Cyclic wetting and drying impact the integrity of cohesive clay materials in geotechnical engineering applications. Boom Clay, frequently used in erosion protective layers, presents a critical case study due to its widespread application and the environmental conditions it endures. This research delves into the effects of repeated wetting and drying cycles on Boom Clay’s erodibility, a process that protective layers often undergo during construction and exposure.

Utilizing the Erosion Function Apparatus (EFA), performed experiments targeted changes in the structure and erosion resistance of Boom Clay under cyclic conditions. The test setup was adjusted, improved, and calibrated. It was observed that these cycles induce alterations in the clay’s erodibility, contrasting with the behavior of untreated samples.

The results demonstrated cyclic wetting and drying increases the susceptibility of the material to erosion, and the rate of erosion, and decreases the threshold of the erosion process. This study enhances our understanding of how environmental stressors influence the long-term behavior of erosion protection materials. It provides engineers and environmental planners with insights for selecting and assessing materials for erosion protection, emphasizing the importance of considering environmental conditions in their design and application.

The comprehensive assessment of the deviatoric behaviour in peats is paramount in ensuring the resilience of soil structures. The understanding of this material is riddled with intricacies, given its highly complex fabric, which includes organic matter such as fibers. This thesis evaluates the capabilities of a recent constitutive model (JMC) to adequately describe the response of peat by means of a flexible combination of constitutive ingredients, such as a non-associate flow rule and mixed volumetric and distortional hardening. The JMC model capabilities are tested with respect to laboratory and field data and compared against well-established models such as the NGI-ADP and Soft Soil (SS) model in a 3D FEM environment. The framework of reference is the measured response of the Bloemendalerpolder test, as it involves long-term consolidation, deviatoric behaviour, and soil-structure interaction. The results of this thesis demonstrate, that although the deformation pattern of the measurements is emulated effectively by the JMC and SS models, all the models show an overestimation of the lateral displacement compared to field data. Despite these limitations of the formulations and the modeling environment, the work shows the potential of the JMC model to reproduce reasonably deviatoric peat behaviour.

Offshore wind power structures are subjected to cyclic loading. Several loads like wind and wave loading are acting on the structure and therefore its foundation. It is important to involve the impact of this cyclic loading into the design of the structure. A cyclic load influences the strength and deformation characteristics of the soil. Due to this the cyclic loading the cyclic shear strength will decrease. This cyclic shear strength depends on several factors which can be influenced by soil properties and shear mechanisms. The aim of this Master thesis is to provide correlations between index parameters and the cyclic simple shear strength to observe how and which parameters influence the shear strength behaviour due to cyclic loading. This research focuses on the results from cyclic simple shear tests and only for cohesionless soil from North Sea sand. The project material was obtained from several offshore windfarm projects by Fugro, namely The Hollandse Kust West ,The Hollandse Kust Noord and The Hollandse Kust Zuid. Available data from these project locations is analysed and translated to cyclic resistance curves. In these curves the number of cycles to reach a nominal strain failure is plotted against a normalised cyclic shear strength, also called a cyclic stress ratio. The strain failure criterion was chosen at 3.75\% and a commonly equivalent number of cycles of 10. The soil parameters that are elaborated for this research are based on characteristics compared to density of the soil, grain structure and in-situ characteristics. These parameters are the relative density, fines content, mean particle size and the normalised cone penetration resistance. The relative density is an unknown parameter in this research and is determined in two ways. First, by considering in-situ data where it was measured along depth and an assumption was made about the real value based on the known depth range of the borehole sample. The second method was based on the initial void ratio measured in the laboratory to obtain a relation between the in-situ measured relative density and the initial void ratio, which is called the theoretical value of the relative density. From this thesis, it was concluded that due to the small range of data it was not possible to provide clear correlations and to observe trends of soil index parameters with the cyclic shear strength. By looking at the total range of the cyclic response of the data in the cyclic resistance curves and the elaborated soil parameters, the relative density and the fines content influence the shape of the power failure lines. A relatively high value of the relative density results in a larger range of different cyclic stress ratio values, i.e. a failure line with a higher gradient. An increment in fines content results in a relatively lower values for cyclic stress ratio and do not differ much by an increment of load cycles. This observation is based on the available results from this research with a relatively low amount of data and a small range of fines content.

Six cyclic settlement models for sand are evaluated to analyse the settlement of automatic stacking crane (ASC) rail tracks at the Rotterdam World Gateway (RWG) container terminal. During Phase 1 of the RWG container terminal settlement of the rail tracks occurred at multiple locations after the ASCs became operational. This has repeatedly led to (unplanned) downtime of parts of the RWG container terminal due to rail track maintenance. Settlements are caused by densification of the sand fill, which is a result of the cyclic load applied by ASCs moving continuously over their rail tracks.

 

The aim of this research is to contribute to prevent unplanned downtime in Phase 2 of the RWG container terminal due to rail track settlements. Also, reliable settlement predictions can be used to determine the intensity and extent of the ground compaction that are needed to meet the settlement requirement of 20 mm for ASC rail tracks.

 

The cyclic settlement models, which have been validated to predict the cyclic settlement of rail tracks and shallow foundations, are obtained from literature. The available soil data include CPT’s, boreholes and standard laboratory soil testing. In addition, settlements of the ASC rail tracks in Phase 1 had been measured for a period of almost one year. The cyclic settlement models are evaluated at six different locations, where the sand is medium to very dense and settlements up to 32 mm have been measured. The load is modelled as a quasi­static load equivalent to a vertical stress of 60 to 90 kPa applied to the ballast-­sand interface. The model parameters of the cyclic settlement models are determined by correlation, (FE) modelling of the first load cycle, extrapolation and estimation.

 

The zone of influence was found to reach around 6 m below the shallow foundation. Densification of the sand fill is substantial within the entire zone of influence. The maximum densification was found not to coincide with the minimum void ratio, it is a variable that depends on the initial state of the sand and the loading and soil conditions. After order 10⁴ load cycles densification of the sand was found to become negligible. To meet the settlement requirement for ASC rail tracks the sand fill must consist of sand layers with a minimum and average relative density of at least 65% and 85%, respectively.

 

Cyclic settlement increases with the number of load cycles, amplitude of the load and extent of the zone of influence and decreases with relative density, stiffness of the sand and volumetric threshold strain. However, correlations used to calibrate the model parameters lead to model predictions that are over- or insensitive to parameters that affect the cyclic settlement. The cyclic settlement predictions of the terminal density model are most reliable and match best with the settlement measurements, for loose and medium dense sand the model predictions underestimate the settlement.

 

Instead of using correlations to obtain the model parameter values and decrease their uncertainty it is recommended to measure the:

·         disturbance of the sand fill underneath the ASC rail tracks due to construction;

·         maximum densification of the sand underneath ASC rail tracks in Phase 1 at locations where rail track settlement has stopped, i.e. where the sand reached its maximum densification;

·         model parameters that characterise the cyclic densification behaviour of sand in cyclic soil tests.

This will improve the reliability of the cyclic settlement predictions of ASC rail tracks constructed on a sand fill. To validate the cyclic settlement models for ASC rail tracks on sand, measurements of the settlement with depth as function of the number of load cycles are needed.

A study carried out to understand the effects on including vegetation as an additional reinforcement to improve streambank stability.

The coefficient of lateral earth pressure at rest (K0) is an important parameter in any geotechnical problem since it provides information on the initial stress state, which governs the response of the soil to the proceeding stress changes. A proper determination of the stresses in any geotechnical problem is required to be able to predict the soil behaviour. Any change in the conditions the soil is subject to can alter the value of K0, how this value changes for conventional stress histories such as loading and unloading is defined well. However, the influence of unconventional stress histories such as creep or drying/wetting cycles is not thoroughly understood. Since every soil is subjected to natural environmental stresses, resulting in creep and unsaturated conditions, it is interesting to look at their influences of the soil stress state expressed by K0.

The goal of this work is to determine if it is possible to better understand what is happening to K0 during creep and under unsaturated conditions and if the prediction of K0 can be improved by accounting for these phenomena, with the focus being on clays. Literature showed that for saturated samples, the value of K0 increases with time during creep. For unsaturated conditions it was found that K0 decreases with an increase of suction.

In order to see if it is possible to improve the prediction of K0, a model needed to be constructed. The starting point of this model was a saturated, elastoplastic model based on the SANICLAY model. The first step in extending this model was to include viscosity which was done by adopting Perzyna’s overstress approach. The model was validated to experimental data on OostVaardersPlassen (OVP) clay obtained from literature and the validation showed that the model was satisfactory in predicting the soil behaviour. Accounting for unsaturated conditions was done by adopting the average soil skeleton approach. Implementation was initially done in the original elastoplastic model. Again, the model was validated to experimental data obtained from the literature, this time unsaturated loading/unloading tests on London clay (LC) were used. The results showed that the model prediction was accurate up to suctions up to 600 kPa. The final step in the model development was to include both Perzyna’s and the average soil skeleton stress approach in the basic model giving an unsaturated elasto viscoplastic model version. Unsaturated creep tests on London clay were used to validate the model but the results showed that an uncoupled stress-suction approach gave inaccurate predictions. The viscous nucleus in Perzyna’s approach was changed to become suction dependent and the results showed that the experimental data could be reproduced reasonably well.

The unsaturated elasto viscoplastic model was then used to analyse K0 during the unsaturated creep tests. The results showed that the model predicted a decrease in K0 with time for low loads and high suctions. For higher loads and low to moderate suctions, the model predicted an initial increase followed by a decrease. For all cases it was found that the value of K0 decreased with suction. The role of anisotropy on the model prediction was analysed by predicting the change in K0 using an isotropic version of the model. This version also predicted a decrease at low loads and high suctions but an initial increase was no longer predicted to decrease. The decrease of K0 with suction was still observed. No experimental data was available to confirm either of the findings, comparing the results with the literature study showed that the decrease of K0 with suction was previously observed. The decrease with time on the other hand was not found in previous work. However, the creep tests in previous works were performed on saturated samples and in general for a shorter time period which could show different results.

It is concluded that by accounting for creep and unsaturated conditions, the qualitative prediction of the soil behaviour can be improved. By accounting for coupled behaviour through an unsaturated viscous nucleus, the currently available unsaturated and time dependent experimental deformation data can be simulated accurately. The prediction of the change in stress state, due to these natural phenomena, is likely to be improved as well since the outcome of the model matches findings from the literature. However, due to the lack of experimental unsaturated time dependent data stating the change in K0, no conclusions can be drawn on the importance of anisotropy and the quantitative model performance. What this work does offer is a good modelling tool to support future experiments or investigations into unsaturated creep behaviour.

The erosion resistance of clay is an important aspect when assessing the integrity of a levee. This is because the clay layer of a dike must resist the forces that are imposed in the case of overflow or overtopping. Currently, the erosion resistance of clay in the Netherlands is being assessed based on the sand content, the liquid limit and the plasticity index. The quality of compaction is assessed by a density requirement. However, the effect of compaction on the erosion resistance of clay is not well understood and although its importance has been widely acknowledged by multiple literature sources, it has never been quantified. Therefore the goal of this thesis is to investigate what the are of compaction on the erosion resistance of clay and how large. To do this first of all a set-up has been designed and built, resembling the Erosion Function Apparatus as suggested by Briaud with some differences. Then 3 types of clay have been tested with differing initial erosion characteristics: one kind which fully complies with the current Dutch guidelines, 1 which does not quite suffice and 1 which does not suffice in any way. These 3 types of clay have been compacted at various energy levels (Modified Proctor, Proctor and 0.5*Proctor density). Also samples with differing water contents have been produced at each energy level and each clay. These samples were subsequently tested in the Erosion Function Apparatus at several flow-velocities in order to establish an erosion curve from which several erosion parameters were determined. \bigskip Having performed the erosion tests and determined the accompanying erosion parameters the results were interpreted in the Proctor curve. Plotting the results in the Proctor curve show that samples compacted at the optimum water content in the Proctor plane perform best, whereas they perform poorer the farther the water content is off from the optimum. Also it shows an increase in the compaction effort will generally result in a lower amount of erosion occurring. This result has been obtained by determining the critical shear stress, critical erosion velocity, the detachment coefficient and the velocity detachment coefficient and plotting these results in the Proctor curve. The first two parameters are measures of the amount of strength the soil has before it starts eroding at all, whereas the latter two are an increment meant to predict how much a soil might erode if the force imposed on the soil is above the erosion threshold. The results consistently showed the detachment coefficients were lowest with the samples compacted at an optimum water content and/or undergone a higher compaction effort (i.e. the soils erode less). On the other hand the critical shear stress/critical erosion velocity was highest at the optimum water content (i.e the soil erodes less), but did not necessarily increase with an increasing compaction effort. This is attributed to the loss of suction. Further, all tested soils showed their optimum erosion characteristics at or close to a degree of saturation of 85 \%. Several other methods were tried, but to no avail unfortunately. Also the rates of erosion were compared between the different types of clay. The clay which is in compliance with the Dutch codes showed the least amount of erosion, the one that did not quite suffice showed some more erosion and the clay that did not comply with the erosion guidelines at all showed the most erosion. However, it was also proved that the clay which did not quite suffice the requirements was (if compacted at optimum water content and at a sufficient energy level) only slightly more erodible than the clay which fully complied with the current Dutch regulations. Finally, the results of the tests carried out in this study were also compared to similar tests performed in the United States and the show consistent similar results. Overall, this study concludes that the use of (currently perceived) unsuitable clay as dike cover can be possible if compacted at or very close to the optimum water content and at a sufficient energy level. Also it can be concluded that compaction at the optimum water content can dramatically reduce the erodibility of a soil or even stop the erosion process. Increasing the compaction effort also reduces erodibility of a soil, although it must be kept in mind that the optimum water content also changes and thus if it is wise to do so. Finally this thesis leaves some suggestions for further research. These mainly concern the effects of cyclic wetting/drying cycles on the erodibility of the soil as well as trying to better understand the effects of the erosion process happening in the field versus the erosion process that is happening in the laboratory.

The current Dutch norms for the macro-stability of ood protection embankments and more specifically the determination of SHANSEP parameters has been subject of debate since its implementation in 2017. The main concerns arise from the apparent over-conservative nature of the guidelines and the apparent underestimation of shear strength of organic and silty clays. In this Master thesis, an assessment of the normalised normally consolidated undrained shear strength is performed for a dike stability reinforcement project between Gorinchem and Waardenburg in the South west of the Netherlands on a problematic clay layer called the `Gorinchem Clay' throughout this thesis. The main goal is the conrmation of the current norms or towards a more optimised and less conservative assessment of the strength parameters and in doing so, discusses one of the most recurrent questions in geotechnical engineering: do some of the measurements from laboratory tests reveal true material behaviour or do the limitations of such laboratory tests produce this particular behaviour? This analysis was performed on Triaxial Compression, Direct Simple Shear and Triaxial Extension tests from which the SHANSEP parameters are derived as input for the available shear strength along a slip surface according to the methodology developed by Ladd (1991) and following the Critical State Soil Mechanics. The structure of this thesis is as follows: rst, a literature study is performed on the strength parameters assessment in chapter 1, the principles of the Critical State Soil Mechanics and the diculties encountered in the determination of the strength parameters from laboratory tests in chapter 2. The laboratory results are then exploited in chapters 4 to 5 according to the current guidelines and more extended methods in which the limitations of the Classical Critical State Soil Mechanics are shown. The next chapters focus on a more fundamental understanding of the considered material consisting of modelling the material behaviour in chapter 6 using a simplied academic constitutive model featuring non-associative elasto-plasticity with mixed volumetric and deviatoric hardening allowing for hardening or softening. The outcomes of this thesis show Critical State conditions as traditionally understood could not be reached reliably for undrained conditions in Triaxial Compression, Triaxial Extension and Direct Simple Shear. The tests showed to be particularly unreliable beyond 10% axial strain in Triaxial Compression and Extension, and beyond 15% shear strain in Direct Simple Shear. The diculties encountered in determining the Critical State friction angle and undrained shear strength were shown to be minimised by performing drained and undrained Triaxial compression tests on slightly over-consolidated soil samples. Additionally, the limitations of the Classical Critical State theory were highlighted and a more advanced constitutive model including a non-associative ow rule and deviatoric hardening was successfully used to predict the behaviour in Triaxial Compression for undrained conditions. The predictions for drained conditions and particularly undrained Triaxial Extension were however limited.

The Netherlands has for long been witnessing problems due to subsidence. While urban infrastructure is mostly safeguarded from differential settlement by deep pile foundations, the same cannot be said about greenhouses. The greenhouses of the Netherlands has enabled the country to become the largest exporter of vegetables, second only to the United States of America. It is imperative that these greenhouses which form the backbone of the agriculture infrastructure of the country, are monitored continuously to minimize unprecedented damage.

This thesis aims to study the feasibility of using time series Interferometric Synthetic Aperture Radar (InSAR) as a means to monitor differential settlement in greenhouse structures. The analysis was primarily done using RADARSAT-2 data. In case of translucent surfaces of greenhouses, it was important to firstly identify the physical targets that are associated to scattering centres. This was done by analysing the statistics of the heights of the scatterers which helps in ascertaining where the radar signal is getting back-scattered from. It was inferred that the persistent and distributed scatterers are primarily identified from objects on the roof and outer walls of the greenhouses.

Moreover, the magnitude of deformation estimated from the scatterers have been corroborated with geotechnical data. It was seen that higher magnitudes of deformation was seen in locations with compressible soil types such as clay and peat close to the ground surface. It was also seen that greenhouse structures are prone to differential settlement when the depths of the piles are insufficient in areas with varying soil types. The effect of thermal contributions has also been studied and it was found that the estimation of thermal expansion does not significantly affect the estimated deformations.

From the study, it is evident that time series InSAR offers an effective means to monitor differential settlements in greenhouses. In order to check for differential settlement in individual greenhouses, it is proposed that a persistent scatter interferometric analysis be done initially and if it is seen that the density of these scatterers is insufficient, the analysis can be followed up with time series interferometry of distributed scatterers. Incorporating multiple track directions of radar data increases the number of greenhouses that can be monitored. Moreover, it was also seen that persistent scatterers were identified from additional greenhouses when Sentinel-1 data was used, despite its poorer spatial resolution. For further study, it is recommended that corner reflectors are used to validate the positions of the targets that are identified as persistent and distributed scatterers.

Mechanical dredging of a stiff clay results in clay lumps. Those clay lumps can be reused for the construction of land reclamation projects, and hereby economic and environmental value is generated. Placement of the clay lumps
in water result in a matrix of clay lumps and an interlump void space. The collapse of the interlump void space will cause large settlements and needs to be overcome before the site can be used for construction. Preloading is
an effective method to close the interlump voids. Stiff clays soften over time due to unloading and swelling. As a result, the strength and stiffness of the clay lumps decrease over time. The presence of discontinuities accelerates
the softening process. It was proposed by Leung et al. (2001) that the interlump void space closes under a reduced
preload of 25 kPa. The interlump void space closes under a reduced preload because the lumps soften over time.
The question rises if closure occurs within a normal construction timespan, i.e., 1 - 2 years, under this reduced
preload. In this study, a combined experimental and numerical approach is applied to determine the influence of soil
characteristics, softening and the presence of discontinuities on interlump void closure. The influence of softening due to chemical and hydro-mechanical swelling is tested by experimental swell-load tests on stiff overconsolidated
Boom clay samples. Additionally, the presence of discontinuities is studied by CT images, and a miniature clay fill test is performed to study the softening time and the rearrangement effect. Furthermore, a numerical study is performed in which the influence of specific soil characteristics on void closure was researched by a sensitivity analysis. By the experimental tests, it is shown that a pore water chemistry change alters the degree of swelling and its compressibility. Furthermore, fissures were identified in the sample material by CT images. The smallest microfissures could not be identified due to the resolution of the images. Consequently, it is impossible to estimate the effect of fissures on the hydraulic conductivity. Therefore, literature data was used to estimate the hydraulic conductivity and this was used as the input in the calculations. In the numerical study, it was shown that MPM could model the softening behaviour over time. The sensitivity analysis showed that the MPM model responds consistently to parameter sensitivity analyses. This leads to the conclusion that MPM can be used as a investigation tool to increase
the understanding of the influence of parameter variability on the final interlump void closure problem. The resulting strains of the numerical model comply with the theoretically calculated strains. It was expected
that the numerical strains were smaller than the experimentally determined strains, due to the presence of interlump voids in the numerical model. But, the numerical strains are larger than the experimental strains. In the sensitivity
analysis, it was found that the final strains and closure time are highly dependant on the soil characteristics. It is likely that the input of the MPM model differs compared to the true sample material and thus different strains result.
The time until the interlump void space closes was determined by a simplified geometry in MPM under a preload of 25 kPa. For an unfissured Boom clay, interlump closure takes place after approximately 16 years while for a
fissured Boom clay it only takes 2 months. Thus, closure of the highly simplified geometry takes place within a normal construction timespan for a fissured Boom clay. It must be kept in mind that the results are highly dependant
on the geometry, lump size and soil characteristics. Therefore, these results can not be generalized into an estimate of the interlump void closure time for any stiff lumpy clay fill. In conclusion, the feasibility of MPM was explored and it turned out to be a promising method to model the interlump void closure problem. Further studies are required to check if the model gives plausible results for more advanced constitutive soil models and geometries. If the results are positive, MPM can be used as a investigation tool to increase the understanding of the interlump void closure time for more refined geometries.

Around 55% of The Netherlands is protected by flood defences like dikes. Flood defences must meet requirements which are based on an allowed flood probability. Safety assessments are performed to check whether flood defences meet these requirements. Uncertainties are explicitly included in safety assessments. Uncertainty in soil behaviour is one of the major contributors to failure probability of dikes. Therefore reduction of this uncertainty in soil behaviour leads to potential savings in reinforcement costs. One of the uncertainties in soil behaviour is the effect of aging on the behaviour of soft clays in compression. Compression behaviour of clay is strongly dependent on the transition from overconsolidated to normal consolidated soil behaviour. This transition phase in plain-strain conditions is indicated by the yield stress. The development of the yield stress due to aging is described by isotach models in terms of creep strains and stiffness parameters. Examples of isotach models are the abc-model and PLAXIS’ Soft Soil Creep model (SSC-model). According to those isotach models, a yield stress induced by aging is similar to a yield stress induced by unloading. This study verifies the development of the yield stress predicted by abc-models based on K0-CRS tests. The test material is remoulded, highly organic, silty clay with a liquid limit of 165% and a plastic limit of 56% (OVP-clay). The determination of the abc-parameters is not straightforward. The value for the creep parameter is easily overestimated due to a slow decrease in excess pore water pressure during a constant stress phase, resulting in stress-dependent strains instead of creep strains. The b-parameter is affected by the gradual transition from overconsolidated to normal consolidated behaviour and by the development of structure during aging. The development of structure is not included in the isotach models and could result in a higher yield stress than predicted by the isotach models. The possible increase of the yield stress due to the development of soil structure is not visible in the test results, but the performed tests indicate softening behaviour after aging phases. The SSC-model simulates no increase in lateral stress during aging. Some studies present data showing that the lateral stresses increase under a constant effective stress in time. Unfortunately, the development of lateral stresses cannot be quantified based on the performed tests due to stick and slip behaviour in the K0-ring during laboratory testing. Based on the test can be concluded that a minimum yield stress in time can be predicted based on isotach models. An important note is that a yield stress is always obtained with respect to a certain isotach which is dependent on the loading rate and the creep parameter. The shift of the yield stress from the imposed isotach to the reference isotach is often significant in laboratory tests, but not taken into account. Effects of development of structure during aging should be further investigated: it influences the behaviour of the soil in compression and affects the yield stress determination.

Reinforcing levees with traditional methods is becoming more difficult in the Netherlands. Often there is a lack of space near the levee or it is desired that the landscape is maintained. Because of this, alternative methods for reinforcing levees are still sought after (CUR-commissie C141, 2007). In this study, the technical feasibility of a possible alternative reinforcement method for regional flood defences is studied: mass stabilisation. Mass stabilisation is a soil improvement technique with which soft soils are mixed with a binder on site and in-place to create a stronger soil layer (Forsman et al., 2015). It is presumed that mass stabilisation requires no extra space at the levee, does not affect the landscape and requires little transportation of materials. For determining the technical feasibility of applying mass stabilisation for reinforcing regional flood defences, three criteria have been examined. First, the ability of mass stabilisation to solve a stability deficit at levees was examined using two-dimensional stability analyses. Secondly, the achievability of the required effective strength parameters in compliance with Dutch standards was examined in the laboratory. Thirdly, the practicability of mass stabilisation at levees was examined using two-dimensional stability analyses. Although the results of this thesis have shown that mass stabilisation is a technically feasible technique for reinforcing regional flood defences, further studies will be required before a definitive conclusion can be drawn. It is therefore recommended that additional analyses into the practicability are carried out and that the relationship of the properties and the variability thereof between soils stabilised in the laboratory and the field is studied.

Peat is formed by biochemical processes and the accumulation of the soil depends on the aerobic and anaerobic conditions. The soil covers five to eight percent of the land surface of the earth. For this thesis a peat coming from a site called Markermeeer, which is a lake between the provinces Noord-Holland and Flevoland in the Netherlands, is used. The fabric of peat consists of a fine dark peat material and fibres. The peat material is made of water and decomposed plant tissue whereas the fibres consist of fragments of (semi-)degraded wood, stems, branches and grass. Furthermore the cell walls of the fibres consists of a primary and a secondary wall which are mostly made of cellulose and lignin. Knowing the fabric of wet peat is of importance to better understand the role of fibres in the unusual behavior of the soil. Previous studies on imaging wet peat with X-ray micro CT has proven to be difficult due to the indistinguishable linear attenuation coefficient of the peat organic matter and water. This coefficient is a function of the interactions of X-rays with the materials present in peat. Flushing a peat sample with lead(II)nitrate have shown to be the most successful approach to show the stems and branches in a peat sample using X-ray micro CT. However, according to \cite{kettridge2008x} this flushing approach showed that the focus was limited on the larger fibres present in the sample because the different materials in peat have almost the same linear attenuation coefficient and therefore this thesis also focuses on two other techniques to image the fabric of wet peat.
Neutron CT was performed on cylindrical peat samples. This technique uses a thermal neutron beam to image a sample and was available at the Reactor Institute Delft. The resolution of this imaging station is 150 $\mu$m. Since the cell walls and the peat material between the fibres in peat have both a large attenuation coefficient for neutrons, it was not possible to distinguish the fibres from the peat material between the fibres, using this technique. To gain a contrast heavy water was used to replace the water because it has a low attenuation coefficient. Flushing a peat sample with heavy water showed the most effective way to do so. The aim of this procedure was to get a contrast difference between the peat material between the fibres and the fibres itself. A triaxial setup was used to flush the sample with heavy water. Heavy water diffused in the water present in the peat inducing a decrease in attenuation of the peat material between the fibres. The reconstructed tomographic images were filtered using a 3D visualization program Avizo version 9.4. Only the air-filled fibres could be observed on the tomographic images whereas the water-filled fibres could not be observed. Samples of the same peat were scanned in a dry and wet state using a X-ray micro CT scanner present at the geoscience section of TU Delft. On the tomographic images of the wet sample, white halo were observed representing the cell walls of the fibres.
X-ray phase CT present at the Ghent University Centre for X-ray CT was performed on a peat sample coming from the same site. This method results in an image contrast using a large fixed distance for low absorbing materials like peat. The aim of this scan was to reveal more fibres than observed with the X-ray micro CT scan at TU Delft. However, edge enhancement did not occur because of the filtering needed during reconstruction of the raw data to visualize different structures in peat. \\ The white halo could be thresholded and filtered in avizo resulting in a 3D image of the rod-like fibres. These fibres were randomly orientated. On the micro CT scan of the sample in a dry state other fibrous structures than the rod-like fibres were observed. These fibres were not shown as white halo on the tomographic images of the wet samples.

In this study, the assessment of the KIJK dike follows the new guidelines given by POV-M 2018 using PLAXIS. The macrostability assessment and design of the dike is examined for two cases. In the first case the dike is assessed based on the in-situ conditions (Green dike) whereas in the second case a cantilever sheet pile wall is used in the design (Blue dike) to strengthen the dike. Prior to the analysis the interpretation of the available laboratory data that were executed according to the new protocol (WBI 2017) and the parameter determination of the necessary input parameters of the considered constitutive models (SHANSEP NGI-ADP, Hardening Soil, Soft Soil models) is established. The undrained shear strength (su) of the SHANSEP NGI-ADP model is based on the SHANSEP concept and the normalised su (S (NC, OC)) and the strength increase exponent (m) parameters are needed. For the HS and the SS models the effective strength parameters φ and c are required.
The FoS is determined with the use of the strength reduction method. Moreover, the CSSM framework is coupled with the SHANSEP (Stress History and Normalized Soil Engineering Properties) concept enabling the incorporation of the effect of stress history and the stress path characterizing the undrained shear strength in the design. The CSSM framework is applied in the design by determining the strength parameters of clays from an axial strain level equal to 25% under triaxial compression whereas for peats at 40% shear strain level under direct simple shearing conditions. In this study the strength parameters were additionally determined from the service conditions strain levels which are the 2% and 5% strain levels for clays and peats, respectively. The necessary strength parameters for the constitutive models are determined in the considered strain levels exposing their influence for both clays and peats. Moreover, the study includes the examination of the constitutive models at a single element level conducted in the Soil Test facility available in PLAXIS. When it comes to the design analysis, it was found that the SHANSEP NGI-ADP model fits properly the su determined from a cone penetration test. The HS and SS models based on the effective strength parameters may result in an undrained shear strength profile which deviates from the su that the soils exhibits in the field.
The study points out that both the strain level dependency of the strength parameters and the selection of the constitutive model at critical loading influence the results in terms of the developed failure plane and the FoS, especially in the case of Green dikes. The study also elaborates the response of the models in regard to the design requirements for the calculation of displacements and the structural forces. Finally, this study answers the knowledge gap regarding how the strain level influences the strength of the examined soils and how this should be translated in the design along with the explanation on the effects of the selected constitutive model on the results.

Backward erosion piping is a dike failure mechanism. It is the internal erosion process by which sand is eroded away from underneath a dike or levee by seepage flow. This erosion process progresses in the direction opposite to the direction of seepage flow and forms a small pipe directly beneath the dike. As erosion continues, this process can lead to dike failure. During this erosion process, the groundwater flow pattern is subject to continuous change, due to the growth of the pipe. Self-potential (SP) monitoring is sensitive to changes in the groundwater flow pattern, because of the electrokinetic coupling between fluid flow and the streaming potential. The SP field due to flow underneath a test dike was modeled with a FreeCAD -> Gmsh -> pyGIMLi workflow. This workflow can also be used to effectively resolve a wide range of standard and customizable geophysical modeling and inversion tasks. After modeling, field experiments were conducted, on the same test dike, to further assess the possibilities and limitations of SP monitoring to track the progress of backward erosion piping. Given that it is essential to have an accurate resistivity model in order to find the location of the SP source, an integrated electrical resistivity tomography (ERT) and SP monitoring system was designed. The electrodes used in this monitoring system were polarizable stainless steel stakes. The reliability of polarizable electrodes was greatly overestimated, as they turned out to give unstable SP measurements. The reason for the inferior reliability of polarizable compared to non-polarizable electrodes, was found through extensive literature research. The reason being that the largest potential in any electrode originates from the contact between the metal and the electrolytes in solution. The metal of polarizable electrodes is in direct contact with the electrolytes in the soil, which have variable concentrations. Therefore, the potential measured fluctuates together with the concentration of soil electrolytes in contact with the metal. The metal of non-polarizable electrodes, on the other hand, is in contact with a solution of its own salt, which has a constant concentration. Finally, piping is not expected to be measurable with SP monitoring, before a large sand boil is visible in the field. Once a positive SP anomaly develops at the sand boil, changes in the SP field due to the growth of the pipe are expected to be too small relative to the SP anomaly associated with water flow through the sand boil. Even though an integrated ERT and SP monitoring system is known to provide useful information about the hydrology of a dike, such a system is not sensitive enough to be able to monitor the development of a backward erosion pipe.

Cyclic loading tends to affect the strength and stiffness parameters of soils, degrade its structure and results in accumulation of the excess pore water pressure. Such behaviour often leads to premature failure of the soils. Over the past few decades, geotechnical research community has developed numerous constitutive models to predict the behaviour of soils with variable degrees of success. The constitutive models based on the concept of bounding surface plasticity have gained much attention owing to the simplicity in describing the development of stiffness. This thesis analyzes the performance of bounding surface SANICALY model in reproducing the stress path and stress-strain behaviour of kaolin clay under undrained cyclic loading conditions .
A driver was developed in MATLAB for the chosen constitutive model to simulate undrained triaxial loading conditions. The performance of the driver was verified against the data published from literature. Further, sensitivity analysis was carried out on chosen model parameters. This was followed by validating the model with the experimental data on kaolin clay. Particularly, model performance was examined with varying initial conditions such as change of over-consolidation ratio, change of initial anisotropy, variation of initial pressure and strain controlled loading. The obtained results from sensitivity analysis have shown to increase the strength and stiffness response of the model with increase in model parameters such as the rate of evolution of the anisotropy, bound for evolution of the anisotropy and change of initial stress-induced anisotropy. In the context of calibrating the model parameters against the experimental data, it was initially noticed that the experimental stress path in monotonic loading was not being reproduced by the model with various combinations of the initial parameters. Owing to such performance the model was subsequently assessed qualitatively. When the model is subjected to different initial loading conditions, certain aspects of the experimental behaviour were qualitatively captured by the model. These include faster rate of accumulation of pore water pressure with increase in the amplitude of cyclic loading, reduction in the rate of development of strains with increase in OCR values, increase in the hysteretic damping with increase in the amplitude of strains. However, with change of OCR there were differences in the development of stress path. Also contrasting results were observed with regard to the development of the stress-strain response with change of amplitude of cyclic loading and initial pressure. Analyzing the model formulations revealed that the chosen model did not take into consideration the fabric anisotropy and hence it explains the deviation of the stress path from the experimental stress path in monotonic conditions. In the chosen model, during the process of cyclic loading, stagnation in the evolution of stress path is observed whenever the plastic volumetric strains stop evolving. It is recommended to incorporate plastic deviatoric strains in the evolution of the bounding surface in order to stimulate the further development of stress path even when the plastic volumetric strains stop evolving. It is also suggested to validate the model against different clays since this thesis focused only on Kaolin clay.

The Dutch flood defences are subjected to the new Dutch safety assessment program (WBI 2017) that incorporates the latest insights and safety regulations. As a consequence of the updated piping assessment model, studies are expected to identify more piping prone areas in the Netherlands, which makes it worthwhile to investigate alternative piping mitigation solutions that could be less expensive. A new potential solution is the Pipingontspanner, which consists of relief wells in combination with a water catchment area. The Pipingontspanner relieves water pressure fromthe aquifer and uses the water as counter pressure against the Uplift mechanism, thereby reducing the likelihood of piping.

The study investigates the technical and economic feasibility of the Pipingontspanner as a piping mitigation measure. A problem analysis was used to identify the components that form challenges in realising the concept and pinpoint the research to the functioning, effectiveness and applicability of the Pipingontspanner concept.

A hydraulic model and a design approach demonstrate the functioning of the concept. The hydraulic model describes the flow underneath the dyke, through the well and towards the basin above it. A critical point for the flow calculation appeared to be the time-dependent interaction between the river and the basin water level, which leaves only numerical calculation methods to describe the problem. The numerical program Modflow was chosen to predict the Pipingontspanner groundwater flow by simulating a flood wave scenario for a green dyke with piping problems and implementing relief wells and a basin. A parametric design approach was followed to create a Pipingontspanner model that could obtain the optimal configuration for a measure that can only be calculated with a groundwater flow model. For the verification of design configurations, relevant failure mechanisms have been included in this model. For the selection of the optimal configuration, a cost-benefit analysis has been used as an evaluation criterion. The Pipingontspanner model creates, calculates, verifies and evaluates the different design configurations.

The effectiveness of the Pipingontspannerwas illustratedwith a sensitivity and a cost analysis. For a variety of subsoil and hydraulic conditions, the sensitivity analysis showed that the safety factor for Uplift increases substantially for higher permeability of the aquifers and slowly growing hydraulic loads. The influence of the cover layer permeability and storage coefficient is negligible on the performance of the Pipingontspanner. On the other hand, the costs analysis showed that well maintenance, well monitoring and basin dyke construction costs are the main cost drivers of the design.
The total cost grows exponentially if the basin width behind the dyke is limited as the number of wells increases for smaller basins.

The applicability of the Pipingontspanner was demonstrated through a case study of a green dyke
in Tiel with piping problems. In addition, the design and costs of the Pipingontspanner in Tiel were compared against a traditional piping berm measure to illustrate the economic feasibility of
the concept. The results showed that the Pipingontspanner is not only able to mitigate the piping problem, but it does so with a smaller footprint and lower total cost than the piping berm. However, the case study also showed that limitations in the current groundwater flow model setup required an adaptation of the case geometry to prevent the drying up of the top layer cells, which would terminate the simulations.

The Pipingontspanner concept has proven to be technically feasible and economically competitive compared to the piping bermunder the conditions of:
1. a permeable aquifer with a minimum transmissivity of 25 m^2/d;
2. a minimum hinterland space of 10 m behind the dyke.

The maintenance and improvement of river embankments plays a crucial role in granting the safety and ensuring the quality of life of the inhabitants of the Netherlands. While the threat posed by storm surges has been greatly reduced since the implementation of the Deltaworks, riverine areas in the west of the country are characterised by the highest risk of flooding.
The use of relief wells is one of the viable solutions to improve the safety of river embankments against instability mechanisms such as uplifting. Throughout the Schoonhovenseveer-Langerak project (SLA), the implementation of a system of relief wells constituted one of the key measures to increase the safety of the embankment structure during high-water events of the river Lek. In the current study, the influence of relief wells was simulated employing a 3D coupled model built with the Finite Element software PLAXIS 3D. Using soil properties from former site investigations, a calibration was performed based on in situ pore water pressure measurements, in order to devise adequate boundary conditions. The safety assessment of the structure was then performed by applying a time-dependent high-water wave and employing strength reduction to yield a Factor of Safety. An intermediate 2D model was employed to examine the evolution in time of the Factor of Safety, the influence of mesh refinement on the safety results, and the shape and the location of the failure mechanism. By means of the 3D model, different design choices of the system of relief wells were tested and their influence on the stability of the structure and on the water discharge could be determined. A comparison between the results obtained in the 2D and 3D models showed that a 3D stability analysis does not necessarily yield a less conservative result than the 2D equivalent.
Comparing the values of the Factor of Safety obtained with time-dependent and stationary boundary conditions showed that the results obtained in the latter case are considerably more conservative, suggesting the potential benefits of a time-dependent fully-coupled analysis.