestigd. Dit geeft wel aan dat bepaalde zaken nog nader onderzocht kunnen worden. Aan de GBW was een oude kade aanwezig die de huidige kade kruiste nabij de locatie van bezwijken. Ook voor andere kades zijn dergelijke vervangingen gangbaar geweest. De exacte samenstelling van de kade op het punt van bezwijken (met twee of drie palen) is ook niet geheel zeker, ondanks diverse inspanningen hier duidelijkheid over te krijgen. Nader onderzoek naar de (invloed van de) opbouw van bestaande kades, zowel archeologisch als modelmatig wordt sterk aanbevolen.
Om voor de Grimburgwal de relatieve invloed van de verschillende factoren nog specifieker te onderscheiden zouden de volgende activiteiten kunnen worden uitgevoerd:
- Onderzoek op houten samples uit de restanten om deze te drogen en te wegen en tevens trekproeven te doen om de buigtreksterkte sterkteklasse vast te stellen. Ook de mechanische eigenschappen van de paal-kesp verbinding kan
worden onderzocht.
- Onderzoek naar de exacte opbouw van de fundering in de bezweken en aanpalende gedeeltes.
- (na bovenstaande) Niet lineair eindig elementen model in 3D van de gehele kade voor het analyseren van de geschetste bezwijkmechanismen uit de RA en daarna variëren met verschillende scenario’s zoals andere geometrieën
Het is in dit onderzoek goed mogelijk gebleken met verschillende ekenmethoden een indruk te krijgen van de stabiliteit van de kade. Doorontwikkeling en validatie van modellen voor gecombineerde berekening van de gehele constructie (vloer, metselwerk, palen) is nodig, omdat deze gecombineerde modellen geen gemeengoed zijn.
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In dit rapport is via een forensic engineering aanpak een eerste analyse gegeven (rapid assessment, RA) van het bezwijken van de Grimburgwal (GBW) op 1 september 2020. Het doel van het onderzoek is om te achterhalen welke lessen zijn te leren van het bezwijken van de Grimburgwal voor de overige kades in Amsterdam. De volgende onderzoeksvragen zijn in dit onderzoek beantwoord op basis van de in dit rapport gepresenteerde informatie en analyses. Hierbij dient te worden aangegeven dat veel duidelijk is geworden maar dat ook nog gegevens ontbreken. Verder zijn alleen analyses uitgevoerd die in deze fase van Rapid Assessment konden worden uitgevoerd. Op basis hiervan is de beantwoording van de onderzoeksvragen als volgt:
Beantwoording vraag 1: Welke faalmechanisme(n) hebben significant bijgedragen aan het bezwijken van de Grimburgwal?
De Grimburgwal kade is in september 2020 in verschillende fasen bezweken. Als eerste was er sprake van een horizontale vervorming van de kade en enige dagen voor bezwijken zijn gaten in het straatwerk aangetroffen. De kade is daarna via een verticale vervorming losgeraakt en uit het vlak richting de gracht gevallen. Hierbij is een sectie van circa 25 m geheel onder water verdwenen. Het bezwijken is aan het oostelijke deel begonnen. Het westelijke deel is als gevolg daarvan
meegetrokken en gekanteld.
Als belangrijkste faalmechanisme is opgetreden een horizontale buigvervorming van de houten paalfundering gevolgd door het breken van de houten palen. De vervorming is het gevolg van een lokaal diepere bodemligging van de gracht. Deze verklaring berust op de verwachting dat er in elk geval onder een gedeelte van de kade slechts twee palenrijen achter elkaar aanwezig waren dan wel effectief functioneerden. Dit is conform het bestek, de duikinspectie en is tevens aannemelijk gezien de zeer korte afstand van de kade tot het pand BG2. Door bestaande scheurvorming was de mogelijkheid tot herverdeling van de krachten langs de kade in langsrichting verminderd, waardoor de sterkere delen (met wèl drie palenrijen) niet meer in staat waren de belasting over te nemen. De analyses zijn uitgevoerd met als aanname een ongedegradeerde houtsterkte. De sterkte van het hout was nog niet vastgesteld bij publicatie van dit rapport.
Beantwoording vraag 2: Wat zijn de onderliggende oorzaken van deze mechanismen?
De oorzaak van de diepere bodemligging is in dit onderzoek niet onderzocht. Naar alle waarschijnlijkheid is het draaien van boten een logische oorzaak hiervoor, omdat de instorting precies nabij het draaipunt voor brug 201 heeft plaatsgevonden. Het is waargenomen dat de diepte van de gracht in de richting van de brug toeneemt en daarna weer afneemt. De herhaaldelijke aanvaringen hebben de kade verzwakt, wat mogelijk verklaart waarom deze precies op de plaats van de schade als eerste is gescheurd. Tevens is dit het smalste deel van de kade, met mogelijk slechts twee palenrijen. Er lijkt geen sprake van belangrijke mate van aantasting van het hout (op basis van inspecties). De trigger (het laatste duwtje) voor de instorting is waarschijnlijk het vernieuwen van het straatwerk geweest van (eerst mei en later) augustus 2020. Het straatwerk zorgt voor een extra belasting op de kade. Dit straatwerk was nodig als gevolg van de reeds opgetreden kadevervorming. De bijdrage van de rondwaterstroming en eventuele droogte/regen is niet bekend, doch kan een rol hebben gespeeld in combinatie met de al opgetreden vervorming en ontstane lekweg van achter de kade richting de gracht voor de vorming van de gaten in het straatwerk. De gebroken hemelwaterafvoer is zeer waarschijnlijk een gevolg en geen oorzaak van de instorting.
Beantwoording vraag 3: Op welke manier zijn de resultaten van dit onderzoek in te zetten voor beoordeling van de veiligheid en het nemen van maatregelen voor de overige kademuren in Amsterdam?
Uit deze Rapid Assessment is gebleken dat de geometrie van de kade een belangrijke bijdrage heeft gespeeld in de gevoeligheid van de kade voor ontgronding/verdieping van de gracht. Door de combinatie van een kade die mogelijk op twee palen en zeker deels op drie palen rust is een uitspraak over de representativiteit van deze kade niet met volledige zekerheid te doen. De kade had volgens afdeling monumenten van Amsterdam (MenA) sowieso een wat afwijkende opbouw en een kade(deel) met twee palen is zelfs sterk afwijkend van het gebruikelijke areaal. Op basis van de bij vraag 1 en 2 beschreven mechanismen en oorzaken die ook voor de kade met drie palen gelden, kan desondanks worden gesteld dat maatregelen die voor de Grimburgwal worden aanbevolen ook voor overige kades van toepassing zijn.
Uit deze Rapid Assessment is gebleken dat de geometrie van de kade een belangrijke bijdrage heeft gespeeld in de gevoeligheid van de kade voor ontgronding/verdieping van de gracht. Hieruit volgen de volgende mogelijke
maatregelen/vervolgacties:
- Controle of er (meer) kades met korte kespen / vloerconstructies zijn en slechts twee palenrijen. Deze zijn kwetsbaarder dan de constructies met meer rijen palen. Deze kades kunnen met voorrang worden getoetst en zonodig versterkt.
- Uitvoeren van een controle op de waterdiepte en ontgrondingen, vooral op routes van scheepvaart en bij kades met twee of drie palenrijen. Er kunnen dan maatregelen genomen worden bij een te grote waterdiepte/ontgronding.
- Locaties met aanvaarschades met voorrang te onderzoeken.
- Systematisch aandacht te besteden aan zettingen achter de kades en problemen met leidingen of riolering, omdat dit kan duiden op het horizontaal vervormen van de kade, al dan niet door een diepere bodemligging. Er kan bijvoorbeeld een registratie/meldingssysteem gebruikt worden voor het optreden van verzakkingen aan de kades/het straatwerk. Als er een opdracht tot herstraten wordt gegeven dient altijd EERST een analyse van de oorzaak van de verzakking te worden gemaakt die het gedrag van de kade meeneemt. Dit kan ook voorkomen dat herstraatwerk tot vergroting van de belasting
op de kade leidt.
- Uitvoeren van metingen (liefst horizontaal aan de kades maar ook verticaal) om de relatie van de vervorming met mogelijk falen te onderzoeken (numeriek of experimenteel). Analyse van de zakkingen van de kades met Insarmetingen
kan helpen om andere “hotspots” in kaart te brengen. Door middel van nadere analyses dient te worden vastgesteld welke grenswaarden een indicatie voor falen geven. Het effect van droogte op de zakkingen in de stad dient hierbij mede te worden onderzocht.
In het onderzoek van de GBW is gebleken dat er veel factoren mogelijk een bijdrage hebben geleverd aan het falen. Enkele daarvan konden niet worden uitgesloten maar zijn ook niet bevestigd. Dit geeft wel aan dat bepaalde zaken nog nader onderzocht kunnen worden. Aan de GBW was een oude kade aanwezig die de huidige kade kruiste nabij de locatie van bezwijken. Ook voor andere kades zijn dergelijke vervangingen gangbaar geweest. De exacte samenstelling van de kade op het punt van bezwijken (met twee of drie palen) is ook niet geheel zeker, ondanks diverse inspanningen hier duidelijkheid over te krijgen. Nader onderzoek naar de (invloed van de) opbouw van bestaande kades, zowel archeologisch als modelmatig wordt sterk aanbevolen.
Om voor de Grimburgwal de relatieve invloed van de verschillende factoren nog specifieker te onderscheiden zouden de volgende activiteiten kunnen worden uitgevoerd:
- Onderzoek op houten samples uit de restanten om deze te drogen en te wegen en tevens trekproeven te doen om de buigtreksterkte sterkteklasse vast te stellen. Ook de mechanische eigenschappen van de paal-kesp verbinding kan
worden onderzocht.
- Onderzoek naar de exacte opbouw van de fundering in de bezweken en aanpalende gedeeltes.
- (na bovenstaande) Niet lineair eindig elementen model in 3D van de gehele kade voor het analyseren van de geschetste bezwijkmechanismen uit de RA en daarna variëren met verschillende scenario’s zoals andere geometrieën
Het is in dit onderzoek goed mogelijk gebleken met verschillende ekenmethoden een indruk te krijgen van de stabiliteit van de kade. Doorontwikkeling en validatie van modellen voor gecombineerde berekening van de gehele constructie (vloer, metselwerk, palen) is nodig, omdat deze gecombineerde modellen geen gemeengoed zijn.