Concrete is one of the primary contributors to global carbon emissions, mainly due to the widespread use of Portland cement. Transitioning to more sustainable binder alternatives is therefore very crucial. The potential advantages of using wood biomass fly ash (BFA) as a sustainable precursor in the manufacturing of alkali-activated concrete is investigated in this study. Several biomass fly ashes (BFA1, BFA2, BFA3, and WA4) were characterised to identify the most suitable candidate for CO₂ sequestration. Focusing on the existence of reactive Ca- and Mg-bearing phases, the selection was based on both chemical composition and phase analysis using XRF and XRD, followed by a free lime content analysis. Among these investigated ashes, BFA1 showed the most interesting characteristics, particularly in regard to its possible CO₂ absorption capacity.
The selected biomass fly ash (BFA1) underwent a two-stage pretreatment process intended to improve its environmental performance as well as to improve its use in alkali-activated concrete. The first step involved a water-interaction treatment. This treatment was primarily meant to prevent the generation of hydrogen gas, an issue that typically originates from the reaction of metallic aluminium in the ash with the alkaline environment. In addition to that, this treatment also enables the immobilisation of heavy metals. The second pretreatment was an accelerated carbonation pretreatment. This is mainly focusing on an increase in the CO₂ sequestration by transforming reactive CaO and Ca(OH)₂ into stable carbonates such as calcite. In addition to carbon absorption, this carbonation process contributed further to the immobilisation of heavy metals.
The effectiveness of each of these methods was assessed. The main assessment criteria were the reduction in heavy metal concentration, removal of metallic aluminium, and overall carbonation efficiency. Both treatments effectively dropped heavy metal levels to below the Dutch Soil Quality Limit (SQL). Furthermore, the combination of both treatments proved most successful for totally mitigating metallic aluminium content. Hereby resolving major challenges for the safety of including BFA into concrete. The carbonation of the ash was carried out through two routes, a dry route and a wet route (L/S = 0.3). The effectiveness of the wet carbonation method exceeded the gas-solid method by achieving complete carbonation in just eight hours. In contrast, the gas-solid approach was significantly slower, and after two months, total carbonation was nevertheless not achieved. The persistent presence of free lime confirmed the limited efficiency of the gas-solid route. The CO₂ absorption capacity of BFA1 was eventually determined to be 6.59% by weight, highlighting the effectiveness of the wet carbonation method in facilitating carbon sequestration.
Combining QXRD, ICP-OES dissolution, FTIR and isothermal calorimetry allowed for the assessment of the reactivity of the pretreated BFA1 samples. Raw BFA showed the most reactivity among the various treatments, based on its largest cumulative heat release from the isothermal calorimetry test. Its finer particle size distribution and lower degree of particle agglomeration were mostly responsible for this increased reactivity. Raw BFA also showed the highest dissolution levels of alumina and silica in the ICP-OES test, further confirming its superior chemical reactivity in alkaline environments. On the other hand, the samples that underwent carbonation treatment showed a reduction in reactivity. This was clear from the lower total heat generated in the isothermal calorimetry tests and the lower dissolution of reactive elements in the ICP analysis. In addition to that, FTIR spectra showed the presence of gel-like structures in both water-treated and carbonated BFA1 samples, demonstrating the initial formation of reaction products. These early reactions reduce the reactivity of the material by consuming some of its available reactive content, thereby influencing the reactivity during subsequent alkaline activation.
Following the pretreatment process of the ash, the water-carbonation-treated BFA was incorporated into alkali-activated concrete mixtures by partially replacing slag at varying replacement levels. These concrete mixtures were developed and tested in order to meet the requirements for the production of sidewalk pavement blocks. Mechanical testing showed that a 25% replacement level of slag with treated BFA was sufficient to satisfy the compressive strength class of C30/37, as defined in the regulation. The reference mix (AAC-REF) reached a compressive strength of 49 MPa, while the mix including 25% pretreated BFA showed a slightly lower strength of 46 MPa. Durability testing showed that the treated BFA significantly lowers freeze-thaw resistance. The mass loss resulting from freeze-thaw after 28 cycles increased from 3.63 kg/m² in the AAC-REF sample to 5.66 kg/m² in the BFA-containing mix at the 25% replacement level, a significant reduction in the long-term durability under freeze-thaw conditions.
Paste samples were prepared to analyse the impact of BFA addition on the microstructure and phase composition. FTIR analysis of the paste determined the degree of polymerisation, showing a slightly increased polymerisation degree in the BFA-containing paste. QXRD and TGA were applied to determine the amount of reaction products formed after alkali activation. Both pastes contained similar types of reaction products, but the reference paste contained a greater amount of amorphous phases. Lastly, SEM analysis was performed to examine morphological and compositional changes due to the incorporation of BFA. BFA incorporation resulted in the development of micro-cracks between the BFA grain and the surrounding matrix and within the BFA particle itself. SEM-EDX point analysis revealed that the reaction gel consisted mainly of C–A–S–H, although the BFA-containing specimens had a larger Ca/Si ratio, likely due to the high calcium level in the BFA and the existence of calcite.
A life cycle assessment (LCA) was conducted to measure the environmental benefits of including water-carbonation-treated BFA, with a focus on lowering its carbon footprint. The results showed that replacing 25% of slag with pretreated BFA reduced CO₂ emissions by 21.28% relative to the reference mix, which consisted of 100% slag. This shows the potential of using BFA as a sustainable alternative precursor in an alkali-activated system.
Finally, this thesis demonstrates that pretreated biomass fly ash can be utilised in alkali-activated binder systems, especially under combined water and carbonation treatments. Although these treatments might slightly reduce the reactivity of the ash, their environmental benefits, including CO₂ sequestration, metallic aluminium elimination, and heavy metal immobilisation, provide strong justification for their incorporation in sustainable building materials.
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Access to tunnel projects is typically achieved through shafts, which can be either temporary or permanent. As tunnel infrastructure becomes more complex and deeper, traditional shaft geometries such as circular or rectangular often fall short in meeting structural and spatial requirements. The caterpillar or peanut-shaped shaft has emerged as an alternative solution, being utilised in recent projects in Brazil, Hong Kong, and the U.K. Despite its growing use, limited information exists in the public domain regarding its structural behaviour and design methodology. This study investigates the structural response and modelling challenges associated with a caterpillar-shaped shaft using both 2D and 3D finite element analysis (FEA) in DIANA software (versions 10.8 and 10.9).
A representative model of a 3-cell caterpillar shaft, 25m in diameter, with a 52m diaphragm wall (d-wall) depth and a 40m excavation depth was analysed. The Y-panels were supported using cross-walls. The model was reduced to a quarter size using symmetry for computational efficiency. Results revealed that the shaft demonstrated high rigidity. This structural stiffness was attributed to the development of hoop forces in the circular d-walls and the presence of cross-walls. However, this rigidity also resulted in minimal soil displacement, which in turn did not trigger the soil arching effect, keeping the surrounding soil in a neutral state.
Comparative analyses were conducted between the 3D model and the 2D modelling approaches - axisymmetric analysis for circular part and plane-strain analysis for Y-panel joining adjacent cells. The axisymmetric model underestimated hoop forces by 15–20% above excavation level and showed significant discrepancies below it when compared with the 3D model. The 2D plane-strain model overestimated deformations and bending moments at the Y-panel junction by 52% (±17%) and 15–25%, respectively. These discrepancies stemmed from the fundamentally different deformation pattern of the caterpillar geometry, which exhibited deformations like an elliptical shaft — contraction along the long axis and extension along the short axis. The study also explored the utility of adding a buttress support at the Y-panel. Introducing a 1m thick buttress reduced bending moments by up to 30%, though further increases in thickness to 2m or 3m yielded diminishing returns.
Overall, this study highlights the structural advantages of caterpillar shafts, particularly their rigidity and reduced need for heavy strut support. However, it also underscores the limitations of 2D modelling in capturing the complex deformation behaviours of such geometries. Accurate analysis and design of caterpillar shafts require comprehensive 3D modelling, with future work needed to develop reliable 2D approximations validated through field data and back-analysis. ... Read more

This thesis investigates the potential of Basalt Fibre-Reinforced Polymer (BFRP) as an alternative to traditional steel reinforcement in concrete structures, with a focus on enhancing shear capacity. BFRP offers advantages such as higher tensile strength, superior corrosion resistance, and environmental sustainability. The study compares two alternative BFRP stirrup designs—braided BFRP rods and laminated unidirectional (UD) BFRP strips—with traditional steel stirrups through uniaxial tensile testing and displacement-controlled three-point bending tests on reinforced concrete beams. The results show that while BFRP stirrups improve the shear capacity of concrete beams, they do not yet match the performance of steel stirrups due to issues like reduced stiffness and stress concentrations in corner sections. However, BFRP stirrups demonstrated potential advantages in terms of weight efficiency, suggesting a promising role in sustainable construction. The findings underscore the need for further refinement in BFRP production methods to achieve consistent quality and better performance in structural applications. ... Read more

his master’s thesis investigates the design and structural behavior of Steel-Concrete-Steel (SCS) composite immersed tunnels, addressing challenges posed by traditional reinforced concrete designs. These challenges - limitations in weight, constructability, and long-term durability, particularly in deep-water environments - are exacerbated by the increasing demand for larger capacity tunnels and the need for efficient, sustainable construction methods. This research explores the impact of construction-induced irregularities, such as interface gaps and shear connector deformation, on the load-carrying capacity and overall performance of SCS tunnels. The study focuses on how irregularities produced during different construction phases affect both the local and global structural behavior of the composite cross-section. The research approach involves four distinct stages. First, a thorough examination of existing literature provides a foundation of knowledge regarding immersed composite tunnels, encompassing material properties, construction stages, and manufacturing processes. Second, a detailed SCS composite immersed tunnel design is developed, incorporating findings from a reinforced concrete tunnel case study to inform geometry and material selection. Third, numerical analysis using DIANA software is conducted. This includes 2D finite element modeling (FEM) of an SCS composite beam and a 2D FEM of the complete SCS cross-section to investigate the interaction between steel plates and concrete core under various interface conditions (strong bond versus slip) and imperfections (gaps, reduced shear connector effectiveness). Both linear elastic and nonlinear material behavior are considered. The findings highlight the critical role of horizontal interface connections and the nonlinear behavior of concrete in determining the overall load-carrying capacity and stability of SCS tunnels. The primary load bearing mechanism identified is the compressive strut within the concrete core; however, the study reveals that the structure’s failure is consistently initiated by the failure of this compressive strut, due to crushing under load. This failure mode underscores the limitations of traditional composite theory design methods and highlights the importance of the strut - and - tie model for more accurate design approaches. Construction imperfections significantly impact structural performance: strong interface connections are crucial for effective load transfer, while webs (vertical steel plates) enhance stability. The research concludes with recommendations to improve design practices, emphasizing the importance of strong horizontal connections and careful construction to maintain compressive strut integrity and avoid premature failure. ... Read more

Due to the recent increase in traffic capacity requirements, the need for larger spans in immersed concrete tunnels has become a pressing matter. Although commonly used in Europe, reinforced concrete has a structural capacity limit when it comes to the transverse span length. The implementation of post-tensioning could allow for longer spans and a reduction in the overall concrete used. This technique is however rarely used in industry due to the complexity of applying it to underwater environments and the varied loading conditions an immersed tunnel element is subjected to. When implementing post-tensioned tendons, the curvature creates additional distributed loads which compensate for the high hydrostatic pressures and backfilling weight present at the final immersed stage of the tunnel. Due to the absence of these loads during the initial stages, prior to transportation, high tensile stresses are exhibited which can lead to severe cracking. Crack mitigation can be achieved by implementing permanent additional reinforcements in the opposite face of the post tensioning tendon. Additionally, another method is installing a temporary system connecting top and bottom slabs to replicate the final loading conditions. The objective of this research is to investigate the governing limitations of implementing transverse post-tensioning and evaluate when it is a structurally viable option. Using finite element modelling, a linear analysis was carried out to evaluate the behavior of the structure after implementing post-tensioning loads in the cross section and to identify the critical areas. The assumptions for the analytical moment distribution were found to overestimate the rigidity of the structure and made for larger moments at the wall-top slab connections, which were adjusted for the remaining parts of the study. An analysis into the effect of adding post-tensioning in the lower bottom slab revealed a substantial improvement in the final stage stress distribution, and was observed to be in full compression. A nonlinear analysis was used to provide insight into the global structural behaviour for both final immersed and dry dock stages. The final immersed stage exhibited linear elastic behaviour, whereas, the onset of cracking was observed at the top of midspan at the dry dock stage. In a further analysis of the critical dry dock stage, the relation between partial prestressing, curvature of post-tensioning tendons and the effects on the cracking behaviour at midspan was explored. The results showed that when maximum curvature and percentage of prestressing are simultaneously present, the crack width limit is reached. When slightly lowering either of these parameters, a substantial decrease in the amount of reinforcement is needed to mitigate these cracks. Lastly, a case study was carried out on the Fehrmanbelt Fixed Link to compare two methods to mitigate cracks at midspan in the dry dock: additional reinforcement and temporary tendons. This study found that implementing transverse post-tensioning was feasible when reducing the curvature of the tendons, reducing amount of prestressing and implementing additional reinforcement, which helped increase structural capacity of the critical areas in the dry dock stage. ... Read more

Non-linear finite element analysis (NLFEA) is a powerful numerical solution method that can enhance accurate determination of the structural resistance for a more efficient design. However, the implementation of NLFEA for the design of reinforced concrete structures is lagging behind as related uncertain- ties have not been quantified adequately yet. Multiple studies have been conducted to evaluate the effect of modelling choices, which has led to the RTD1016 Dutch Guideline. This guideline enables a better quantification of the uncertainties related to NLFEA for the proposed solution strategy. In this research, a full probabilistic approach is applied to improve the quantification of uncertainties related to NLFEA, and thereby enhance its application for the design of reinforced concrete structures. In particular, to the ultimate limit state (ULS) of simply supported beams subjected to both ductile and brittle failure modes. To achieve this goal, 48 benchmark beams were selected from literature for calibration purposes. Material induced uncertainties of concrete and reinforcement were incorporated through an optimized Latin hypercube sampling strategy. The beams were modelled in a 2D plane in software program Diana based on a total strain crack model and Von Mises plasticity. A displacement-controlled analysis was performed to determine the numerical ultimate resistance. In total, 1104 analysis were performed of which the model uncertainty was quantified and the failure mode was determined by the ductility index. Based on this, a global reliability method was defined as a function of the failure mode. A comparison with existing reliability methods was made in terms of accuracy and robustness. Furthermore, a standalone multivariate non-parametric Bayesian network (NPBN) was developed that allows for extensive reliability assessment possibilities. The research has shown how a full probabilistic approach with benchmarking can be applied. A reliability method as a function of the failure mode was proposed that showed improved efficiency compared to existing reliability methods (GRF,PRF,ECOV). For a 50 year design lifetime, a mean unity check of 77% was attained for the ductile failure mode and 66% for the brittle failure mode. For specific types of concrete and reinforcement, even higher efficiency can be obtained by reduced coefficients of variation of the material parameters. Furthermore, a NPBN was constructed which describes the NLFEA behaviour of reinforced concrete beams. Additional research is necessary to improve the model application, but it has been demonstrated how such a model can be established and used for reliability assessment of reinforced concrete beams. The findings of this study suggest that the design of reinforced concrete beams by NLFEA can be applied for efficient design, while respecting safety standards. The full probabilistic approach enabled an improved quantification of the design resistance. Thereby, this research contributes to the implementation of NLFEA for the design of reinforced concrete structures. ... Read more

Het Noordzeekanaal verbindt de Haven van Amsterdam met de Noordzee via het sluizencomplex van IJmuiden. In de huidige situatie is de Noordersluis maatgevend voor de maximale diepgang van de scheepvaart op het Noordzeekanaal. Met de geplande realisatie van Zeesluis IJmuiden begin 2022, zal de Noordersluis niet langer maatgevend zijn voor de maximale diepgang. In de nieuwe situatie zal de bodemligging van het Noordzeekanaal maatgevend zijn voor de maximale diepgang van de scheepvaart. Het Centraal Nautisch Beheer en Rijkswaterstaat wensen de scheepvaart capaciteit van het Noordzeekanaal te bepalen afhankelijk van de beschikbare waterdiepte en de daarbij passende diepgang van schepen. Uit eerdere onderzoeken bleek de Velserspoortunnel maatgevend te zijn bij eventuele verdieping van het Noordzeekanaal. Daarnaast werd geconcludeerd dat de spoortunnel in de huidige situatie niet voldoet aan de eisen omtrent de minimale tunneldekking. Verder bleek de dynamische kielspeling boven meerdere Noordzeekanaal tunnels te klein te zijn. Tevens werd duidelijk dat door de optredende bodemsnelheden, veroorzaakt door schroefwerking en retourstroom, de kritische snelheid van de tunneldekking overschreden wordt. Aangezien de constructieve veiligheid van de tunnels gelegen onder het Noordzeekanaal niet in het geding mag komen dient meer inzicht te worden verkregen in de veiligheid van de tunnels bij scheepvaart calamiteiten. De rol van de dekking op de tunnels is hierbij cruciaal. In het onderzoek is daarom ingegaan op de optimalisatie van de tunneldekking bij scheepvaart calamiteiten. Aan de hand van literatuuronderzoek is inzicht verkregen in de scheepvaart calamiteiten. Vervolgens is de verkregen kennis toegepast op de casus van de Velsertunnels, door de tunnels te toetsen aan de vereiste veiligheid bij de calamiteitsbelastingen. Tot slot zijn alternatieve tunnelbeschermingen onderzocht voor de Velsertunnels waarbij het meest geschikte type bescherming verder is uitgewerkt. ... Read more

Increased traffic loads and ageing of concrete bridges and overpasses in the Netherlands make it necessary to reassess these existing structures. Consequently, the current condition and capacity of many concrete structures need to be evaluated. Residual capacity could be discovered during reassessments of concrete slabs due to a phenomenon called compressive membrane action (CMA). CMA is the formation of internal compressive arches caused by the lateral restraint. As a result, the load is not only transferred by bending action but also by arching action. Research has shown that the ultimate capacity can be significantly increased due to the occurrence of CMA. The goal of this study is to examine the influence of geometrical nonlinearity on this increase in capacity. Also, accurate quantification of the capacity enhancement for a variety of concrete slab variants can be scientifically useful and increases the knowledge on CMA. The study is confined to one way reinforced and restrained concrete slabs.

A new analytical model is presented to quantify the capacity enhancement due to CMA and the geometrical nonlinear (GNL) effect on the capacity. Calibration of the analytical model is performed with a finite element model in DIANA FEA. Also, the finite element model validates the analytical results and is used to study the failure mode of a restrained concrete slab in detail.

The enhancement factor – defined as the enhanced capacity divided by the conventional capacity – turned out to be varying between 1.35 and 4.7 for a large variety of restrained concrete slabs. Thus, the ultimate capacity of restrained one way slabs is significantly increased due to CMA. However, the capacity enhancement would have been even greater if geometrical nonlinearity was not accounted for. Geometrical nonlinearity reduces the increase in capacity because the formed compressive arches will tilt as a result of deflections, therefore leading to a relative decrease of the resisting arching moments. This GNL reduction effect varies between 3% and 37% according to the finite element model. The calibrated analytical model sufficiently estimates this effect with a maximum deviation of about 12%. An important finding was that the enhancement factor is larger for deep slabs than for slender slabs, while the reduction of the ultimate load due to geometrical nonlinearity is larger for slender slabs than for deep slabs.
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A submerged floating tunnel (SFT) can be a promising solution for crossing a deep or wide waterway. This tunnel concept will consist of an immersed tube, either attached with anchor cables to the seabed or attached to pontoons floating on the water surface. The reliability of the tether-stabilized SFT is assessed in this research. A suitable target reliability is determined in order to design a full probabilistic SFT. Subsequently, a calibration of partial factors from Eurocode is performed. The robustness of the structure is also analyzed and improvements are suggested. Important failure mechanisms are defined as yielding and slackening of the tethers, longitudinal failure and transverse shear failure of the tube. A first-order reliability method (FORM) and a Monte Carlo simulation (MC) are performed for the limit state functions of these mechanisms. Design parameters are determined so that a target reliability index of 3.8 is met, because of consistency with Eurocode. Consequently, the design points from FORM are used to calculate partial factors for different loading types. The calculated factors and the general partial factors from Eurocode are compared. Slackening of the tethers proved to be the governing failure mechanism in this analysis. The resistance against slackening depends on the force equilibrium, whereas the resistance of the other mechanisms depends on structural strength. The influence factors from the FORM analysis indicated that permanent loading parameters were dominant, i.e.\ concrete density, water density and tube diameter. It was found that for the strength (STR) mechanisms, the factors from Eurocode result in an overly safe design of the SFT. The calculated partial factors for unfavorable permanent load and variable load are significantly lower than the corresponding general factors from Eurocode. For the equilibrium (EQU) case, Eurocode is not safe to be applied. The general partial factor for the unfavorable permanent loading is insufficient. The robustness of the structure is assessed by considering important scenarios. Excessive leakage has large consequences and will result in global structural failure. However, it has a low probability of occurrence. Mitigating measures are available to prevent failure due to leakage. Furthermore, an SFT will be constructed at a specific location. Wave conditions and geolocation need to be taken into account to reach an optimal design. At a depth of 30 meters, the impact of waves becomes insignificant. Lastly, failure of a single tether should not result in failure of adjacent tethers (i.e. progressive failure). A redundant system can be created by installing more or higher quality tethers. Consequently, when all four tethers of one element fail at the same time, this does not result in longitudinal failure. Overall, it was demonstrated that the reliability requirements of the SFT can be met in the design. Moreover, the design can be optimized by a full probabilistic calibration of partial factors. ... Read more

Existing reinforced concrete (RC) structures can be strengthened using Strain Hardening Cementitious Composites (SHCC). The ability of SHCC in exhibiting a ductile response under tensile load due to strain­-hardening after crack initiation makes it a viable material to be used in, both, construction and retrofitting of concrete structures. The main objective of this research is to study the shear behaviour of SHCC-strengthened RC beams using NLFEA. The shear behaviour of benchmark RC beams is analysed first. The analysis of the selected RC beams analysed using Damage-based shear retention function results in accurate predictions of peak load if a fine mesh size resulting in 30 or more elements in the height of the beam is used. However, the failure type is predicted inaccurately for both coarse and fine mesh sizes due to lack of consideration for aggregate interlock in Damage-based shear retention function. The analysis of the selected RC beams using Al-Mahaidi shear retention function results in accurate predictions of peak load if a coarse mesh size resulting in 20 elements in the height of the beam is used. The failure type is also predicted accurately using Al-Mahaidi shear retention function with the stated mesh size. The consideration for aggregate interlock implicitly in Al-Mahaidi shear retention function in the form of shear retention factor allows for accurate prediction of both peak load and failure type. After analysing the shear behaviour of RC beams, the shear behaviour of a reinforced SHCC beam is analysed using Al-Mahaidi shear retention function since it can predict both failure load and failure type accurately for RC beams. In comparison with experiment, the peak load for the reinforced SHCC beam is underestimated and the failure type is also incorrectly modelled. Use of embedded reinforcement results in excessive cracking along the reinforcement, causing convergence issues at a load lower than the experimental peak load. Such excessive cracking is not observed in RC beams since cracks more localized in concrete as compared to SHCC, which exhibits multi-cracking behaviour. Therefore, the shear behaviour of selected reinforced SHCC beam using Al-Mahaidi shear retention function is not accurately modelled. After the analysis of shear behaviour of concrete and SHCC separately, their behaviour is studied in the form of SHCC-RC hybrid beams. The solution strategy consisting of Al-Mahaidi shear retention function is used, and different types of hybrid interface are modelled. The results show that peak load and failure type are accurately predicted when a numerically perfect bond is modelled at hybrid interface for hybrid beams exhibiting no debonding during experimentation. This is in case of a mesh size resulting in 20 elements in the height of beam used. The peak load and failure type, however, are inaccurately predicted when delamination is modelled at the hybrid interface for hybrid beams failing due to delamination during experimentation, irrespective of the mesh size considered. This is due to the inability of the Coulomb friction interface model in recognising significant delamination at the hybrid interface as a reason for the failure of the hybrid beam. ... Read more

Recently experiments were conducted at the Technical University of Delft on the size effect of concrete. The size effect is a term used for the relative decrease in shear capacity with an increase in height of the structural member. The beams observed in the experiment failed much sooner than was predicted. These test results have implications for the Heinenoordtunnel, the roof of which shares many of the characteristics of the beams that were used in the experiments. The question is posed what happens to the Heinenoordtunnel in case of a fire, when also considering the recent tests on the size effect of concrete. The Heinenoordtunnel is analysed with a numerical model. First however, in order to account for the observed size effect, the beams from the experiment are recreated. A study is performed on the effect of various parameters on the numerically obtained failure load, cracking load, crack pattern and deflection in order to find a set of parameters to approximate the observed size effect. It was found that a significant reduction in tensile strength and fracture energy is necessary to obtain a better approximation of the experimental results. However, despite these changes the numerical model still overestimates the shear capacity. This information is used to create a model of the Heinenoordtunnel. A situation without a fire load is analysed and validated. The model is compared with the analytical IBBC-TNO method. Consequently, the model is subjected to a fire load. The fire is modelled using temperature dependent properties and by determining the temperature ingress for a 2 hour RWS fire. A significant shear crack is found present due the fire load, the location and shape of the crack suggesting onset of shear compression failure. The model however is still considered to be in equilibrium and so failure has not actually occurred in the model. A comparison with an analytical model suggests that a shift in bending moments from the increase in temperature results in a shift of shear capacity in the roof. It is concluded that, while the numerical model does not fail, some caution is advised for the translation of these results to practical application. The change of material parameters found in modelling the size effect tests still leads to an overestimation of the shear capacity. On the other hand, the situation that was modelled was an extremity. In the model of the Heinenoordtunnel the absolute physical maximum water load was assumed in conjunction with an extreme fire. It is recommended to check the fire load with computational fluid dynamics modelling, to see if the fire load could possibly be less severe than assumed. ... Read more

Strain Hardening Cementitious Composite (SHCC) is a new type of concrete that is able to control the crack-width in concrete structures. The application of this material in the tension zone of hybrid concrete structures is thus of interest in practice. The interfacial behavior for these hybrid structures is something that is of importance for the composite action. Therefore, a study is performed to investigate the behavior of the interface in hybrid SHCC-concrete beams. This is done by performing an experimental study on beams with a joint at midspan. Several parameters such as the interface roughness, coupling reinforcement cover, curing method and protruding reinforcement have been investigated. The hybrid SHCC-concrete beams are tested in a four-point bending configuration to obtain a constant-bending moment region along the interface. The crack propagation along the interface and through the SHCC/concrete has been evaluated with the use of Digital Image Correlation (DIC). Furthermore, also a numerical analysis has been performed using DIANA FEA based. The numerical analysis was used to help to determine the experimental campaign. Furthermore, also several experimental samples have been modelled to study the behavior of the beam in more detail and to make recommendations for future modelling of the interface. Based on the experimental results, the influence of the interface roughness resulted in an increased bond at the interface. This is both seen in an increased bearing capacity and a reduction of the interface and joint opening at equal loads. The profiled interface and holed interface resulted in an increased capacity due to the mechanical interlocking at the interface. The bearing capacity of these samples increased by 78.4% and 54.7% respectively compared to the reference sample (13.9 kN). In case of the profiled sample, no delamination of the interface occurred as the sample failed due to a horizontal crack at the level of the coupling reinforcement. This can be attributed to the localized tensile stresses at the reinforcement level. The last sample with a different interface roughness consisted of epoxy and sand (1-2 mm). For this sample, the bearing capacity increased by 51.8%. The failure of this sample was a combination of interface delamination (initially at the joint) and a horizontal crack through the coupling reinforcement. For all the samples with a different interface roughness, no yielding of the coupling reinforcement occurred. Several beams with a smooth interface have also been tested by adjusting the reinforcement cover, curing method and applying protruding reinforcement. The influence of the reinforcement cover didn’t have an effect on the bearing capacity. However, the interface and joint opening reduced significantly as the eccentricity of the reinforcement bars also reduced. Furthermore, also the effective height increased resulting in lower reinforcement stresses. The influence of a different curing method was investigated by placing the sample in a humidity-controlled (50 %) room to investigate the effect of shrinkage. The results showed that the bearing capacity remained similar to the reference samples. This can be attributed to the fact that the bond internally was still good, caused by the restraint of the reinforcement bars. However, the deflection of the beam increased as a result of the reduced stiffness by the shrinkage induced cracks. Finally, one sample consisted of stirrups at a distance of 50 mm from the joint to take up the tensile stresses at the interface (e.g. due to reinforcement eccentricity). The results showed an increase in bearing capacity by 102.7% compared to the reference sample. However, also in this case, no yielding of the coupling reinforcement occurred. The failure of the sample is also caused by the delamination of the interface. Furthermore, as a result of this delamination, fracture of the top part of the SHCC occurred at the location of the protruding reinforcement due to the rotational restraint of the stirrup. The 2nd part of the study consisted of a numerical analysis. A Coulomb-friction model with an interface tensile strength cut-off is used to model the interface. Based on this, a good correspondence between the experimental results and FE results is found in terms of the bearing capacity and failure mode for the sample with a smooth interface(delamination). However, the crack propagation of the flexural cracks through the concrete and SHCC was substantially different.
The sample with a profiled interface has also been modelled in Diana FEA. This is done by implementing the profiled interface manually in the FE model. The model showed a good correspondence with the experimental results in terms of bearing capacity and interface and joint opening. The crack propagation in the concrete is also similar to the experimental results. The model however doesn’t show any flexural cracking in the SHCC layer as a result of the limitation of DIANA FEA. Also, an additional study is done to replicate the behavior of this beam using a smooth interface. Based on this model, it is recommended to use a perfect bond at the interface. For both these models, the FE model wasn’t able to replicate the strain hardening behavior of SHCC. This is due to the limiting material models in DIANA. ... Read more

During construction in urban areas often noise and vibrations are not tolerated. For the installation of the foundation piles in these cases there is often chosen for screwed displacement piles. These piles can be installed without nuisance for the vicinity, but do induce large soil displacements during installation. The soil displacements can cause increased soil pressure on adjacent structures. Because urban areas are getting more densely built, these problems will occur more often in the future. The effects can be minimilized when they are known in advanced. This research looks into the possibility of predicting the installation effects of screwed displacements piles on adjacent structures. This is divided in the prediction of the soil displacement in a finite element analysis and in the effect of the soil displacement on adjacent piles. This is done with two case studies. First data from tests in Shanghai is used to create a finite element model. After this measurements are done in Rotterdam to verify this model for Dutch cases.
Combining all the analysis of the measured and modelled displacements showed that the displacement is depended on the stiffness of the soil layers, the initial displacement at the edge of the pile and the distance from the pile. The soil parameters influence the initial displacements in each soil layers. When the initial displacements are correctly determined with tests, it is possible to predict the soil displacement due to the installation of a screw pile with a finite element model. No conclusion could be made on the effect of the soil displacements on adjacent structures. ... Read more

Frozen soil is a powerful tool for engineering purposes due to its increased strength, stiffness and decreased permeability. Water between the soil particles bonds them together, making it possible to use frozen soil bodies as impermeable barriers and load-carrying structures. Furthermore, during a freeze and thaw cycle different processes cause deformations in the frozen and unfrozen soil. For example; frost heave, consolidation of the unfrozen zone, creep and thaw settlements. These phenomena are often called frost actions. Artificial ground freezing (AGF) is regularly used during the construction of cross passages between bored tunnels. The frost actions are expected to increase the loads acting on the lining of the main bored tunnels. This thesis investigates if a quantitative measure of loads due to frost actions on the main tunnels lining can be given with a numerical model, supporting the physical understanding of frozen soil. The objective is to determine if loads due to AGF may be a governing load case on segments of the bored tunnel lining.

Load situations that influence the interaction between frozen soil and the tunnel lining have been identified for the construction of cross passages using AGF. These load situations are based on the principles of ground freezing, construction stages in cross passage construction with AGF, the behaviour of frozen soils and case studies. The following five load situations are identified: frost heave, enclosure of water in the frozen heart, excavation, construction of the lining and thaw weakening.

The load situations have been investigated for one of the cross passages of the Westerschelde tunnel. The studied cross passage was constructed with AGF at a depth of -28,5 m in boom clay. The monitoring program for the studied cross passage of the Westerschelde tunnel was very extensive. Different types of monitors have been used to measure the soil stresses, deformations, water pressures and temperatures in the soil near the cross passage during construction. Before construction, several frozen and unfrozen soil test were carried out on the boom clay. The constitutive model used in the numerical calculation is the frozen and unfrozen soil model of Plaxis. The model requires seventeen model parameters. Furthermore six thermal parameters and three parameters for the soil freezing characteristic curve are necessary. Not all these parameters could be determined directly from the laboratory test, therefore correlations and default values were used as well. The determined parameter set is optimized and validated with help of available laboratory tests. Simulating these simple soil tests gave the opportunity to explore the capabilities of the model. In later stages the optimisation and validation of the parameters turned out to be crucial to obtain a plausible soil response in the large scale models of the cross passage.

The frozen and unfrozen soil model is only available in a two-dimensional version. Therefore, two numerical models have been made representing the construction of the cross passage: one axisymmetric model and one plain strain model. The model results have been compared to the measured data and to each other. The frozen and unfrozen model is able to describe important features of frozen soil behaviour. For more complex engineering challenges, like cross passages, some assumptions in the model are made that influence the capability of the model to simulate certain load situations. The fact that the deformations are independent of the temperature gradient has a large influence on the lining displacements, but also on pore water pressures inside the frozen cylinder. Beforehand it was already known that the constitutive model is rate independent and thus not capable to take creep into account.

Four of the load situations could be qualitatively analysed with the two numerical models .The enclosure of water in the heart of the frozen cylinder could not be simulated with the numerical models. On the other hand, soil stresses due to frost heave and excavation gave a good quantitative measure. In this research one case is extensively investigation, therefore this research is non-statistical. Henceforward, the conclusion cannot be drawn that this quantitative measure of frost heave stresses can also be obtained for other cases. A qualitative measure of loads due to frost heave in construction with AGF can certainly be given with these numerical models. Although not all loads due to AGF could be taken into account (i.e. creep, enclosure of water in the frozen heart), one of the most important load situations (i.e. frost heave) could be quantitatively defined for the boom clay. This load situation is worth investigation in AGF projects, since stresses can become 2.5 times higher than initially measured soil stresses. At the start of the project the boom clay was given a frost-susceptibility index of negligible to low. Even with this mild index the stresses due to frost action increased significantly. This factor and index are probably not the same for other soil types. However, this study shows that such large stress increases are a real possibility during cross passage construction with AGF.
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The hyperloop system is a new transportation mode, which consists a magnetic levitating capsule-like hyperloop pod and a vacuum tube. Due to small air hindrance, the hyperloop pod is conceived to have a maximum speed of 333 m/s. If such a hyperloop system is to be built underground in soft soils, the hyperloop speed can easily reach the wave propagation speeds in the soil. Strong wave radiation is expected when the hyperloop is travelling at wave propagation speeds, which are called the critical speeds.

The first objective is to analyse the dynamic influence from the hyperloop. A linear elastic half-space with an infinitely long concrete tunnel buried at a certain depth has been modeled. The excitation of the system is a hyperloop modeled as a moving constant load acting at the tunnel invert. In this thesis, a so-called indirect boundary element method (BEM) is applied. Indirect boundary integrals are formed which rely on the fundamental solutions for the interior medium, the two-and-a-half dimensional Green's functions. These 2.5D Green's functions are essentially the steady state solutions of the half-space subjected to a spatially varying line load. The space is assumed to be infinitely long and invariant in the direction parallel to the axis of the tunnel.

Before implementing the BEM model, two improvements have been made to the 2.5D Green's functions: a better convergence of the Green's function surface-related terms and a better satisfaction of stress-free boundary conditions at the free surface. The accuracy and correctness of the boundary element model using the improved Green's functions have been verified by intensive case studies. Firstly, the scattering of 3D harmonic seismic P waves by a cavity and a tunnel in a linear elastic half-space is analysed. Results are validated by comparing to those from literature. Secondly, the BEM model is employed for the moving load problem. The embedded concrete tunnel is modeled using the Donnell's theory for thin shells. A coupled form of the indirect boundary integrals is formulated. Using the same model parameters, the results obtained by the BEM are in good agreements with those from literature. Moreover, a parametric study has been conducted to study the effect of moving load velocity, tunnel depth and thickness of concrete lining on the dynamic response.

As a second objective of the current thesis work, the BEM model is compared with a finite element method (FEM) based model, developed by Movares B.V. The models are compared in both accuracy and computational efficiency. In the FEM model, the moving load is considered as a series of consecutive short pulses. The contributions from all the pulses are synthesized using a convolution. Furthermore, since the space is invariant in the direction parallel to the tunnel axis, it is possible to apply just one stationary impulse load in the finite element model. Using this method, a constant moving load and a moving load with acceleration are modeled. The FEM results are found to have close agreements with those by the BEM. Besides the Rayleigh wave speed in the soil, a second critical velocity which is related to the wave propagation in the tunnel is found. Furthermore, the case where a hyperloop runs constantly at the Rayleigh wave speed is more crucial than the case where the hyperloop accelerates and passes the Rayleigh wave speed. ... Read more

A series of accelerated corrosion tests were done by SGS INTRON (commenced by Combinatie Aanpak Maastunnel, constructor of the Maastunnel project) to investigate for a proper repair material for the deteriorated concrete floor in the Maastunnel at Rotterdam. Five types of fiber reinforced mixtures with different tensile behavior ranging from SHCC (strain-hardening cementitious composite) to ON06 (similar tensile behavior as normal concrete) were designed. Different cracking behavior was observed due to the different tensile behavior in the repair mortar. It is of utmost importance to investigate if the cracking behavior can be simulated by numerical modeling and if in the future, the parametric analysis might be performed without large experimental series.

Two types of numerical models are implemented in this master thesis, namely the lattice model and the continuum model. The lattice model can simulate the crack pattern of different materials in the accelerated corrosion test. The continuum model cannot show the behavior of decreasing number of cracks with decreasing fracture energy in the strain-softening materials due to the bifurcation problem brought by its incremental solution method. However, the lattice model shows the trend to underestimate the crack width of the strain-softening material. The continuum model has better performance in predicting the crack width. Also, the lattice model can predict the influence of the repair mortar-substrate bond strength on the crack pattern. Meanwhile, the continuum model always shows a complete failure in the repair mortar-substrate interface.
The boundary conditions at the bottom edge are observed to influence the direction of the bottom cracks. The edges of the specimen are kept free in the experiment. However, the repaired area is constrained by the surrounding concrete in reality. This indicates that the laboratory test may not represent the cracking behavior of the concrete floor in the tunnel accurately. SHCC is observed to be more sensitive to the repair mortar-substrate bond strength than material with lower stain capacity. Extra caution on the bond quality is advised while applying SHCC in a concrete repair system.

From the material point of view, with increasing fracture energy and strain capacity, more but thinner cracks can be performed in the accelerated corrosion test. SHCC material can perform the distributed crack pattern with a maximum crack width of 0.1mm which is ten times smaller than normal concrete. This behavior of SHCC is very suitable for being applied to a concrete repair system. The distributed cracks with smaller crack width can effectively limit the possibility of further corrosion. Besides SHCC, some strain-softening (under direct tension) materials can also show the deflection-hardening behavior in the bending test and produce the distributed crack pattern. SHCC and fiber reinforced concrete with deflection-hardening behavior in the bending test are suggested to be used in the concrete repair system. ... Read more

Areas are getting more and more populated causing new infrastructure lines to be constructed below the surface. A bored tunnel is one of the possibilities to create this subsurface infrastructure, but the construction process of a bored tunnel is a complicated one. Many loads and aspects are present in this construction process that can be divided into six phases. In each of these phases, different loads and aspects are acting on the tunnel lining or the surrounding soil which can cause the lining to deform.

The increasing complexity and demands of problems have led to the use of the finite element method. A computer based method which allows one to model the problem.

For finite element modelling, numerous programs are available of which several claim to be able to model bored tunnels. However, it is not yet clear what the exact differences between the different programs are. With many aspects to be modelled, many differences between programs occur, either in the soil, the tunnel lining or a combination of both.

This research has focussed on the possibilities of modelling the different construction phases of bored tunnels in two widely used programs: DIANA and Plaxis. Simple two dimensional (2D) models were created to which the different construction phases were added before continuing with three dimensional (3D) modelling. This approach has led to a good assessment of the possibilities and limitations within these two programs.

DIANA is not yet suitable for modelling the construction process of bored tunnels completely. The construction phases are modelled undrained to account for the relative short time they are acting. A consolidation phase in which the pore pressure can dissipate cannot be modelled in DIANA, which is essential for modelling the construction phases.

Plaxis, on the contrary, is not able to model joints in the segmental lining appropriate for 3D. In 3D, Plaxis only allows to model a joints as "fixed" or "free". In DIANA different theories can be applied to the joints, including Janssens. For 2D, both programs have a rotational springs besides the free and fixed connection for modelling the joints.

For the model in which the material models were changed, the difference with the main impact between the two programs occurred, especially for the bending moment. This means the Modified Mohr-Colomb material model in DIANA is different than the Hardening Soil model in Plaxis.

Including the construction phases leads to more favourable internal lining forces for tunnels, something of which clients should be convinced. However, not until the models have been benchmarked with measured data from a tunnel project.

While 3D models have been investigated in this research, they should be extended in order have a better understanding of the different 3D phenomena that are present in the construction of bored tunnels. This will both assess the possibilities of modelling this process and more potential differences between programs can be investigated.

Besides extending the 3D models, other programs should be investigated on their capabilities too. In order to come to a proper assessment of the possibilities, program experience is strongly recommended. These programs should also be compared with measured data for benchmark purposes. ... Read more

Watertightness and leakage prevention is crucial in every tunnel joint design. Immersed tunnels have been utilizing the Gina-Omega gasket solution since the 1960s to prevent leakage from occurring. However, after almost 50 years of service life, leakages were detected in the immersion joints, which leads to an investigation into the source of the problem. It was uncovered that the bolts of the Gina gaskets had failed due to the immense load exerted by the sand above them. It was apparent that the load on the gaskets builds up due to an increase in soil stresses. A hypothesis was formed regarding this phenomenon; the sand inside the joint gap was exposed to almost 50 years of loading and unloading cycles from the expansion and contraction of the tunnel elements due to seasonal changes in temperature. This, in turn, densifies the sand inside the joint gap, which results in rising soil stresses. A 1:3 scaled physical model of the joint gap was designed and constructed to test the validity of this hypothesis. The model joint gap is equipped with a static lining on one end and an actuated lining on the other, hence the device is able to imitate the annual joint contraction and expansion cycle. A barrel containing sand is fixed onto the top of the model joint gap and acts as a reservoir of sand, allowing for more sand to enter the joint gap. The device is also equipped with 2 load cells and an LVDT, which allows for the measurement of horizontal soil stresses and vertical gasket displacement respectively. Two holes, with a flap covering each of them, are installed on the side of the model joint gap, which allows for a penetrometer test to be conducted on the joint gap sand. Multiple experiments with varying configurations and test conditions were performed. The results show that although the multiple loading and unloading cycles apply the same displacement for every cycle, the soil stresses increase with time. The Gina gasket show an apparent “walking effect,” where the gasket moves continually inwards. While a similar test conducted without the presence of sand fails to produce any “walking effect.” Penetrometer measurements show that the soil increases in density over time. The investigation is continued further with finite element analysis using the geotechnical modeling software PLAXIS. The joint gap part of the device is modeled in the program and is subjected to loading conditions and configuration similar to the physical model. Results of the simulation show indications of gasket “walking effect,” as well as stress-strain behavior similar to the experiment results. A comparison analysis between the results of the physical and finite element model is subsequently conducted. The finite element analysis allows for the calculation of the sand vertical stresses, which is previously unable to be measured during the experiment. It is observed that the vertical stresses rapidly escalates due to the high friction between the sand and the lining wall. A further analysis is conducted to estimate the force required to push back the displaced Gina gasket. Due to the good agreement between the results of the physical and finite element models, a modified version of the finite element model is used to predict the resulting pushing-back forces. Finally, the research is concluded with the validity of the previous-mentioned hypothesis. It was also confirmed that the soil undergoes densification, proven by the penetrometer readings as well as further validated by the stress-strain behavior of the sand. It is also proven that at higher horizontal strain, the increase in density, stresses, and the “waking effect” is more pronounced. The force analysis produces values of force needed to push back the Gina gasket. It was concluded that pushing back the entire Gina gasket upwards would require a high amount of force. However, affecting only 1/3 of the gasket bottom area would result in the gasket merely being pushed aside while failing to push the soil upwards. ... Read more