The engineering response of fibrous soft soils, such as peats, is significantly influenced by the presence of fibres. Interpretation of laboratory tests performed on such soils is complicated due to the reinforcement provided by the fibres, which results in the material behaving as a composite, with properties different from traditional inorganic soils. The fibre matrix interaction complicates the estimation of geotechnical parameters such as shear strength and frictional properties from standard laboratory tests. Some empirical criteria to account for the contribution of fibres have been proposed, but there is no general consensus in practice. In this work, a numerical approach using finite element method and accounting for the kinematics of fibres is used to gain physical insights into the role of fibres in the mechanical behaviour of soft soils. Numerical replicas of common geotechnical laboratory tests were simulated as boundary value problems by superimposition of a soil matrix with a fibrous network. The contribution of fibres to the mechanical response of the soil sample is quantified for various stress and strain paths relevant to the engineering practice. The influence of two constitutive stress-strain relationships for the fibres upon stress reversal are compared and results suggest that fibres engage before they return to their original configuration, thus implying an influence of their compression history that could be linked to local tensioning of the fibres after compression. Further, laboratory tests on peat are used to validate the numerical approach, modelling the big fibres explicitly in the model and using an advanced model for the matrix, capable of reproducing behaviours such as distrotional hardening expected from the microscopic fibres in the matrix. The comparison highlights the effectiveness of the proposed numerical approach in accurately capturing the mechanical behaviour of natural fibrous peat as observed in laboratory tests. This approach provides valuable physical insights into the composite behaviour of peat. Furthermore, it offers a novel tool for interpreting extensive experimental data on peat samples, enabling the derivation of critical geotechnical parameters essential for the design and assessment of infrastructure founded on this unique soil.

The Netherlands faces an urgent need to reinforce its flood defences against rising sea levels. This study investigates the potential of using locally sourced Dutch clays as a sustainable alternative to imported materials for dike reinforcement. A comprehensive experimental campaign was conducted on three types of laboratory-compacted Dutch clays, with clay content varying from 42% to 23% and differing levels of erodibility. Soil characterisation, microstructural analysis (ESEM), and mechanical testing through oedometer and triaxial tests assessed the impact of composition on engineering properties. Results showed that compression behaviour is primarily influenced by the clay content, while shear response, exhibiting both contractive and dilative tendencies, remained consistent across the different compositions. Triaxial tests revealed that samples acted as representative volume elements (RVEs) only up to a certain strain level, beyond which localised deformations occurred due to end-restraint. Additionally, mixing local soils with erosion-resistant materials proved effective in improving the erodibility of unsuitable soils, thereby reducing the quantity of imported material needed. Finally, a new advanced model, JMC-clay bounding surface, was calibrated and validated across the range of soil compositions. The model accurately captured pre-failure and failure behaviour, demonstrating its applicability for predicting geotechnical structure performance and stability.

The Netherlands, known for its extensive coastal regions and significant portions of land below sea level, faces escalating flood risks due to climate change and sea level rise. To address these threats, the country relies on the construction of dykes founded on soft soils like peat. Traditionally, peat has been regarded as the critical soil layer for infrastructure stability; however, recent studies suggest that the interface between peat and clay is crucial for assessing failure mechanisms. This research focuses on the mechanical response of peat-clay interface samples to better understand the interaction between these two materials.

The study integrates comprehensive laboratory experiments with advanced numerical modelling to investigate the peat-clay interface. The findings underscore the critical role of the clay layer in the mobilised shear strength, which is essential in dyke stability assessment. The study highlights that accurate modelling of the interface requires considering the volumetric fractions of peat and clay and the reinforcing effects of fibres. This research enhances the understanding of peat-clay interfaces in dyke systems and contributes to improving geo-infrastructure assessment.

With changing climatic conditions, droughts are expected to increase in duration and intensity. Under drying conditions, desiccation cracks can form in clay soil, which is often used as dyke cover and in some cases also for the core of the dyke. These desiccation cracks have a potential impact on the stability of dykes. The main objective of this study is to gain a better insight into the potential influence of desiccation cracks on the macro-stability of Dutch river dykes. With the help of PLAXIS 2D software, a numerical model is used to study the hydraulic response and stability of a dyke with desiccation cracks. It is chosen to simulate the effect of cracks by adjusting soil parameters of the cracked layer. Steady-state scenarios as well as time-dependent scenarios, where high water events are simulated, are studied. The influence of precipitation is also taken into account. For the steady-state case, a maximum decrease of the factor of safety of 5.6% is found. In the time-dependent scenarios, the factor of safety decreases up to 10.8% if a cracked zone with a depth of 2 meters is present. The magnitude of this decrease is mainly depending on the hydraulic conditions and the crack parameters chosen to simulate the effect of cracks. Both inner and outers slope failure is observed. The largest difference between the factor of safety of the cracked and the uncracked dyke occurs when the dyke fails due to outer slope instability as the factor of safety decreased most in the case of a rapid drawdown. From these results it follows that cracks can have a negative impact on the macro-stability of a dyke, but the difference is not unambiguous. Precaution is advised if significant cracks develop at dykes, but smaller cracks are not likely to lead to failure immediately.

Land subsidence is a complicated phenomenon, relevant in the Holocene coastal-plain of the Netherlands, that can be triggered by many different mechanisms and components. To cope with the effects of land subsidence, detailed information on which mechanisms are causing subsidence and its variation over time and space are needed. Subsidence studies predominantly focus on grass and agricultural areas with organic soils above the groundwater level. In this study, subsidence is also evaluated for dikes and the urban environment. It is attempted to model subsidence behaviour of four different projects, grass plots within the Krimpenerwaard polder, locations along the N3 Dordrecht, an embankment constructed for the A5 Badhoevedorp and locations along the Markermeerdikes, and to verify these model results with subsidence measurements available. Existing one-dimensional models for different subsidence components are used in the evaluations. For grass and agricultural areas it is found that subsidence behaviour could be modelled and the results lie within the range of uncertainty of the subsidence measurements used. However, the equations used are often empirical and do not include all relevant influencing factors and couplings. Compression by degradation of organic material as a response to drainage of organic soil is the main subsidence component in these areas. Anaerobic degradation of organic material is also included in the modelling approach, a component that is often neglected in other subsidence studies. Two infrastructure related projects provided insight to subsidence components in the urban environment. Compression by loading is the main subsidence component relevant in this area type, and thus settlement models are suitable to approximate the subsidence behaviour. However, a slight underestimation of creep shows that the existence of a small additional subsidence component for a situation with soft soil layers pushed below the water table could not be excluded. Unravelling subsidence behaviour from dikes showed to be more complicated, where an interplay between different components is relevant. Based on the evaluation in this study compression by loading and compression by an oxidation or shrinkage component are indicated the main subsidence components. Many uncertainties and assumptions used in this evaluation influenced the results. For all area types disentangling subsidence into contributions of separate components based on a total measured subsidence signal causes large uncertainties, as modelled subsidence contributions for individual components could not be verified.

Chalk putty is a soil-like material that is formed when intact chalk is disturbed. Recently, it has become of special interest due to the development of offshore wind farms in the North and Baltic Seas, where in a number of sites piles are driven into chalk strata. Several pile tests reported in the literature show increased shaft capacity up to 4 months after driving. Although it is a widespread remark in the scientific publications that chalk properties improve over time, laboratory replications of this phenomenon are scarce and the results are inconclusive. The aim of this thesis is to develop a specimen preparation technique yielding representative specimens as well as feasible for commercial purposes. Thereafter, the evolution of the characteristics of chalk putty over time is assessed through laboratory tests. Triaxial with Bender Elements, Direct Simple Shear, Constant Rate of Strain and Thixotropy have been conducted on specimens aged up to 3 months. Finding that the behavior of chalk putty cannot be classified among the classic types of soils. On one hand, Constant Rate of Strain tests have shown that the response is comparable to silty soil, with a gradual transition between re-loading and virgin compression lines, leading to difficulty in the determination of the yield point. On the other hand, in shear, the critical state friction angle is in the range of dense sand. Regarding the volumetric response, there is an initial compressive response, succeeded by a constant volume phase during destructuration and eventual dilation or contraction at larger strains. Furthermore, it was determined that the material possesses thixotropic properties when mixed with de-ionized water. Finally, no increase in the shear strength or was recorded while the initial shear stiffness decreases with aging time -due to unconfined preservation of the samples. However, the ultimate dilatancy of the specimens increases over time, this may be a result of dissolved calcium carbonate during crushing and saturation which re-precipitates or re-cements around the grains.

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

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

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

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

Four of the load situations could be qualitatively analysed with the two numerical models .The enclosure of water in the heart of the frozen cylinder could not be simulated with the numerical models. On the other hand, soil stresses due to frost heave and excavation gave a good quantitative measure. In this research one case is extensively investigation, therefore this research is non-statistical. Henceforward, the conclusion cannot be drawn that this quantitative measure of frost heave stresses can also be obtained for other cases. A qualitative measure of loads due to frost heave in construction with AGF can certainly be given with these numerical models. Although not all loads due to AGF could be taken into account (i.e. creep, enclosure of water in the frozen heart), one of the most important load situations (i.e. frost heave) could be quantitatively defined for the boom clay. This load situation is worth investigation in AGF projects, since stresses can become 2.5 times higher than initially measured soil stresses. At the start of the project the boom clay was given a frost-susceptibility index of negligible to low. Even with this mild index the stresses due to frost action increased significantly. This factor and index are probably not the same for other soil types. However, this study shows that such large stress increases are a real possibility during cross passage construction with AGF.

Tunnel construction projects in the western part of the Netherlands are typically built in soft, heterogeneous ground conditions with low stiffness and high groundwater tables. The tunnelling technique used is the ‘slurry shield’ method, which uses a tunnel-boring machine (TBM). The TBM consists of a steel cylinder with a cutting wheel at the front. At the face of the TBM, called bore front, a pressurised slurry is used as face support. The aim of a static tunnel face stability assessment is to determine the horizontal effective stress and pore water pressure acting at the tunnel face both during boring and a standstill. While the pore water pressure distribution along the tunnel is clear, the estimation of the horizontal effective stress distribution is difficult. At the moment many calculation models have been developed to calculate the latter, but none ever made it into a standard. The first part of this thesis consists of a comparative study between the Ruse-Vermeer (2002), Jancsecz-Steiner model (1984) and DIN4126+4085-model (2008). A theoretical background analysis has been performed for each model; by using a case study analysis, the effect of the models’ differences on the minimum support pressures has been determined. Thereafter, this theoretical analysis has shifted to a more ‘practical’ project analysis by including external aspects like project aspects, soil conditions and contract requirements. From the practical project analysis it resulted that at the entrance zones, the soil cover is limited and thus additional surcharge is required. The placement of this additional surcharge lead to consolidation in the subsoil which can subsequently influences the static face stability assessment. Within this research a model is proposed in order to incorporate a one-dimensional vertical consolidation analysis within face stability assessment. During tunnelling a continuous process of slurry penetration and excavation takes place and therefore, the interaction between the support fluid, advancement of the TBM and the soil needs to be taken into account. The combination of these two time-dependent effects can possibly lead to the build-up of excess pore pressures in front of the tunnel face which has a high influence on the minimum support pressure. The second part of this research investigates the possibility to include the time dependent effect of slurry infiltration and excavation within a face stability assessment e.g. transient face stability assessment. The last part of this research is based on the role of slurry TBM parameters on ground deformations. Due to overcutting and the tapered shape of the TBM, a joint (annulus) is present between the TBM and the soil formation. In this research it is assumed that the annulus is hydraulically connected to the face. This means that the annulus is pressurized with bentonite slurry originating from the face. Due to the slurry pressure gradient and hydraulic connection with the annulus, not at each location the exact geostatic stress can be reached and ground deformations will inevitably take place.

Four kilometers west of the Norwegian island Fedje, mercury, leaking from the U-864 wreck, has highly polluted a seabed area of approximately 40 000 m2. The bow section of the wreck is located on a stability critical slope of 15 degrees. In 2016, Van Oord used a flexible fall pipe system to install a counter fill to stabilize the wreck and simultaneously cap a portion of the contaminated area. In the future, the Norwegian government plans to cap the complete area to prevent further dispersion of mercury. This research aims to gain more knowledge about the spreading of sediments during such an operation occurring under realistic hydrodynamic conditions. A more complete understanding of the process will allow for better risk assessment of future capping operations. To this end, the unique data set gathered during the counter fill project has been analyzed.

In order to predict extreme flow events, the bottom currents are decomposed. By decomposing the erratic velocity signal, tidal (25 %) and inter-tidal residual current (38 %) components are identified and understood. However, the driving force for the remaining intra-tidal part, which contains the highest current anomaly events, cannot be identified. Evidence is found for the occurrence of internal waves providing a possible explanation. Due to a lack of data, a quantitative prediction cannot be made. Consequently, the exact maximum currents at the site are unpredictable, but stayed below 0.4 m/s during the project.

During the installation work, high turbidity clouds have been measured. The origin is investigated by analyzing the particle size distribution and the mercury concentration of sediment samples; said samples are drawn from the capping material, the installed capping layer and sediment traps placed around the wreck. The findings indicate that the clouds are caused by a loss of clean material and are not from the contaminated seabed. This is supported by modeling the dispersal of clean particles from the flexible fall pipe. The promising results regarding the use of a flexible fall pipe for capping layer installation are not only applicable for the U-864 area but also for other polluted offshore areas.