The study focused on the use of associated and non-associated plasticity models in FEM simulations, with particular focus on the role of the dilatancy angle. The dilatancy angle was shown to significantly influence both the shear strength, the deformation behaviour of soils and the mesh dependence of model outcome, particularly in situations with confined or constrained deformation. To evaluate this, three cases were analyzed: Direct Simple Shear (DSS) test, slope stability, retaining structure. ... Read more

In the borough of IJsselmonde in the city of Rotterdam, there are significant problems with the tram track geometry. The subsoil consists of soft soil types, such as peat and clay. At certain locations, settlements result in severe irregularities of the vertical alignment. At the roundabout at the Groeninx van Zoelenlaan, the track deflections are so severe that the protective shield underneath the front end of the tram touches the surface level and the emergency brakes are activated, which is very uncomfortable for the passengers. The tram track structure is only 18 years old, which is significantly lower than the intended life span of 30 years. This research aims to balance and optimise the sustainability and durability performance of the tram track structure and therefore, the following research question is defined: “To what extent can the durability and sustainability performance of a tram track structure on soft soil conditions be improved while preserving the vertical track geometry?”

PLAXIS 2D and PLAXIS 3D simulations are performed to investigate the structural performance of the tram track superstructure for the current situation and for a situation where light-weight filler materials are used. This study reveals that Rockwool, foam concrete and EPS are suitable as a foundation to reach the intended life span of 30 years. When considering the carbon footprint over 30 years, installing EPS and Rockwool result in a reduction of 3.4% and 1.5% respectively. Further CO2-reduction can be obtained when embedding the track in olivine ballast and producing rails from recycled steel in Electric arc Furnaces. This leads for a track built on Rockwool or EPS to a carbon footprint reduction of 72.3% and 74.4% respectively. When taking into consideration that Rockwool can be used as water buffer and therefore contributes to a more climate-resilient neighbourhood, this is considered to be the most sustainable alternative. So overall, when Rockwool and EPS are used, the sustainability and durability performance of the tram track structure on soft soil improves while preserving the vertical track geometry.
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The current Hardening Soil small-strain (HSsmall) model, developed by Benz (2007), is capable of capturing the behaviour of a wide range of soil types, including their small-strain response. However, under specific conditions, the model exhibits overshooting, which is the overestimation of the material’s elastic stiffness upon the closure of a small unloading-reloading (UL-RL) cycle interrupting monotonic loading, rather than recovering the stiffness corresponding to the onset of the cycle, as observed in real soil behaviour. Overshooting leads to underestimated deformations and consequently non-conservative results.
To address this issue, PLAXIS (part of Seequent, the Bentley Subsurface company) proposed two formulations: the Continuous Brick (CB) formulation, which replaces the small-strain component of the HSsmall model, and a Memory-Surface-Based (MSB) formulation, which extends it. Both have been implemented in the source code of the existing HSsmall constitutive model. However, their implementations had not yet been verified in the literature, nor had an assessment of overshooting, the motivation behind their development, been conducted.
This research aims to close that gap and to determine which of the two formulations provides the most suitable approach. Implementing the best-performing formulation into the HSsmall model results in a new more robust state-of-the-art model.
Accordingly, the following research question is posed:
‘To what extent do the Continuous Brick formulation, as a new formulation for small-strain stiffness, and the Memory-Surface-based formulation, as an extension to the existing small-strain stiffness formulation within the Hardening Soil small-strain model, reduce the overshooting observed in the current formulation?’
This question is addressed through a structured test plan consisting of two main components: (i) single stress point simulations to verify whether the formulations behave as expected at the most fundamental level, and (ii) a boundary value problem to evaluate their performance under more numerically demanding conditions representative of practical applications in the pre-failure range, where small strain stiffness strongly influences the magnitude of deformations.
It was found that both formulations reduce overshooting to a negligible level. However, their effectiveness decreases in simulations involving nested cycles, as both formulations exhibit the limitation of retaining the memory of only a single UL-RL cycle. Although both formulations perform well, the MSB formulation proves to be the most suitable approach: it is easier to interpret, appears more robust, retains the small-strain component of the original HSsmall model, and yields a response consistent with the HSsmall model under monotonic loading.
Extending the HSsmall model with the MSB formulation leads to more accurate deformation estimations in geotechnical problems especially within the pre-failure range, as deformations will no longer be underestimated, without introducing additional model parameters. Moreover, adopting this formulation will not have major consequences for the end user, since its behaviour is consistent with that of the HSsmall model, except that overshooting no longer occurs.
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Subsidence is a significant geohazard affecting many areas globally, including the western part of the Netherlands. Subsidence is the lowering of the ground relative to the surface level, and it occurs both on the scale of a single structure and larger areas. The shallow subsurface in these areas, characterized by compressible soil, contributes to subsidence, which may be initiated or intensified by additional drivers. Subsidence can cause damage to buildings, leading to increased costs and social concerns

In the current state of the art, less distinction is made regarding the difference in effects between different subsidence drivers. This study aims to provide insights into the relative influence of different drivers on the damage to existing buildings with a shallow foundation. More specifically, it aims to understand how various conditions and factors influence the damage parameters associated with soil deformation. This approach allows for analysing the interactions between the soil and the structure, ultimately contributing to a better understanding of the relative influence of the different drivers.

Numerical models in PLAXIS 2D have been carried out to compute settlements for various scenarios, involving different soil scenarios, building scenarios, and subsidence drivers. The primary emphasis of numerical modelling lies on the soil rather than on the structure itself. Various damage parameters have been established based on the numerically calculated settlements. The purpose of this analysis is to determine the impact of settlements on the building based on these damage parameters.

Several aspects related to settlement occurrence and its impact on buildings were investigated. The study provides multiple outcomes regarding the interaction between soil scenarios, different drivers of subsidence, and the presence of an existing building with and without a partial basement. This was achieved by considering the influence of each of these variables on the damage parameters, with emphasis placed on the relative influence of the different drivers. The approach includes a method that compares the influence of the situation with the existing building and the situation without the existing building (greenfield situation). This comparison shows the settlement behaviour caused by the presence of the building load and its interaction with the soil. Additionally, the combined effects of the soil scenarios, different drivers, and building scenarios on the resulting damage to an existing building are considered.

To conclude, for the soil scenarios considered in this study, the soil scenario with a weak spot (SS3) has the most unfavourable effect on an existing building. For the drivers considered in this study, when evaluating the relative influence of the different drivers, the global groundwater lowering (D2glo) has the most unfavourable effect. Considering the combined effect, the soil scenario exerts the greatest influence on the resulting damage parameters for the evaluated scenarios, followed by the type of driver and the building scenario. ... Read more

In response to the urgent need for sustainable energy sources to combat climate change, offshore wind power has emerged as a promising solution. However, the installation process of offshore wind turbines, particularly the driving of monopile foundations, presents challenges, notably concerning underwater noise pollution and its environmental impacts. This research studies the efficacy of an alternative approach to traditional installation methods: the vibratory pile driving, renowned for its minimized noise impact. It focuses on its effects on the long-term performance of monopiles under cyclic lateral loading, through numerical simulations. By addressing certain uncertainties, the aim of this work is to contribute to optimizing offshore wind turbine installation practices and ensuring the stability and performance of monopile foundations in challenging marine environments.

Two models are integrated and merged to address the previous objectives. The first model simulates the dynamic behaviour of the soil after vibratory installation effects. Meanwhile, the second model analyzes monopile response to lateral loading induced by environmental factors like wind and waves. The OpenSees software is employed for the computation of 3D finite element analyses, and the soil, represented as dry, initially dense, Karlsruhe fine sand, is modeled using the SANISAND constitutive model, which relies on the Critical State Soil Mechanics framework, to accurately capture stress and state-dependent behaviour. Only half of the monopile's embedment depth is evaluated, due to computational constraints.

Both the behaviour of the soil after the vibro-installation process and after the lateral loading are evaluated. Significant vertical and radial displacement occurs during pile driving, leading to settlement around the pile shaft and mudline as soil densify. Horizontal displacement patterns indicate an initial outward movement followed by lateral drawing-in towards the pile shaft, driven by soil compaction and rearrangement induced by installation vibrations. Notably, post-installation, there is a marked increase in relative density around the pile shaft, enhancing soil strength and friction, particularly near the pile tip. This densification, along with changes in mean effective stress, significantly affects soil behaviour and sets the stage for subsequent lateral loading.

After the lateral loading stage, the influence of installation on pile response becomes apparent. Post-installation soil conditions profoundly impact lateral displacement patterns, with vibro-installed piles exhibiting larger displacements during initial loading cycles compared to wished-in-place piles. Throughout lateral loading cycles, localized soil densification and remoulding further influence stiffness and displacement patterns. Notably, the relative density changes reflect these alterations, showing the intricate interplay between installation effects and lateral loading response. Overall, the results emphasize the necessity of considering installation processes in predicting pile behaviour accurately.

While this study provides valuable insights into the behaviour of piles in dry sand conditions, it also underscores several limitations that necessitate further research. Future investigations should address these limitations to provide more robust insights into the behaviour of offshore wind monopiles and inform more effective design and installation practices in the renewable energy sector. ... Read more

This thesis describes the investigation of the lateral monopile response in weak rock and the effect of a zone of crushed rock that is induced by driven installation. The goal of the investigation is to study the impact of the weaker zone around a monopile on the lateral response. The thesis focuses on the modelling of a monopile in a finite element model to examine the effect. The modelled monopile is compared to field tests that were done in previous research to be able to benchmark the model. To full-fill the research objective several steps were taken. The research can be divided into four parts to get to the final conclusion, being a literature review and three different models that are set up. The goal of the literature review and the two model in 2D are to act as a basis for the setup and inputs of the 3D model. The 3D model can then assess the effect of a crushed zone on the lateral capacity and stiffness response of the pile.
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The Netherlands is prone to flooding as more than a quarter of the country lies under sea level. To combat flooding and ensure that the country remains dry structures such are levees and dikes have been installed. However, older water retaining structures are more than ever failing the stringent safety standard assessments. These older conventional reinforcement measures, including berm constructions, are not only costly but require an expanse of ground to ensure performability.

Backward erosion piping is an internal erosion mechanism during which shallow pipes are formed in the direction opposite to the flow underneath water-retain structures as a result of the gradual removal of low cohesive material by the action of water. This mechanism is an important failure mechanism in both levees and dams where a cohesive layer covers a sand layer. Although failure resulting from backward erosion piping is not common, several levee failures in the United States, China and the Netherlands have been attributed to this mechanism.

There are mitigation measures known to stop the backward erosion mechanism. One such measure is the placement of a seepage wall, to create a physical barrier directly in the flow path trying to reach the lowest region of the hydraulic head. A review of the literature showed that current design rules only consider groundwater flow calculations when determining the likelihood of hydraulic heave, one of the failure modes within the backward erosion process. Hydraulic heave in the backward erosion piping context is closely linked to the quicksand condition, essentially stating that once the effective stress is zero, the sand particles become suspended, liquifying a solid layer. The absence of an assessment of the effective stresses during the design process in conjunction with hydraulic heave has contributed to the main research question addressed by this thesis; How does a restricted exit for groundwater flow affect hydraulic heave compared to Terzaghi’s free exit situation?.
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Crane hardstands serve as crucial platforms for supporting heavy lifting equipment and ensuring operational efficiency. However, crane hardstands are not infinitely stiff and will deform upon loading. Uneven loading during wind turbine installation will result in differential deformation of the crane hardstand, causing the crane to tilt. A small tilt of 0.3°, which is equivalent to around 30-60mm differential settlement depending on the crane, will create safety hazards, causing construction to be discontinued. As a result accurate deformation predictions are required to design a sufficient crane hardstand.

This research is conducted to investigate the influence factors of the deformation of a crane hardstand, evaluate the current prediction method and to improve the accuracy of future deformation predictions, so the hardstands can be designed more efficiently.

The research begins with a literature study on the above surface influences on the magnitude of the load and the corresponding soil behaviour of the soil profile beneath the hardstand. Furthermore the current prediction method is analyzed to dictate shortcomings. The expected influences found in the literature study are examined with full scale monitoring and testing cases. Finally, a sensitivity analysis is performed on the current prediction model to specify the parameters with the biggest influence on deformation for different variants. These parameters are then assessed on how a more accurate determination might influence the predicted deformations.

The numerical simulations are carried out using advanced finite element analysis software Plaxis, specifically the HS(small strain) model. This model enables the investigation of various factors affecting hardstand deformation, such as varying soil stiffness, load distribution, and foundation characteristics.

The biggest shortcoming of the current prediction method is found to be the exclusion of time dependent behavior. And the most influential soil parameters of the HS(small strain) model after the addition of a consolidation phase to the model are found to be the stiffness and permeability parameters. The deformation prediction is done for the entire range of uncertainty of these parameters (5-, 25-, 50-, 75-, 95- percentiles) to quantify prediction accuracy improvements were these parameters determined witch precise. For both peat and clean clay the permeability coefficient is found to, when determined more accurately, have a 50% chance to result in a predicted deformation reduction of between 40 to 60 %, while a more accurate prediction of the stiffness parameters E_oed E₅₀ E_ur has a 50% chance to result in a predicted deformation reduction of between 65 to 75%

The findings of the research can be used by engineers to test the effectiveness of their own hardstand deformation prediction method and provide advise on the benefits extra soil investigation might lead to.



Keywords: Crane hardstands, deformation analysis, differential settlement, cyclic loading, Hardening soil small strain, FEM-modeling, sensitivity analyses. ... Read more

Terps are artificial dwelling mounds mostly found in the northern regions of the Netherlands, built to provide safe ground against water in such areas affected by flooding, storm surges, and high tides, before the development of dikes. In this thesis, the stability of these terps was evaluated via FEM models and analyses performed on PLAXIS. Results highlight considerable stability risks when the terp slopes are subjected to considerable external loading, such as those exerted by heavy agricultural vehicles being operated in proximity of these.
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For supporting Offshore Wind Turbines (OWT), monopiles are currently the most common foundations. The role of a foundation is to transfer safely the loading to the ground. The wind and the wave loads are considered cyclic because they repetitively apply on the OWT. The North Sea is sand dominated in many areas. During cyclic loading, permanent strains develop in the surrounding soil while the soil stiffness and strength are irreversibly affected. Through time, the accumulation of strains can lead to the soil failure. Thus, assessing the behaviour and stability of monopiles under cyclic loading is essential. To model the response of monopiles under lateral loading, the traditional design procedure is the use of p-y curves that express the lateral soil resistance in function of the pile deflection. The p-y curves are nowadays recommended to be calibrated on FE models. The Stiffness Degradation Method (SDM) of Achmus et al. (2009) is a numerical strategy that assesses the behaviour of a monopile under cyclic loading. The method estimates the cyclically degraded stiffness based on the results of a static analysis. The soil stiffness is degraded based on a semi-empirical power law that accounts for the number of loading cycles, the stresses in the soil after the static analysis and two model parameters calibrated on cyclic triaxial tests. The SDM was successfully implemented in PLAXIS 3D via a practical routine coded in Python and the use of soil clusters around the pile. The soil stiffness is degraded by updating the soil material within the clusters. The study model was verified by comparing results with the published reference system of Kuo (2008) for two piles with embedded length to pile diameter ratios of 2.7 and 5.3. The results indicate that the study model provides a stiffer pile-soil response than the reference model because the soil stiffness is overestimated. The degraded stiffness overestimation is attributed to the initial stiffness mismatch and the use of soil clusters. The impact on the short pile is greater than on the long pile because the short pile opposes less resistance to the loading and is thus more affected by the stiffness difference. The study model was validated against three 1-g pile tests for homogeneous uniform and multi-layered dense sand. The numerical results are in agreement with the test data. In the absence of cyclic triaxial tests, the two model parameters were directly calibrated on the pile head displacement of the experiment. The two model parameters have a significant impact on the stiffness degradation. Thus, model parameters from literature were classified from the highest to the smallest estimation of pile lateral displacement. The results of codified and published approaches (DNV-GL-0126; Duhrkop, 2009; Garnier, 2013) were compared with the results of the study model. The study model and the method of Garnier (2013) are in agreement. They both account for the loading amplitude, the number of cycles and the pile geometry. The codified procedure and the method of Duhrkop (2009) estimate higher lateral displacement compared to the study model. Finally, the 1D model was successfully calibrated with the highest displacement estimate of the study model. With this procedure, the 1D model accounts for the number of cycles, the pile geometry and the loading amplitude. The study model provides a less conservative approach for determining the pile lateral displacement under cyclic loading. The calibration of the 1D model on the pile deflection curves of the study model is a promising procedure which will require further research. ... Read more

The stability of breakwaters in seismically active areas is not always guaranteed. A new approach to model a breakwater subjected to an earthquake is with PM4Sand. The goal of this research is to find if breakwaters subjected to earthquakes can be correctly modelled with PM4Sand.
To investigate PM4Sand a breakwater subjected to an earthquake on centrifuge scale is modelled in Plaxis. The soils of the centrifuge test are modelled with PM4Sand and UBCSand. After calibrating the soil parameters and incorporating the proper earthquake signal results are generated and compared. The investigated results focus on the settlements of the caisson, deformations of the breakwater and generated Excess Pore Water Pressures underneath the breakwater due to the earthquake. Comparing the results from the numerical models with the centrifuge test result show that both the UBCSand model and PM4Sand can give comparable results for the settlements and deformations. However, both UBCSand and PM4Sand were not able to give the correct EPWP development underneath the breakwater.
Due to the incorrect behaviour of the EPWP underneath the breakwater resulting from the numerical models, this research is not able to conclude that PM4Sand can be used for modelling breakwaters subjected to earthquakes. Further research is needed to investigate the development of EPWP underneath the breakwater during an earthquake. Focus points of future research can be: the influence of the amplitude of an earthquake signal on the EPWP, the influence of modelling a centrifuge test on the behaviour of the EPWP and the influence of the initial static shear stress on the EPWP development. ... Read more

During the boring of a tunnel in soft soils with a slurry TBM, support pressure is used to achieve equilibrium at the face of the TBM. When this equilibrium is not reached, when the face support pressure is too low or too high, settlements will occur. In this research settlement and pore water pressure measurements are used to monitor the behavior of the soil and estimate the stability of the tunnel face. During boring of the tunnel, the exact stability of the face is not known. The TBM driver has to rely on the provided stratigraphy data, the advised face support pressures range provided by the geotechnical engineers and the experience of the tunnel boring team. Monitoring is not yet used to determine the face stability during construction. To do so, field data from a case study at RijnlandRoute is compared with analytical and numerical models. Sensitivity of the measurement equipment, and of both the analytical (DIN) and numerical model (Plaxis 3D) with respect to soil parameters, are considered. It has been found that the strength parameters of the layer in which the face is located have the highest influence on the minimum face support pressure. For the maximum face support pressure this is the volumetric weight of the entire soil profile above the face. For comparing the (soft soil) field data with numerical results, a Plaxis 3D model is built, and it is determined that HSsmall is a suitable constitutive model to capture the interaction between face stability, tunneling operations and soil behavior. It is shown that the tail void injection influences the settlements above and in front of the cutter head, but this influence is discarded and replaced by a wished in place lining, to simplify the numerical model. A scenario analysis on the sensitivity of soil parameters shows settlements do not vary significantly between the characteristic low and high values. In this analysis correlation of parameters is taken into account. The failure mechanism for the minimum face support pressure coincides with the active cave-in failure mechanism found in literature. For the maximum face support pressure the failure mechanism found in Plaxis 3D does not coincide with the expected hydraulic fracturing failure mode. The continuum representation of the soil in Plaxis 3D does not allow a hydraulic fracturing like failure mechanism to develop. Instead, a blow-out approximately 10 m. in front of the cutterhead occurs. This behaviour better resembles the failure mechanism of a EPB TBM. The field data gathered from the case study shows a thrust wave in front of the cutter head in both settlement and pore water pressure measurements. This thrust wave reaches up to 20 to 40 meters in front of the TBM. The heave induced by the thrust wave reduces the amount of settlements after the TBM passage. Excess pore pressures, induced by a high thrust wave, affect the face stability negatively. The excess pore pressure mainly depends on the advance rate. The higher the advance rate, the less time the pore pressures have to dissipate, leading to an increase in excess pore pressure. The accuracy of the settlements measurement devices is 0.8 mm., and of the spade cells 1.0 kPa. In general, the field data shows settlement curves corresponding to the Peck (1969) Gaussian curve (in lateral and longitudinal direction). Comparing the case study settlements with the numerically generated settlement curves show similar trends. The field data shows lower settlements than the numerical results. This can be due to the presence of excess pore pressures, the accuracy of the TBM data or its interpretation. A method to increase the accuracy, which is expected to result in a better fit with the numerical results, was found in a late phase of the research. This method takes into account the settlements which are induced by the thrust wave in front of the TBM. Comparing the numerical and analytically determined limit support pressures, it is found that the minimum face support pressure are similar in both methods. As similar failure mechanisms are found, numerical modelling seems a reliable way of determining the face stability. However, due to the limited range of applied support pressures available from the TBM data set, limit states could not be fully analyzed. To assure the reliability of a numerical model to determine the actual face stability based on surface settlements during the construction phase, additional research must be done. It is suggested to extend the methods used in this research with physical modelling. For maximum face support pressures, the analytical and numerical models do not coincide. Hydraulic fracturing cannot be modelled in Plaxis 3D. It is recommended not to use numerical modelling to determine the face stability based on settlements at face pressures higher than the face pressures at which an equilibrium is achieved. ... Read more

Throughout the years the requirements of sheet pile walls have changed. Therefore reassessment of these structure's reliability is of importance. In this thesis, Bayesian updating is used for the reliability updating task. Updating processes require measurements of the structure under known conditions. Although for practical and economical reasons failure measurements of structures are seldomely available. Therefore the research focusses on whether it is possible to update a sheet pile wall's reliability using service domain measurements instead. Parameter updating methods generally return the statistically most likely parameter values for producing the observations. Using a theoretical sheet pile wall case, it is tested if the Bayesian updating method is able to effectively return the true soil parameter values as the most likely parameter set. The results show that the Bayesian updating method is very capable of approaching the used observations with the updated model response. Also the updated values of the most influential parameters show evolution in the direction of their true values. But the method does have difficulties with returning the true soil parameter values, even when applied to a theoretical case and with the use of elaborate observation configurations. Recommendations are given on further research concerning the use of the Bayesian updating method, the different influences on its performance and method application limitations. ... Read more

Numerical modelling of pile foundations can be done in several ways. In commercial finite element packages two main options are available; using volume elements with interface elements between the pile and soil domains and the embedded beam approach. The embedded beam element was first proposed by Sadek and Shahrour (2004) and considers a beam element that can cross a solid element at any arbitrary location with any arbitrary inclination. This has several advantages to the volume pile method, such as the need for fewer elements and the mesh uncoupling of the pile and soil, which make this method much more efficient and leads to a significant reduction in calculation time. However, the embedded beam element also deals with a number of limitations and drawbacks. This research focuses on overcoming the mesh sensitivity, which is caused by the stress singularity that is introduced in the soil by the beam element. Also, the inability to take into account the pile surface will be resolved, aiming to improve the lateral pile-soil interaction.

The idea of Turello et al. (2016) of an embedded beam element with explicit interaction surface is extended and generalised leading to a new embedded beam formulation. In the proposed model the beam displacements at the interaction surface are obtained by a mapping scheme that takes into account Timoshenko beam theory and which is generalised to model inclined piles as well. A constitutive equation that describes the relation between the interface stresses and relative displacements between the pile and soil is defined along the shaft and at the foot of the pile. Along the shaft of the pile a shear stress limit is defined based on the Mohr-Coulomb failure criterion in order to incorporate plasticity in lateral direction. Furthermore, a more practical and efficient assembly procedure is proposed.

Validation of the proposed method proofs that the proposed model leads to a significant mesh sensitivity reduction in case of axially loaded models compared to the existing implementation. The overall response of laterally loaded piles is improved considerably as well. However, the proposed method is still unable to capture lateral interface behaviour in order to model soil slippage around the pile. Furthermore, it is recommended to formulate a generally applicable foot interface stiffness and to optimise the code in order to reduce the computation time. The description of the interaction surface opens up many new possibilities for future research, such as modelling the true cross-section shape.
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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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Offshore wind farms are rapidly developing in Western Europe as an alternative means of clean energy. The foundation of wind-turbines in the shallow water of the Northern Sea is almost exclusively (over 87%) performed by short rigid monopiles, as an efficient and relatively cheap solution. Scour formation, meaning the soil removal around the pile due to the actions of the waves and currents, is a potential hazard for the functionality and the integrity of the structures supporting the turbines. This thesis investigates the effect of scour formation in the soil-pile capacity and examines the possible contribution of the scour protection layers in the stiffness of the system both under static and cyclic loading. The methodology followed included 18 main static and cyclic centrifuge experiments and 9 basic cyclic numerical analyses. The physical modelling included an aluminum pile, equipped with strain gauges in order to calculate bending moments and ultimately derive the “p-y curves”, which was embedded in a dense dry Geba sand. Numerical analyses have been conducted in PLAXIS 3D, with a symmetrical fully drained model, in prototype scale, with the same set of materials (Geba sand, aluminum pile).
The results have shown that scour formation can diminish the lateral soil capacity in the ultimate limit state (ULS). Scour depth is the most critical characteristic of the scour hole geometry, but the scour width at a specific depth can also make the difference between structure’s integrity and failure. Therefore, the term “local” scour is deemed insufficient, as it cannot be described by a unique geometry. A valid design against scour is recommended to include a series of analyses with combinations of scour depth and width in realistic ranges. Experimental derived “p-y curves” have been compared with the ones proposed by the API method, concluding that the API overestimated both the initial response and the ultimate capacity of the soil reaction. It is proposed to update the existing regulations for the rigid piles and add rotational springs to simulate the effects of the considerable shear developing in the tip of rigid monopiles. The effect of the scour width in the “p-y curves” was observed to be limited in the shallower depth of the pile, as going deeper resulted almost to the same soil response regardless of the type of scour. The cyclic centrifuge experiments focused on the load type, investigating different scenarios of “one-way” and “two-way” load patterns. It was shown that the “one-way” case is more favorable in terms of accumulating deformations compared to the “two-way” case which experienced higher residual displacements, as long as the same maximum load was applied. This was attributed to the smaller dissipation of energy and hence destruction of the soil structure by the “one-way” loading. However, when the maximum load applied in a “two-way” experiment is considerably smaller than the equivalent of the “one-way” test, smaller deformations observed in the “two-way” test, implying the significance of the maximum load. The last section of this thesis, the numerical analyses, focused on the scour protection effect in the mechanical properties of the soil-pile system when subjected to cyclic loading. It was shown that a typical protection layer can highly increase the stiffness of the system and hence decrease the accumulated displacements. The length of the protection layer is not crucial, as change in its magnitude does not alter considerably the reduction of the accumulated displacements, in contrast with the thickness which has a larger impact on the stiffness of the system. It is concluded that scour protection layers can considerably increase the soil resistance around the monopile, allowing for smaller embedment length, so their contribution in the soil-pile stiffness should be taken into account for a more economic design.
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The production and injection of fluids from and in reservoirs leads to changes in the in-situ stress in the subsurface. This can cause reservoir compaction, subsidence, fault reactivation and / or seismicity. As these effects may greatly influence society it is of importance to find accurate methods to describe them so they can be predicted or even better, mitigated.

This thesis discusses a new analytical approach to calculate stress in the subsurface which incorporates the effects of differential compaction on the initiation of fault reactivation. This new approach is named Differential Compaction Loading (DCL) and the reason for its development is due to discrepancies observed between calculations using the Mohr-Coulomb failure criterion, also known as Poro-elastic Loading (PEL), and field observations.

Geomechanical modelling was performed to assess fault failure sensitivity to a range of geometrical aspects as well as reservoir and fault properties. From this analysis, focusing on the reactivation pressure at which failure first occurs, an empirical sense of sensitivity was established. It was found that for the examined variations in the geometry the fault dip angle resulted in the largest spread in reactivation pressure. For the examined reservoir and fault properties, the friction angle was found to have the largest sensitivity.

With these results it was possible to improve the estimates of essential parameters within the analytical approach, yielding a better fit between analytical and modelled solutions. These solutions lie closer to field observations. Hence, the new method of DCL shows a great improvement in calculation of stresses in the subsurface, compared to the method of PEL. This calibrated analytical approach allows for a quick assessment of the fault stability within a reservoir. Additionally, through the results from the geomechanical model new insights were obtained into the way stresses change and behave when a reservoir is depleted. The rotation of the principal stresses for each level of depletion was quantified and a new definition of the critical fault angle, the dip angle which will fail first, was derived. This links the depletion pressure and related rotation angle directly to a value of the new critical fault angle when DCL is present.

Ultimately, these new insights into fault failure behaviour of boundary faults could be a useful tool in the step towards prediction and mitigation of production or injection related seismicity.
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This study was conducted in the framework of the Leendert de Boerspolder stress test. It focuses on the investigation the performance of the available constitutive models in describing the coupled hydro-mechanical response of peats and organic clays as observed during the stress test. The soil models that are considered are the Mohr-Coulomb model, the Soft Soil model and the Hardening Soil model as are implemented in PLAXIS, a standard commercial finite element code.
The evaluation of the models is performed in two steps. First, the performance of the constitutive models is evaluated by simulating the laboratory tests as single soil element tests with PLAXIS SoilTest facility. Based on the comparison of the numerical results with the laboratory data it is concluded that the HS performs the best compared to the SS and the MC model. In order to achieve good fit it is found that it is necessary to drastically reduce the failure stress ratio, $Rf$ to an average value of 0.15 in contrast to what is mentioned in literature for soft soils. In terms of one-dimensional compression stress path both the HS and SS model are deemed to perform similarly. The MC model is found to reproduce poorly the laboratory tests due to the assumption of linear elasticity - perfect plasticity.
Subsequently, soil models are evaluated through a fully coupled hydro-mechanical simulation of the Leendert de Boerspolder stress test in PLAXIS 2D. The evaluation is done through the comparison of the measured to computed displacements and pore water pressure. It is found that the prediction of the HS model is ``soft'' for peat while the stiffness degradation in the organic clay results in excessive lateral displacements. Response of the SS is found to be better considering both displacements and pore water pressure. The best description of the stress test was found to be possibly by using the SS for the organic clay and the HS for peat. The performance of the MC model is quantitatively good however qualitatively is deemed to be poor. Furthermore, the influence of (a) the soil anisotropy and (b) the interface between organic clay and peat layers are pointed out as factors influencing the outcome of the simulation.
Based on this study, it is concluded that in general the Soft Soil model, at this stage, is recommended for use for both soils. The Hardening Soil model should be used for peat but with caution and mainly when the deviatoric strains are deemed to be important. In this case the calibration should be done focusing on triaxial tests, therefore compromising the oedometric response. Moreover, a high secant stiffness should be considered to describe peat. That might be justifiable due to presence of fibers which under tensioning provide additional stiffness. Moreover, results suggest that the use of the HS model for the organic clay is not justifiable. Finally, the Mohr-Coulomb model should be used only as a rough approximation. ... Read more