The study delves into the crucial role of undrained shear strength in the stability of civil engineering projects involving soft soils. It explores the enhancement of this strength through (pre)loading in conjunction with Prefabricated Vertical Drains (PVDs). The Stress History and Normalized Soil Engineering Properties (SHANSEP) framework is employed to forecast changes in strength across various phases - before, during, and after loading. Validation is conducted using Cone Penetration Tests with pore pressure measurements (CPTu), and the Nkt strength factor is employed to correlate CPTu data with undrained shear strength. The research combines laboratory experiments and field data to predict undrained shear strength via SHANSEP, which is subsequently validated through CPTu measurements and monitoring. Two distinct datasets are examined in the study. The first dataset involves dike projects at 'de Markermeerdijken' in the Netherlands, where PVD-assisted surcharges of 5 meters are implemented. SHANSEP is utilized to predict strength levels before surcharge, post-surcharge removal (after >90% consolidation), and post-removal phases. These predictions are cross-validated using CPTu data and laboratory tests. The second dataset focuses on a reclamation project involving Marine Soft Clay strength in the Philippines. Strength correlations with CPTu tests during consolidation are established, and comparisons are made with data from piezometers and SHANSEP predictions. The analysis uncovers that SHANSEP's precision is contingent on several factors: 1. The uncertainty surrounding Pre Overburden pressure (POP), leading to inaccuracies in initial strength predictions. 2. The effectiveness of preloading being compromised by limited load area relative to depth. 3. Submergence of the surcharge beneath the phreatic surface, resulting in diminished preload and gained strength. 4. Incorporating creep into predictions necessitates a reduction in SHANSEP's S factor. 5. Accurate predictions rely on high-quality laboratory tests. SHANSEP demonstrates its ability to effectively forecast undrained shear strength in soft soils, with its predicted surcharge strength increments aligning with CPTu values. However, challenges are identified in predicting and verifying strength during consolidation due to the dominating influence of excess pore pressure. The study's key findings recommend the inclusion of adjustments for preload submersion, consideration of load distribution, and the evaluation of partially drained CPTu data using parameters such as Bq and qt to enhance the verification of SHANSEP-predicted strengths. The study emphasizes caution against relying solely on CPTu or piezometer data for ensuring reliability and accuracy.

Mangroves can provide coastal protection by attenuating waves, currents, and trapping sediment (Menéndez et al., 2020; Bao, 2011). The effect of mangroves on the hydrodynamics depends on their size, location, density, distribution and morphology of the vegetation (Mendez and Losada, 2004). Mangrove loss during storm events will therefore impact the coastal protection. The soil-root interaction and influences of soil properties in the resistance of a mangrove tree against failure is neglected in most studies of flood protection by mangroves, due to a lack in understading. This thesis improves the understanding of the soil-root interaction by deriving the schematization for multiple failure mechanisms in comparison with field observations. Firstly, the wind and wave forces are modelled to determine the different loads in the forest. Secondly, a novel schematization was developed of different failure mechanisms considering the soil properties and root system. All results are developed for a case study of a fringe forest in Demak, existing of Avicennia marina rooted in a silty, saturated soil. To be able to determine the difference between the seaward and landward edge of the forest, the width of the forest is increased to 500 meter. Also, due to a lack of information on the mechanical strength of Avicennia m., the mechanical properties of Rhizophora m. are used, which will overestimate the resistance due to the higher wood density of Rhizophora m. (Manguriu et al., 2013). The results show that the drag forces were largely influenced by the tree architecture (such as vegetation width and height), forest density and inundation of the tree. Larger wind and wave forces, and therefore a larger moment occured if the width of the vegetation increased. If the height of the tree increases, a smaller part of the canopy is submerged. This leads to a decreased area that is exposed to waves and therefore a declined wave force. A higher tree does enlarge the area subjected to wind forces, resulting in an increased total moment. Overall, the 2.8-meter tree analysed in this thesis experienced the largest horizontal forces and moment at the seaward edge of the forest due to the relatively large water depth at this location. Furthermore, the maximum horizontal force and moment shift to the landward edge for larger trees due to the increase of wind contribution. To investigate the stability under these loads, the different failure mechanisms need to be inspected. The failure mechanisms of root breakage and slippage are not important because of the high safety factors and no observations in the field. The failure mechanism trunk breakage also showed a high safety factor, contradictiory to field observations pulling willow trees. The model results and field observations showed that a combination of upwind soil uplift and bending of roots was found most likely to occur. For this failure mechanism, the roots are schematized as beams with a spring-support on the leeward side of the trunk. The roots are exposed to the weight of the overlying soil and the overturning moment. The model showed that an increase in the participation angle α or root diameter, or increase in root length results in larger stresses in the windward roots. The amount of uplift enlarges when the root diameter or root length increases or the participation angle decreases. The modulus of subgrade reaction and modulus of elasticity influence the distribution of the overturning moment over the leeward and windward roots. The largest difference between the moment inside the leeward and windward roots is found with a high modulus of elasticity or high modulus of subgrade reaction. This difference is in agreement with the contrasting reaction between soil types as stated in literature (Dupuy et al., 2007). The results show the dependence of the different failure mechanisms on soil and root properties, but also on the forest architecture. This research shows that the incorporation of mangrove stability in wave attenuation models cannot be neglected. Using an effective stress approach, instead of a total stress approach as in this thesis, would provide extra information, such as soil behaviour in time during loading. It is therefore recommended to measure pore pressures under static or cyclic loading, resulting in the progression of the effective stress over time. The inclusion of more accurate soil descriptions would enable reducing uncertainty in nature-based solutions and enables to describe the soil-root reaction to loading over time.

Tailings dam failures are one of the most destructive phenomena in terms of the number of victims and the environmental impact generated. Over the years, different causes have been identified, with flow liquefaction being a prominent factor to consider when assessing the stability of tailings deposits. Due to the complex nature of these events, numerical models are considered an important alternative for the analysis and design phases of tailings dams. These models require the application of appropriate advanced soil constitutive models to reproduce the relevant features of the soil behaviour and, thus, obtain results that are a valid representation of the observations in reality. In this work, the capabilities and limitations of the Clay And Sand Model (CASM), originally proposed by Yu (1995, 1998) and NorSand model (Jefferies, 1993) are assessed using the finite element software PLAXIS in the analysis of flow liquefaction. The criteria considered in the assessment consist of the accuracy of the models to predict the response of the soil under different monotonic loading conditions, the efficiency of the models for their application and the consistency of results obtained from boundary value problems compared with reality. Both CASM and NorSand are models developed within the critical state soil mechanics framework and use the concept of the state parameter (Been and Jefferies, 1985). However, they present differences in their formulations and assumptions. The performance of the model is explored in this study at three different levels: the material point level, simple boundary value problem level and complex boundary value problem. Different laboratory tests are simulated for the material point level, analysing a single stress point. For the boundary value problems, the construction of an embankment and the well-known Tar Island Dyke flow liquefaction event are simulated. Before applying the models, the implementation of CASM into PLAXIS is widely verified, and additional verification exercises for the NorSand implementation are also presented. Parametric analyses are performed to have a better insight into the effect of the parameters in the response of the models. Moreover, CASM and NorSand calibrated for the same tailings materials are used for the simulations. Given that most of the model parameters can be estimated through very well established procedures using experimental data, in principle, the calibration of both models is relatively straightforward if the required experimental data is available and when the data is limited, correlations can be used to estimate the parameters of CASM assuming normally consolidated conditions, which is the state at which tailings are often found. The results of the thesis, at the material point level, show that CASM presents limitations to quantitatively predicting the response of dense materials. This is associated with the formulation and assumptions of the model. In the analysis of the boundary value problems, lower strengths and higher stiffness in terms of stress-strain response are obtained with CASM compared with NorSand. Despite these differences, the results of this thesis demonstrate that both CASM and NorSand can successfully reproduce flow liquefaction at the different study levels.

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

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.

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 extraction and processing of mineral and metal ores in the mining industry comes paired with large amounts of mine waste, also known as tailings. This waste consists of various chemicals, acids, and heavy metals which are used during these extraction processes. The tailings are usually stored off in tailings storage facilities (TSF) in a loose state, gradually consolidating over time. TSFs founded in seismically active areas are susceptible to liquefaction due to earthquake loading. Historic data show that about 35% of dam failures are due to liquefaction of the tailings, thus increasing the need to study the liquefaction responses of tailings under cyclic loading. Extensive studies have already been conducted using cyclic direct simple shear (DSS) and cyclic triaxial (CTX) tests as these tests under constant-volume conditions can evaluate the change in pore pressure within a soil accurately. Previous study shows what importance the relative density and sloping ground conditions, known as drained shear bias, have on the cyclic resistance to liquefaction of the tailings. However, in practice the pore water is not bounded within the material and excess pore water can flow out through installed drains. A round-robin program, issued by the University of Western Australia (UWA), requested a study on the liquefaction response of a particular fine-sand tailings material. Inspired by this round-robin program, an interest raised in studying the cyclic shear response of tailings by using a direct shear box to investigate the cyclic behaviour under partially drained conditions. With use of the direct-shear apparatus of Wille Geotechnik, a test program has been set up to study the influences of relative density and drained shear bias under stress-controlled cyclic shearing and under constant normal load conditions. Results of the experiments met the expectations that denser soils have a 9.6% higher cyclic resistance ratio (CRR), and samples with applied drained shear bias have a 32% lower CRR compared to samples tested with level ground conditions. Furthermore, samples which underwent post-cyclic shearing showed strain-hardening responses and yielded higher shear stresses compared to the monotonic test, indicating that the constant normal load further densified the samples during cyclic shearing. However, during the experiments, it was quickly found out that the loading frequency was not being applied optimally making it not possible to analyse influence of partially drained conditions . This study showed promise on its capabilities to study cyclic shear loading on a soil. For future work, it is suggested to perform similar tests under a uniform loading frequency with the use of a shear box to evaluate its capabilities to study on partially drained conditions. It is also recommended to conduct tests under constant volume conditions, to evaluate the shear-box apparatus’ capabilities to study the liquefaction response of a soil due to excess pore pressure generation.

Long-term settlement is unavoidable but by good prediction accompanied risks can be reduced. Nowadays linear isotach models belong to the state of the art but result in very small strain rates when small load increments or unloading is applied. For that reason this study has focused on the validation of those predictions using InSAR. Different causes of settlement have been studied and it was found that fluctuations in groundwater do not affect the final settlement. Another influencing factor is the presence of organic matter and its degradation. This reduction of organic matter is caused by many factors and is yet too complicated to quantify and include in predicting models. In spite of the idea that predicted strain rates by linear isotach models are too small, InSAR results show that the measured displacement rates were actually smaller than predicted some years after construction for road segments of the A2 and that a new proposed model could be used to match these displacement rates better. The C+S model by Vergote et al. (2021) is an isotach model which uses non-linear isotachs which would reduce the strain rates faster than linear isotach models and viscoplastic swell is included which also reduces the total strain rate. Results on incremental loading tests show that this model captures the behaviour in unloading stages better than the linear isotach models but for conventional incremental loading tests the two models do not differ much. For field scale embankment scenarios the results show that the C+S model with non-linear isotachs and viscoplastic swell included will predict a longer swell period with more swell, lower total strain rates after the swell period with a faster decay and in the and less residual settlement.

Soil liquefaction is a phenomenon in which an otherwise stiff, loosely packed, cohesionless soil loses its strength and behaves behaves like a viscous liquid in response to a change in stress conditions. This study focuses on liquefaction under monotonic load (i.e. static liquefaction) and not cyclic liquefaction. While the role of state variables such as relative density in liquefaction is well established, the importance of intrinsic soil properties (ISPs) is less clear. ISPs include grain size gradation, mineralogy and grain shape The critical state soil mechanics framework can be used to link these properties to liquefaction susceptibility. One approach to do so is the "Relative Contractiveness" (RC) concept proposed by Verdugo & Ishihara (1996). This thesis investigates the role of ISPs in soil liquefaction and tests the RC concept through a combination of statistical analyses, four case studies and a new experimental study. The statistical analysis shows that an increasing fines content generally leads to greater relative contractiveness and especially at lower stress levels, indicating increased sensitivity to liquefaction. Particle shape plays a multi-faceted role in liquefaction susceptibility, as increased angularity may increase compressibility but also increase resistance to particle rotation and hence reduce the likelihood of flow behaviour. The mineralogy of soils was difficult to statistically analyse as the information is usually not given, but extra care should be taken when dealing with sands that are not made of quartz, as most index methods are based on quartz. The case studies exemplified varied applicability and benefit per case. The Ijmuiden case demonstrated the limitations of field tests and critical state determination. It did indicate medium to high relative contractiveness for the tested soils. The Nerlerk berm failure demonstrated the importance of fines content in liquefaction susceptibility, as only the finer of the two soils used for the hydraulic fill liquefied. However, the geometry and differences in deposition method also played a role. For the Hollandsch Diep case environmental factors are ought to play a more important role in liquefaction rather than that the soil is intrinsically exceptionally susceptible to liquefaction. The Bangabandhu bridge case highlighted the limitations of compressive loading based methods as the soil was particularly weak in tensile loading. It also highlighted the importance of mineralogy and grain shape, as the presence of plate-like micaceous particles drastically reduced its strength. The new experimental study investigated a soil from the Eastern Scheldt estuary in the Netherlands, a region historically notorious for liquefaction flow slides. Surprisingly, the sampled soil was not prone to liquefaction at all, showing strong dilative tendencies under triaxial compression. In conclusion, this study suggests that the relative contractiveness concept could be used as a screening method for assessing liquefaction risk, rather than a deterministic method for designing parameters. However, further studies with extensive and consistent material characterization and critical state determination are needed to verify the validity of the relative contractiveness concept. Discrete element modelling of soils could also provide future opportunities for advancing our comprehension of the role of ISPs in liquefaction susceptibility.