Monopiles with large diameter (larger than 6 m) and low aspect ratio (less than 6) are increasingly used in offshore wind farms. These foundations demonstrate a rigid response under lateral loading. The validity of the existing design methods, that are based on small diameter flexible piles, has been questioned by both the industry and researchers. In addition, the monopiles are subjected to both lateral and vertical loads. The influence of vertical load on the lateral design of short rigid monopiles in clay soil is not clear. This study aims to perform a comprehensive study on the influence of vertical load on the lateral response of monopile foundations in clay soil.

All analysis in this study was performed using 3D finite element modeling in PLAXIS 3D software. The NGI-ADP constitutive model was adopted to simulate the nonlinear mechanical behaviour of clay. Considered in the analysis is a short rigid pile with a diameter of 10 m (L/D = 3) and a long flexible pile with a diameter of 2 m (L/D = 15). The analyzed clay soil profiles consist of a normally consolidated clay soil and an overconsolidated clay soil with a constant undrained shear strength profile equal to 30 kPa. For each pile in each type of clay soil, a pure lateral loading scenario is performed first to assess the validity of current design methods. Subsequently, a combined loading scenario is performed to assess the influence of vertical loading on the lateral behaviour of rigid monopile in clay soil.

Results of the pure lateral loading scenario suggest that current design methods heavily underestimate the lateral capacity of rigid monopile foundations in both clay soil profiles analyzed. According to the findings of this study, it can be concluded that current design methods are not fit to provide an accurate assessment regarding the lateral load response of rigid monopile in clay soil. In order to correctly assess the lateral load response of rigid monopile in clay soil, a method consisting of a 3D finite element model akin to the model used in the research or a PISA design model is advised. A potential third design method, the 1D rotational spring model, is also proposed.

Results of the combined loading scenario suggest that the presence of vertical loading causes a decrease in lateral and moment capacity of the rigid pile in both clay soil profiles analyzed. However, the influence is negligible when the vertical load magnitude is smaller than 50% of its bearing capacity. To quantify the influence of vertical load on a monopile foundation, a series of load analysis were performed on a real offshore wind turbine with a 5MW power capacity. It was found that the vertical load on a typical monopile foundation in clay is around 27% of its bearing capacity. According to the findings of this study, it can be concluded that the influence of vertical load on the lateral response of rigid monopiles in clay soil is limited and can be ignored in foundation design.
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The numerical modelling of a cone penetration test (CPT) has long been a challenging task due to the large deformations associated with the penetration of a CPT. Recent developments in advanced numerical methods have shown promising results in overcoming these difficulties by using the Material Point Method (MPM). In this thesis it is researched whether the MPM is able to reliably produce CPT results in dry sand by using a state-dependent constitutive model. Calibration chamber (CC) tests are modelled for dry sand and results are compared with experimentally performed CC tests in the laboratory. Features regarding the numerical setup and applied boundary conditions which quantitatively influence modelling results are identified and assessed before the model is validated to real CC test data. Validation results show that the model is able to accurately produce cone resistance values for different types of sand for soil states that can be categorised as moderately-dense to dense. Last, it is shown how parameters within the constitutive framework affect the model output and a quantification of the sensitivity of the parameters to model results is presented. ... Read more

Wind energy is now a popular competitor among other energy sources all around the world. The offshore wind industry has progressed in recent years, with larger wind turbines being installed in deeper oceans. The construction of such large-scale wind farms necessitates more modern foundation design technologies to increase operational safety while also lowering total structure set-up costs. The environmental load applied to offshore piles are of great complexity. Currents, wind, waves, and even earthquakes are very common dynamic loads in an offshore loading environment. Of course, when a wind turbine is working normally, it also has significant operation loads. The design of offshore wind turbine support structures often involves some universal criteria, e.g., the pull-out capacity of jacket structures on piles. Wind turbine foundation capacity is determined by the qualities of the offshore soil as well as the properties of support structure configurations. Therefore, it is necessary to take account of the potential cyclic impacts of soil-structural interaction to guarantee dependable responses of the wind turbine structure. This thesis aims at evaluating soil−structure interaction of offshore wind turbine foundations under cyclic loading, with emphasis on the tension capacity of axially loaded displacement piles, under different load conditions (cyclic-to-average ratios) on Fontainebleau NE34 sand in France. A newly developed constitutive soil model SANISAND-MS (2018) is applied to model sand stress-strain evolution. In this thesis, the soil is assumed a homogeneous linear elastoplastic material for the sake of simplicity. The SANISAND-MS constitutive model used in this thesis can capture sand ratcheting after considerable cyclic loading cycles. Furthermore, drained and undrained compression triaxial tests performed at DTU GEO−Lab were used to calibrate the model parameters of the constitutive model for Fontainebleau NE34 sand. The finite element model adopted here is built in an open-sourced platform, the OpenSees. The Small-strain approach is adopted in the finite element modelling part. The pile is simplified as a wished-in place which does not include the installation effect and the time effect after the installation and before cyclic tests. Finally, the modelling results are compared to the experiment results recorded by Tsuha et al. (2012). Clear stable, metastable, and unstable response types are recognized in the model results. However, the initial stress state of the sand at the soil-pile interface differs a lot compared to the experiment results. This is the consequence of not including the pile installation effects in the finite element modelling. Recommendations are given to use large-strain soil modelling techniques to include the pile installation process. ... Read more

Glacial lake outburst floods (GLOFs) are outbursts caused by the failure of glacial lake moraine dams. Longer ongoing processes, such as moraine dam degradation, or instantaneous events, such as landslides, can trigger dam failure. GLOFs have a catastrophic downstream impact leading to significant economic damages and more than 12000 casualties worldwide until 2015, with Bhutan and Nepal being impacted the most. Climate change causes increasing temperature and precipitation, leading to the expansion of glacial lakes and the destabilisation of glaciers, slopes and moraine dams. Consequently, GLOFs are likely to become more frequent, and glacial lakes require continuous monitoring and analysis to understand and predict GLOF-related hazards.

Since glacial lakes often lie in inaccessible mountainous regions, on-site monitoring is challenging and remote sensing proposes a safe and cost-effective solution. Satellite radar is unaffected by nighttime and clouds, enabling continuous displacement measurements. Interferometric synthetic aperture radar (InSAR) using Sentinel-1 data from 2014 to 2021 was applied at six Himalayan glacial lake areas (Imja, Lunana, Barun, Rolpa, Thulagi and Lumding) to identify potential GLOF hazards and to investigate InSAR's capability as a monitoring tool. Optical, meteorological and topographical data were used to aid in interpreting the InSAR observations; linking displacements to potential hazards and evaluating the limitations of an InSAR-based analysis.

Significant deformation was detected at the terminal moraines of Imja, Thulagi, Rolpa, Lunana and Barun Lakes; on lateral moraines at Rolpa and Lunana Lakes; and on rock glaciers at Imja, Rolpa, Barun and Lunana Lakes. In addition, significant seasonal variation could be distinguished, showing the impact of temperature and precipitation on geomorphological processes and potential hazard developments at glacial lakes. InSAR-related limitations arose in regions with significant topographic variations, extant snow or vegetation covers, and rapid displacements.

This study demonstrates the capability of satellite InSAR as a glacial lake monitoring tool. An InSAR-based analysis is instrumental in highlighting areas from where GLOFs could originate, requiring mitigation measures or further investigation to map the impact of failure. By extending the research frame over multiple years, continuous and long-term monitoring could demonstrate the climatic influence on displacements and GLOF trigger developments. ... Read more

The control of infiltration and seepage of water is one ofthe most challenging tasks in water management and civil­-engineering and, inan attempt to control this, methods for forming a water­impermeable layer inthe soil have been widely practised in soil engineering (Laumann et al., 2018;Proto et al., 2016). The use of natural processes to modify the engineeringproperties of the subsurface could help to develop cost-­effective, robust andsustainable engineering technologies and is attracting increasing attentionfrom the industry (Zhou, 2020). This research aims to reduce the permeabilityby using aluminium (Al) and organic matter (OM) precipitates mixed ex­-situwith porous media to create a horizontal barrier. The Al­-OM precipitates wereexpected to clog the pore space with a reduction in permeability and hydraulicconductivity as result. To find out if it is feasible to use Al­-OMprecipitates mixed ex­-situ with porous media for a permeability reductivelayer, the Al­-OM precipitates and the permeability of the medium wereresearched.  To characterise the flocculation reaction, experiments onthe flocculation were performed. The yield of the reaction was obtained byadding certain amounts of Al and OM solutions to form particular amounts of drymass of flocs. This experiment confirmed the hypothesis that 85% of the addedmass of OM will result in dry mass of flocs. The concept of a critical metal tocarbon ratio (M/C­-ratio), indicating flocculation regardless of the inputconcentrations, was tested by measuring the pH over an increasing M/C-­ratio.From this titration curve, the found critical M/C ratio is between 0.023­0.031,and the pH stabilises at a level lower than pH 4. This result proves theconcept behind the numerical scenarios describing the titration of OM solutionswith Al3+, with a critical molar M/C ratio independent of the inputconcentrations (Veerkamp, 2018; Zhou, 2020). After determining the yield of thereaction and concluding that the concentration of Al and OM was not ofinfluence, the by-products of the Al­-OM reaction were quantified. The ionicstrength of the supernatant of an increasing density of flocs in solution wasdetermined by measuring the electrical conductivity (EC). The results showedthat the ionic strength increased linearly with an increasing density of flocs.The relationship between the ionic strength and the density of flocs was coupledto the linear relationship between concentrations potassium chloride (KCl) andits EC. From the results, the measured EC can be used as a tracer since the K+and Cl­ are non­reactive.  The hydraulic conductivity measurements were conducted by afalling head test to be able to make an indication of the change inpermeability when adding the Al­-OM precipitates to the sand. To find theoptimal method to mix the Al, OM and porous media, the influence of differentmethods of producing, adding and mixing the materials on the permeabilityreduction is explored. In the first mixing method, the Al and OM were added insolution, the solution containing Al-­OM precipitates was centrifuged until thereduced ionic strength was at an EC value less than 700 µm/cm. The hydraulicconductivity measurements were used to obtain the relationship between thehydraulic conductivity reduction over an increasing concentration of flocsretained by one kilogram of sand. The hydraulic conductivity was reducedexponentially over an increasing concentration of flocs up to a magnitude of 3.The results imply a large variability in the achieved reduction dominated bythe amount of retained flocs. The second method is using Al and OM in powderformat and adding them to the dry sand and adding 500 ml of water to thismixture. This mixing method resulted in a completely different floc structure.The flocs produced by mixing in solution have a shear dependency feature, whiledry mixing created particles that have a constant size. For this method, theincrease of the reduction is linear over an increase of concentration of flocsretained by the soil. The highest reduction for this method was found to be ofa magnitude of two, measured at 50 grams of flocs retained by one kilogram ofsand. Finally, this research gave proof of principle of using Al­-OMprecipitates mixed directly with sand could reduce the permeability up to amagnitude of 3. These results present a new road to research on this Al-­OM­-sandmixture’s strength parameters and compaction over time over an increasing flocdensity, since these parameters are critical for using the layer in practice.   ... Read more

Deployment of renewable energy is essential to reach a carbon neutral economy. Offshore wind farms have caught the interest of many developed countries since they are an essential source of green energy. The interest of this thesis lays in the design of the foundation used for offshore wind farms, in particular the interaction between the monopiles and the scour protection layer. An optimised one-stage installation process for the scour protection is investigated, which consists of first placing the rock armour and then driving the monopile through the entire scour protection layer. This is an efficient method to reduce the installation time and the operational cost.
The purpose of this research is to identify the limitations which are related to the penetration of the monopile through the stone armour. The scour protection material is composed of large diameter rocks, which can hinder the penetration of the pile, or even damage the tip of the pile. A restricting ratio of the mean size of the rock (d50) to the thickness of the monopile wall (w) is explored, as well as an investigation of the effect of penetration resistance on different material and geometry characteristics. The first research method employed is a literature study, which proves that the analytical formulation of the axial capacity provided by the available standards is inappropriate for the application of this thesis. Thus, two other research methods are identified, which include designing an experimental small-scale test and a Discrete Element Model (DEM) of a penetration test. ... Read more

In this study, the excavation process of a low pressure vertical impinging jet in cohesive soil has been investigated. Mass flow excavation is a hydraulic, low pressure, subsea excavation method whereby a large volume flow is applied to the seabed through which the seabed is eroded, and the soil is transported.
The erosion process of a cohesive soil by impinging jets depends on many variables (e.g., jet flow velocity, standoff distance, grain size, undrained shear strength). The erosion processes, especially for dynamic pressures of lower than two times the undrained shear strength, are not fully understood. It is still unknown what the influence of different soil and jet parameters are. As a result, it is still unknown what the expected scour rate will be during a mass flow excavation process of cohesive soils. The goal of the research is to quantify and be able to predict the production of a mass flow jet on cohesive soils. For this purpose, the relevant parameters of cohesive soil erosion are investigated, and fluid velocity profiles of the jet are related to clay bed failure mechanisms. Special attention is put on the effect of the consolidation coefficient on the erosion process. Based on literature and experimental research, different soil failure mechanisms have been investigated. The main result of this study is a relation between the main jet and soil parameters, and the erosion velocity development. ... Read more

The suction pile foundation is a large steel cylinder with an open end and sealed top. This foundation type is widely used in the oil&gas industry and wind energy. Experimental investigation and numerical investigation are the main two methods to understand the performance of the suction bucket foundation. The experimental studies are important and basal for design, but it is time-consuming and costly compared to numerical studies. However, the accuracy of the finite element method(FEM) in geotechnical problems highly depends on whether the soil constitutive models can correctly predict soil behaviour. In our study, we investigate four constitutive models: the Mohr-Coulomb model, the hardening soil model, the NGI-ADP model, and the hypoplastic model. To some extent, advanced soil models can better present soil behaviour than traditional ones. However, it needs more laboratory test data to calibrate the advanced model parameters. The Mohr-Coulomb(MC) model is the most common soil model, which is an isotropic model include few parameters. The hardening soil model exceeds MC model by introducing the stress-dependent stiffness and distinguishing between loading and reload- ing. The NGI-ADP model is mainly used for undrained analysis, and it is an anisotropic model which can exact match with undrained shear strength and stiffness for various failure surfaces. The hypoplastic model has no distinguishing between elastic and plastic strain. It is an inelastic(dissipative) and incrementally nonlinear soil model without the requirement of a yield surface. A soil investigation report of Block 17 offshore Angola is used to calibrate the aforementioned constitutive models. The cone penetration test(CPT), ball penetration test(BPT) and a series of laboratory tests(i.e. direct simple shear, triaxial, and oedometric tests) are exacted from the soil report and interpreted for calibration. Parameter determination procedures for constitutive models are explained. Subsequently, the consistency of the parameter set is validated by numerical simulation of direct shear and triaxial tests. The numerical experiments for a suction pile foundation whose out diameter equals four and aspect ratio equals three are carried out. Four loading cases(i.e. horizontal, vertical tension, vertical compression and vertical-horizontal-moment (VHM) combining loadings) are included in the finite element analysis. The suction pile performance(i.e. deformation and capacity) are compared among the using of different constitutive models. Additionally, the compliance matrices of the suction pile for different soil models are obtained for the structural engineer. The analyses indicated that NGI-ADP model could be the best choice for undrained analysis of suction pile foundation. This model has a robust calibration process, and well simulate the anisotropic strain-stress relationship. Additionally, the finite element results are conservative when modelling by NGI-ADP model. However, this model can not predict the right pore pressure build-up and stress-path, which may be improved by using a well-calibrated hypoplastic model. ... Read more

In the face of climate change, urban stormwater management practices are subject to an uncertain context. More frequent and extreme rainfall events are expected in many urban areas. The extent how climate change will affect weather patterns is however uncertain. This challenges contemporary stormwater management practices. A post-disaster reconstruction of an urban area would potentially be an opportunity to anticipate climate change uncertainties from the initial design phase of new urban development. This would allow for taking advantage of the disaster recovery by rebuilding a more resilient stormwater system than was present before, in which the uncertainties are anticipated. A resilient stormwater system would thus be able to cope a wide range of plausible futures, rather than the average. As academic literature lacks insight on re-establishing resilient urban stormwater management in a post-disaster reconstruction, this thesis aims to gain more insights into what extent resilient urban stormwater management has been established in a Tsunami reconstruction, with regard to uncertainties on climate change and urban development. A case study is conducted in which is focused on a) the conducted design approach, b) the realm of conditions for which the implemented stormwater system would be vulnerable and c) which solutions could be implemented to reduce the vulnerability, and reflected upon opportunities of an interdisciplinary approach from a stormwater management perspective. The selected study area is a town called Ötsuchi Town, in Northern Japan, Iwate Prefecture, which was severely hit during the 2011 Tsunami. The disaster necessitated a comprehensive reconstruction. To gain more insight into what extent a resilient urban stormwater system has been implemented, an exploratory modelling and analysis approach has been applied, which is also known as Scenario Discovery. Prime drivers of Scenario Discovery are exploring the system's performance for wide-ranging plausible futures by sampling the system myriad times with aid of computational modelling, and, the use of regional sensitivity mapping algorithms to understand the dominant factors that lead to insufficient system performance. The latter can be used as input for seeking vulnerability-reducing strategies. In this research, the Patient Rule Induction Method (PRIM) algorithm is used to understand the conditions for which the stormwater drainage system capacity would be exceeded, for an acceptable flooding level of 0.2 meter. Two open-source tools were combined: the conceptual stormwater flow model EPA-SWMM, and, python library EMA-workbench which provides the tools needed for applying Scenario Discovery and supportive analysis. After the 2011 Tsunami a separate half-open sewer system was established in the reclamation area of Ötsuchi Town. No natural-based solutions, or Blue-Green infrastructure were implemented, even though the residential is adjacent to steep mountain slopes. The uncertainty parameters over which is sampled, include rainfall intensity, rainfall duration, external runoff coefficient, imperviousness, the hydraulic conductivity, Manning's roughness coefficient and depression storage. The Scenario Discovery results show that the system has been built robust. For almost all locations no flooding occurrence arise from the sampling, even for very extreme rainfall events (>100 mm/hr, for a duration of 60 min). However, when the runoff coefficient of the external runoff from the mountain would be above 0.58, a small area in the northern part would be vulnerable. The dominant factors, are found with PRIM and the relative weights are with very high precision and recall found by pre-processing the sampling data with Principal Component Analysis. It showed that for the vulnerable area, the hydraulic conductivity of the soil and the proportion of paved area are decisive in whether or not flooding occurs. Paying attention to certain thresholds, would significantly reduce the vulnerability of flooding occurrence. Given the high volumes for which flooding would occur, a preliminary feasibility study has been done for both a vegetative swale and infiltration trenches, in combination with a storage retention area in the lower lying area, such that the pressure on the stormwater drainage system can be released. The vegetative swale reduce the vulnerability significantly. The infiltration trenches seem to be effective for the lower rainfall duration and rainfall intensity. PCA-PRIM outcome showed that when implementing infiltration trenches in the vulnerable area, the impervious rate becomes relatively less sensitive in comparison with outcomes of PRIM with the reference situation. More information on mountain runoff, soil properties and land use would enhance the accuracy of the results. In addition, since, no information is known yet on initial losses of mountain runoff, more information on runoff flow processes from the mountains would enable to perform joint probability analysis with the results of this research, to better assess whether or not the time is now to implement additional measures. From the results of this research it seems that the urban stormwater drainage of Ötsuchi Town has been rebuilt very robust, however when the system would fail, no additional measures outside the grey infrastructure are taken. An interdisciplinary approach could have encouraged a more resilient stormwater management. As this research shows that multiple commutative reconstruction measures could be found from a stormwater management perspective, the obtained information with applying Scenario Discovery, would have been useful to encourage an interdisciplinary approach during the reconstruction process. It is therefore suggested to explore the usage of Scenario Discovery on urban stormwater management even further, such that it enables to take advantage of the disaster reconstruction to establish a resilient urban stormwater management. An example would be to investigate whether the relative sensitivity of the uncertainty parameters would change when a less robust system would be examined. For example, for an urban drainage system for which the threshold would already be exceeded for lower rainfall intensity and duration than 100 mm/hr, and 60 minutes respectively. In addition, a coarse representation of rainfall events are considered in this research. Therefore it would be suggested to examine the applicability of region-specific storm profiles in combination with an exploratory modelling and analysis approach, to better assess the implication of the PRIM outcomes for implementing resilient urban stormwater management strategies. ... Read more

Due to often existing subsea infrastructure or challenging seabed conditions, engineers are forced to design newnearshore pipeline trajectories that are not always in a straight line. Since subsea tie-in operations are often complex and relatively expensive, alternative methods like pulling pipelines into a curve are worthwhile. By means of this research fundamental knowledge of pipe pull operations into curved trajectories is acquired. This thesis research focuses on the soil - structure interaction between the sliding pipeline and the seabed. A more thorough understanding is obtained of the geotechnical processes that create lateral resistance of the pipeline against the forces applied by the installation vessel. The main goal behind this thesis is to expand the capabilities of pipeline installation by exploring the feasibility of curved pipe pull projects. The research is based on three fundamental elements: 1. An extensive theoretical study 2. A validated numerical soil-structure interaction model 3. A successful physical test campaign of thirty-two experiments The theoretical study of the thesis focuses on the physical processes that play a role during curved pipe pull operations. Research is conducted to gather knowledge of the three main subjects of research, being: 1. Structural behaviour of a concrete covered pipeline 2. Geotechnical behaviour of soil under lateral pipeline displacement 3. Geotechnical behaviour of soil under axial pipeline displacement Since this was the first known research into curved pipe pull operations, influences of dynamic environmental loads are excluded from the scope of research. The second stage of the research focused on creating an engineering model that enabled to predict to what extend partially embedded pipelines can be pulled into a curved trajectory on sandy seabeds. After considering multiple simpler structural models, the final result was a non-linear spring supported tensioned bending beam model. Within this computational prediction model: the lateral soil resistance is modeled by means of a P(Y) spring model, while the pipeline is represented by the tensioned bending beam. The applied lateral soil resistance model is computed based on researches ofWang et al. (2018) and Verley and Sotberg (1994). After themodel was completed, a purpose made test facility was created to pull scaled pipelines into a curved trajectory. During the test-campaign, five different model pipelines were tested on a scale range of 1/11.8 to 1/5.3. The physical test campaign provided valuable experimental data of thirty-two successful drained curved pipe pull tests. By means of the acquired set of experimental data the model was able to be validated. Since the model pipelines differed in diameter, specific weight and bending stiffness, the influence of those parameters is examined. From the fact that the majority of the model pipelines was pulled into a radius that was 0-30% lower than the computed radius,we can state that the numerical model predictions gives awell defined upper limit of the controlled pull radius of pipelines. Along the classified critical curved pulls, the imposed lateral force during the experimentswas within 20% of the maximum predicted force in 86% of the physical tests. From this observation we can conclude that the numerical model captures the maximum lateral pull force with a high accuracy. ... Read more

The thesis presents a numerical study on dredging induced undrained instability and subsequent static liquefaction of submarine landslides. For the study, a pre-existing hydro-dynamic uncoupled submarine slope numerical model, developed by Molenkamp (1999), has been modified to incorporate a fully hydro-dynamic coupled interaction between submerging water mass and submarine slope. The modified model is able to simulate transient quasi-static and dynamic phenomena up-till and including the immediate post-liquefaction behavior of submerged slopes of loose undrained homogenous fine sands in a 2 dimensional Updated Lagrangian (UL) finite element (FE) frame of work. To simulate soil behavior under dredge loading applications the model incorporates a Monot soil constitutive model and for submerging water behavior a Lagrangian expression of Navier stokes for nearly-incompressible visco-elastic, irrotational, fluid model.
The study primarily addresses the effect of dynamics of submerging water on the liquefying submerged slope. The research findings suggest that the dynamic motion of submerging water barely affects the occurrence of instability. However, it may decrease the rate of post-instability liquefied flow as compared to the commonly sorted uncoupled scenario, where dynamics of submerging water mass is ignored and only constant hydrostatic pressure heads due to water level is considered at the slope interface. Moreover, the findings suggest that about 50% of the loss in the potential energy of soil is consumed by the potential energy of the submerging water at the very initial stages of post-instability and that the contribution of kinetic energy of water amounts to mere 3.4%.
Next, as a secondary issue, the study also provides a valuable insight into the effect of the liquefying slope on the motion of the submerging water mass. The findings show a surface impulse wave formation post-instability, moving along the direction of landslide. Moreover, it shows a development of a distinct circular motion of fluid along the slope interface. Other than this, the thesis also attempts to provide some similarities and differences between the current findings and the published conventional research studies which make use of basic slide shapes such as viscous or rigid sliding wedge blocks.
Finally, the thesis also addresses some numerical shortcomings such as the hour-glass effect, the shake-down by the procedure to define the “initial state” effect etc., and thereby providing necessary recommendations useful for future computational modelling work. ... Read more

Almost four million people in the Netherlands who live below sea level are protected by 17000 km of dikes against the water. Since deformation can be an indicator of a developing failure mechanism, information about this parameter can help authorities to detect weak spots in the dutch water defense structures.
In this research, Radar Interferometry (InSAR) is used to estimate deformation time series on the vegetated part of the dike. Using Point Scatterers (PS), it is already possible to estimate deformation time series with mm precision on objects with a coherent reflection over time. Apart from objects, also surfaces provide a reflection of the radar signal. These so-called Distributed Scatterers (DS), which can for instance be found on dikes, are often strongly affected by temporal decorrelation. Although conventional approaches to detect and estimate these DS do not work, there may still be coherent information for interferograms with small temporal baselines. In this research project, a methodology is presented to detect coherent DS on the vegetated part of a dike and to estimate their deformation time series.
To do so, the phases of PS are subtracted from the DS on the vegetated part of the dike to reduce the phase noise due to different states of the atmosphere. The estimated coherence matrix is then used to only select coherent interferograms which are used to unwrap the phases with respect to PS. Using data obtained from the TerraSAR-X (X-band) and Radarsat-2 (C-band) satellites, it was shown that DS could be detected on some parts of the grass-covered inner slope of the Marken dike. ... Read more

Landslides are destructive and recurrent natural disasters that cost annually significant social and economic losses all over the world. These events can be induced by natural factors as earthquakes and extreme rainfall, as well as by human intervention, including construction and mining. A primary resource to conduct landslides studies for prediction, risk assessment, and mitigation are historical databases with accurate location of individual events. To increase the location accuracy of those past landslide events, and optimize conventional time- and cost- consuming mapping routines, this study aims to develop an automatic landslide detection method from free-of-charge optical satellite imagery (Sentinel-2) and global Digital Elevation Model (ALOS World3D-30m DEM) using Object-based Image Analysis (OBIA) in combination with Machine Learning (ML). Existing works have successfully used earth-observation datasets for the generation of landslides databases. Most of them apply rule-based techniques using features thresholds that are not global and therefore perform poorly when applied to new regions where the method was not developed. This study presents a first attempt of an automatic method that generalizes to landslides occurring over the entire world without knowledge of their cause or triggering factor. To obtain a robust method that can deal with the complex characteristics of landslides (e.g. diversity of shapes/sizes, land cover, illumination and spectral variability), we explored OBIA, an image processing technique that has demonstrated better performance than the pixel-based approach, specially when the target objects are bigger than the cell resolution. The developed method consists in cloud-free images acquisition and determination of suitable features for image segmentation and image classification. For the image segmentation, we developed a two-step approach that consists in an initial segmentation using k-means and the Red/Green Difference (RGD) as input feature to create homogeneous segments and isolate landslides from non-landslides. This first approach leads to oversegmentation of non-landslide areas and, consequently, to an imbalanced dataset. The second step consists in a merging algorithm using Normalized Difference Vegetation Index (NDVI) as input feature to merge homogeneous non-landslide segments and balance the dataset. These two-stages include the setting of parameters as the number of clusters (K) and NDVI thresholds that were experimentally derived. Once the segments are created and the dataset is balanced, a non-parametric supervised classification using Random Forest (RF) was applied to identify landslide segments; the main advantage of this classifier is that it can deal with different statistical distributions of features and can handle imbalanced datasets. Using a training and testing set of 70% and 30%, our method achieved a precision of 83%, recall of 83%, and f1-score of 83%. We found that topographic features have less influence than spectral ones; however, their exclusion decreases the model performance in about 10%. Our method is built using entirely open source technologies allowing its applicability and re-usability. For future work, we propose to use our method to detect new landslides and increase the number of training samples. Additionally, we recommend to explore a complementary approach to the merging algorithm to reduce the number of non-landslide segments, balance the dataset, and keep accurate classification results while more training images are added to the model. ... Read more

Offshore wind has achieved many milestones considering this relatively new and emerging industry. Its huge advancement is mostly driven by lowering the overall project cost and renewable energy targets which are set by the EU. One of the potential cost savers can be found in the foundation structure design in which the monopiles are the most used choice. The area from which the design of these structures can be improved is quite complex and it can be related to the field of Dynamic Soil-Structure Interaction (DSSI). Offshore Wind Turbine (OWT) has several damping sources but in the last few years soil damping phenomena has attracted a lot of interest within the offshore wind research community because it is believed that potential benefits can be achieved if the contribution of this damping source can be estimated with reliability. Offshore wind design codes do not provide a methodology for the estimation of this damping type and therefore there is a need for the research. For the purpose of soil damping estimation, a geotechnical Finite Element (FE) software – Plaxis 3D was utilized. Firstly, the chosen soil model was verified through the three-step verification process which involved a comparison between experimental field data and numerical data from Plaxis. Once successfully verified, the soil model is utilized further in the design of the simplified OWT structure which is exposed to the dynamic wave and wind loads based on the serviceability and ultimate limit conditions. What is of the particular interest leads to the free vibration phase and monopile positioning (position of structure at the start of free vibration) from which the necessary information regarding the decay of displacement amplitudes is obtained and further analysed with the logarithmic decrement method for damping estimation. Adding on soil damping, as part of research additional topics were also considered and they relate to soil behaviour and natural frequency. The final results are compared to the other research papers and they are found to be well within the proposed literature damping range which highly depends on a certain combination of monopile geometry, soil type and loading conditions. From the research it was concluded that soil damping value increases as the wave and wind loads increase. Also, it was proven that a damping potential in offshore wind exists and that eventually soil damping should be implemented in the future offshore wind design codes. ... Read more

PLAXIS study of a primary flood defence in the Netherlands. The influence of water level, precipitation, and wave overtopping on the position of the phreatic surface is being investigated. The hydraulic loads are applied to a clay dike with a cracked top layer and a sand dike with a cracked clay cover. The findings are related to the current deterministic methods to estimate the location of the phreatic surface. ... Read more

The static liquefaction tank (SL-tank) is a unique large-scale facility that provides a means to simulate the conditions of a submerged soil slope under failure conditions. Static liquefaction (SL) may be defined as a significant loss of soil’s shear strength due to a substantial development of excess pore water pressure under monotonic loads. This complex soil behavior is commonly related to case studies such as submarine flow slides.
The main objective of this project is to use a finite element method (FEM) based on a Hypoplasticity (HP) framework to numerically simulate the experimental data obtained from the Static liquefaction tank facility of Delft University of Technology. A set of Elasto-Plastic (EP) constitutive models were chosen as an initial reference to simulate the SL-tank and afterwards compare their results to the HP framework. The numerical results were analyzed mainly in terms of excess pore water pressure and relating them to experimental data. Additionally, effective stresses, strains and stress paths were examined from the given model outputs.
A fine-cohesion less soil called Geba sand was used in the experimental procedure of the SL-tank, as well as in elements tests performed within this study. The model parameter determination and calibrations were performed by means of element tests, empirical correlations, theoretical formulations, and best-fits from experimental data. Soil behavior at low stresses is of fundamental importance for the performed experiments and numerical simulations in this work.
A potential instability behavior from the given numerical simulations was studied by means of an adopted instability line (IL). The IL criteria is a framework which is commonly illustrated in stress paths as a boundary that delimits a potential susceptibility to soil collapse. Element test data from fine-loose sands, as well as the numerical outputs from this work were used for estimating a potential IL applicable for the scope of this Master’s thesis project.
Results of this investigation showed clear limitations of the hypoplasticity constitutive laws in generating sufficient excess pore water pressures and deformations to trigger static liquefaction. Additionally, boundary effects in the assumed fixity conditions were a main potential issue regarding inaccurate results. Nevertheless, an enhanced model response (HP) was observed in comparison to Elasto-Plastic models.
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