Advanced models for soft organic layers encountered in the shallow subsoils in the Netherlands have been developed recently at TU Delft. The models are based on high-quality laboratory data on peats and soft organic clays. The constitutive effort mostly focussed on some partially unexplored features, such as the role of fibres, extension stress conditions, and the dependence of hardening on deviatoric plastic strains, besides anisotropy. Although the models have proven to be able to reproduce and predict the behaviour over a variety of triaxial probe tests, validation at the field scale is lagging behind. On the one hand, field soil response encompasses diverse stress paths and histories not replicable in the laboratory. On the other hand, the role of the advanced features introduced in the models on the engineering structure response needs to be quantified. We back-analyse a well-documented full-scale test performed in the Netherlands, the Leendert de Boerspolder stress test, where the role of different soft soil layers both on the pre-failure and failure response has been investigated. Comparison between numerical simulations and available monitoring data is used to demonstrate the contribution of advanced models to the understanding of the engineering response of soft soils.@en

Accurate evaluation of undrained shear strength of soils is crucial in geotechnical design and assessment. In the practice, undrained shear strength is obtained most frequently from CPT data, dividing the net cone tip resistance by a cone factor, 𝑁𝑁!". For organic soils, values between 8.6 and 15.3 are reported, depending on the stress history. The cone factor can be conditioned to the results of laboratory tests, although uncertainties remain on the variety of stress paths followed by the soil elements around the tip of the cone, compared to the ones tested in the laboratory. Non-uniqueness in the definition of the cone factor may lead to either unsafe or over-conservative choices, partly undermining both the reliability and the sustainability of the design. This contribution analyses numerically the inversion technique used to determine the undrained shear strength of organic clays, exploiting data from an extensive in situ and laboratory investigation. The adopted constitutive model was calibrated on the results of laboratory tests. Cone penetration tests were simulated performing coupled hydro-mechanical numerical analyses via G-PFEM, developed in the last decade at CIMNE-UPC. The role played by initial stress state and previous stress history upon stress distribution at failure, cone factor and sleeve friction is discussed. The numerical results suggest how the sleeve friction could be used to condition the cone factor depending on the over-consolidation ratio and demonstrate how combining the different available CPT readings with the aid of numerical results may reduce the uncertainty in the estimation of undrained shear strength.@en

A large part of the Dutch regional dyke network is classified as drought-susceptible given that both the dyke body and the foundation layers consist of soft organic soils. The erratic weather conditions over recent years, which included prolonged drought, new temperature records and intensified rainfall, are linked to an increased number of accidents related to dyke degradation. As global warming continues to exacerbate extreme weather, there is a growing concern on the impact of changing climatic conditions on this type of regional dykes. Poor understanding of climate induced soil degradation processes poses a serious challenge in the development of adaptation strategies. The challenges are caused by the large variety of interplaying factors, dynamic environmental actions and the complex description of coupled degradation processes with varying spatial-temporal scales. This study demonstrates the potential use of field monitoring to overcome some of these limitations. Field monitoring data on ten Dutch regional dyke sections, with varying geometry, stratigraphy and vegetation are presented. The data provide insight into changes in dyke hydraulic state as a function of atmospheric conditions and allow to infer possible climate induced soil physical degradation mechanisms depending on dyke characteristics. To fully evaluate the impact of degradation on the water protection system, ancillary monitoring data are required, able to quantify the mechanical implications of climate induced state variations. The design of dedicated monitoring set up on three selected dykes, which will serve as representative case studies for the development of geotechnical assessment methods, is eventually presented.@en

A large part of the Dutch regional dyke network is classified as drought-susceptible given that both the dyke body and the foundation layers consist of soft organic soils. The erratic weather conditions over recent years, which included prolonged drought, new temperature records and intensified rainfall, are linked to an increased number of accidents related to dyke degradation. As global warming continues to exacerbate extreme weather, there is a growing concern on the impact of changing climatic conditions on this type of regional dykes. Poor understanding of climate induced soil degradation processes poses a serious challenge in the development of adaptation strategies. The challenges are caused by the large variety of interplaying factors, dynamic environmental actions and the complex description of coupled degradation processes with varying spatial-temporal scales. This study demonstrates the potential use of field monitoring to overcome some of these limitations. Field monitoring data on ten Dutch regional dyke sections, with varying geometry, stratigraphy and vegetation are presented. The data provide insight into changes in dyke hydraulic state as a function of atmospheric conditions and allow to infer possible climate induced soil physical degradation mechanisms depending on dyke characteristics. To fully evaluate the impact of degradation on the water protection system, ancillary monitoring data are required, able to quantify the mechanical implications of climate induced state variations. The design of dedicated monitoring set up on three selected dykes, which will serve as representative case studies for the development of geotechnical assessment methods, is eventually presented.@en

Measurements taken on a historical dike in the Netherlands over one year showed that interaction with the atmosphere led to oscillation of the piezometric surface of about 0.7 m. The observation raised concerns about the long-term performance of similar dikes and promoted a deeper investigation of the response of the cover layer to increasing climatic stresses. An experimental and numerical study was undertaken, which included an investigation in the laboratory of the unsaturated behavior of a scaled replica of the field cover. A sample extracted from the top clayey layer in the dike was subjected to eight drying and wetting cycles in a HYPROP™ device. Data recorded during the test provide an indication of the delayed response with depth during evaporation and infiltration. The measurements taken during this continuous dynamic process were simulated by means of a finite element discretization of the time-dependent coupled thermohydraulic response. The results of the numerical simulations are affected by the way in which the environmental loads are translated into numerical boundary conditions. Here, it was chosen to model drying considering only the transport of water vapor after equilibrium with the room atmosphere, while water in the liquid phase was added upon wetting. The simulation was able to reproduce the water mass balance exchange observed during four complete drying–wetting cycles, although the simulated drying rate was faster than the observed one. The numerical curves describing suction, the amount of vapor and temperature are identical, confirming that vapor generation and its equilibrium is control the hydraulic response of the material. Vapor generation and diffusion depend on temperature; therefore, correct characterization of the thermal properties of the soil is of paramount importance when dealing with evaporation and related non-steady equilibrium states.@en

Cone penetration tests, CPTs, are extensively used in the Netherlands to assess the stability of fourteen thousand kilometres of dykes protecting the country from flooding. On the regional dykes, site testing is planned and executed only from spring to autumn. The data collected in the drier season of the year must be used then in safety factor calculation for dyke stability with reference to the worst expected conditions, including the highest weights and the highest water pressures over the year. Inferring reliable values of the shear strength in a different season implies understanding the unsaturated response of the dyke material and the effect of variable water content on the CPT response. In previous studies referring to CPTs in unsaturated soils, it was observed that both the cone resistance and the sleeve friction depend on suction, however, only the cone resistance was used to determine the shear strength in combination with water content or suction probes installed into the ground. In this contribution, we analyse an extensive set of data, coming from repeated CPTs performed over one year on the Maasdijk near Oijen in the Netherlands. The data are elaborated to investigate whether the entire set of data can be exploited to try to derive the water content and the constant water content shear strength at the same time, if the test is repeated in different seasons. @en

This study characterises the effects of naturally varying organic content on the compression and shear behaviour of a marine silty-clay from the Netherlands. Index properties and mechanical properties are determined through laboratory tests, including oedometer and multistage loading-unloading triaxial stress paths. The results indicate a significant impact of the organic content on the compression response, with both the loading and reloading indexes increasing as the loss on ignition increases from 3% to 7%. Additionally, the study suggests a directional response of the compression behaviour, with the loading index increasing with the stress ratio. The influence of the organic content on shear strength appears to be less significant. No brittle response is observed during shearing, and a similar ultimate stress ratio is attained by all samples. However, a unique critical state line can only be identified for samples with similar organic content, as its intercept and slope are found to increase with increasing organic content. The experimental results from stress paths at constant stress ratio reveal an anisotropic pre-failure plastic deformation mode, which depends on the previous stress history and loading direction. This suggests that the stress–dilatancy relationship cannot be formulated as a unique function of the stress ratio. The high-quality experimental data presented in the study enlarge the database on soft organic soils in view of the development of advanced constitutive models.@en

Ground borne vibrations generated by the passage of underground trains may change over time due to objective causes, such as increasing weight and speed of trains or ageing of the infrastructure components, as well as a variation in the dynamic response of the soil surrounding the tunnel. Among the possible causes of changes in the soil dynamic response, its hydrologic state has been seldom investigated. In this contribution, the role played by the conditions of the soil above the water table is addressed, starting from a case history in the city of Milano. Two-dimensional plane strain numerical models have been developed for the infrastructure. The models were calibrated on the results of two geophysical investigations performed at the same site in the city centre, but at two different times, which allowed distinguishing different dynamic responses. The system was excited by a synthetic load time history, matching a reference dynamic load spectrum included in Italian recommendations. Limitations of using this input on a 2D plane strain model were assessed by comparing the computed vibrations with experimental acceleration records collected on the tunnel. The results of the two numerical models are compared with those of a simulation performed assuming fully dry conditions above the water table. Overall, the set of analyses shows that even small changes in the dynamic response of the soil, interpretated as a consequence of variable saturation, may result in a change of a few decibels in the acceleration levels. Much larger accelerations are predicted on average with the simpler dry model, clearly showing the advantages of a more accurate modelling strategy.

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A relevant part of the geotechnical infrastructure in the north of Europe and overseas is built on soft organic soils, including peat. Peat is extremely vulnerable to climate-related hazards as increased temperature accelerates drying, shrinkage and decomposition of the organic matter. Peat exhibits dramatic changes in volume with changes in water content. As the material deforms, the pore space evolves and changes the water retention response. The evolution of the pore space leads to a hysteretic relationship between suction, water content, and void size distribution. In this work, data from free shrinkage-swelling and suction measurements on natural fibrous peat subjected to drying and wetting cycles are presented and discussed. The water retention and shrinkage behaviour of the samples are modelled by accounting for capillarity and considering the evolution of the pore size distribution. X-Ray computer tomography was used to explore the change in the pore space upon shrinkage and drying. The experimental evidence shows that peat experiences distinct shrinkage zones including one where accelerated contraction occurs. Such behaviour is explained as a consequence of the interactions of an aggregated fabric. This is supported by the conceptual modelling approach that highlights the pivotal role of the evolving pore space.

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Monitoring structural behavior of earth structures during construction and in service is a common practice done for safety reasons, consolidation control and maintenance needs. Several are the techniques available for measuring displacements, water pressures and total stresses, not only in these geotechnical structures but also at their foundations. Materials testing has been used for calibrating models for structural design and behavior prediction, and these models can be validated with instrumentation data as well. Relatively recent investigation on the behavior of these materials considering their degree of saturation focuses on monitoring the evolution of water content or suction as function of soil-atmosphere interaction, necessary to predict cyclic and/or accumulated displacements, and has huge potential to predict the impact of climate changes on the performance of existing geotechnical structures. This new need justifies the investment on developing sensors able to be used for in situ monitoring of water in the soils, such as those presented here. Testing and monitoring becomes even more important nowadays when, for sustainability purposes, traditional construction materials are replaced by other geo-materials with unknown behavior and long-term performance (mainly accumulated displacements). Existing experimental protocols and monitoring equipment are used for such cases, however new techniques must be developed to deal with particular behaviors. Three case studies are presented and discussion is made on monitoring equipment used and how monitored data helped understanding the behaviors observed.

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Element testing of soft soils is challenging due to the large strains attained in the pre-failure range. Besides the heterogeneity of natural samples, the set-up configuration is the main driving factor for non-homogenous response. Stress, strain and pore pressure non-uniformities induced by the loading system affect the observed behaviour and complicate proper interpretation of the results. Among the difficulties encountered in the interpretation of laboratory data, the unexpected decrease of the stress ratio frequently observed on Dutch organic soft clays on the wet side of critical state is investigated by numerically back-analysing the triaxial test set-up. A 3D finite element simulation using an advanced constitutive model for soft clays developed at TU Delft was performed to clarify the nature of the response. The results indicate that a decrease in the deviatoric stress up to critical state may be interpreted as a true feature of the soil response. However, the response at large strains is very much influenced by the triaxial shear apparatus, in particular, by the rotation of the top cap which triggers geometrical instability. Practical recommendations are given to limit the effects of the set-up configuration on the determination of the undrained shear strength to be used for field applications.@en

Increasing frequency and intensity of extreme weather events in the Netherlands is raising attention on the unsaturated response of geo-infrastructures, promoting research projects to provide an overview of the impact of unsaturated conditions on the response of shallow soil layers and embankments, and to better address maintenance and mitigation measures. As part of this effort, we discuss the results of standard laboratory tests performed on initially unsaturated samples retrieved from the field and tested in natural conditions, as well as after controlled drying and wetting. The variation of the "undrained"(i.e. at constant water content) shear strength with the degree of saturation obtained from the laboratory tests aligns well with CPT measurements performed in the field. An elastic-plastic constitutive model with mixed isotropic-rotational hardening developed for saturated soft soils was extended to unsaturated conditions by following a robust approach previously developed for compacted clayey soils. Coupling between the mechanical and the hydraulic behaviour is provided by the water retention curve. The model nicely captures the response observed in the laboratory, until extreme dry conditions, which possibly alter the structure of the soil, the peak stress, and the brittleness after failure. The model is capable of reproducing the effects of the previous hydraulic history on the stress-strain behaviour observed from the laboratory tests over a wide range of degree of saturation.

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Peats are encountered in waterlogged deltaic areas, where degradation is delayed by favourable environmental conditions. The recent increase in frequency and severity of droughts is expected to accelerate peat degradation, in turn increasing subsidence and flood risk, urging better understanding of the response of peats to drying events. To this aim, compression tests on natural and reconstituted peat samples were performed, supported by X-ray micro-computed tomography. The peat fabric was found to be the key factor in the response to drying, with fibres playing the most significant role. Drying in peats starts affecting the macro-fabric, with an irreversible reduction in volume and disruption of the fibrous network occurring under saturated conditions until a threshold void ratio is reached, below which desaturation occurs of the intra-fibres and intra-peds pores. The first drying stage dramatically decreases the compressibility, while the hydraulic conductivity is hardly affected due to the enlargement of macropores. Secondary compressibility is affected by the peat fabric besides the organic content. The total organic content does not change substantially during drying; hence, it is not the best proxy to describe the consequences of drying on the response of fibrous peats. The fibre content can be better used to serve the aim.

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A recent increase in frequency and severity of exceptional climatic events is of concern for the stability of natural and artificial slopes. These undergo continuous evaporation and infiltration cycles, which change the suction distribution and trigger shrinkage, swelling, cracking, and surfi-cial erosion, overall decreasing the soil strength. To assess the impact of these climatic stresses, the determination of water retention properties is a priority. Although advanced techniques have been proposed in the last few decades to this end, simpler commercially available techniques allow col-lecting data for a larger number of samples in a shorter time, thus enabling a basic description of the water retention properties for a larger database of soils. Data on two silty sands, coming from very different climatic environments in Europe, were collected with a combination of two simple commercial devices, and the results were modelled with a van Genuchten’s law. The fitted pa-rameters were found to correlate well with the amount of fines, irrespective of the different origin and composition of the two soils. Eventually, the limitation of the approach is discussed based on the results of cyclic drying–wetting tests.

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Recent research effort carried out at Delft University of Technology to improve the experimental knowledge and develop a comprehensive modelling approach for fibrous organic soils is summarised. Experimental results and numerical analyses are combined to discuss some contradictory results which have delayed advanced characterisation of peats. Part of the apparent inconsistencies commonly found in the literature is due to the influence of the testing apparatus, including rough platens and membrane restraint, which inhibit homogenous deformation modes and alter the response of the samples compared to the true material behaviour. The consequences of non-homogenous deformation are particularly relevant on peats due to the unique combination of their exceptionally low stiffness and high strength. An elastic–plastic constitutive framework was developed starting from repeatable reconstituted samples of peats, taking care of reducing end restraint to a large extent in the experimental setup. The results suggested that an elastic–plastic model for peats should include a non-associated flow rule and a mixed volumetric–deviatoric hardening law. The role played by different fibres at the laboratory scale is discussed, and the additional reinforcement offered by bigger fibres on the observed behaviour of natural peats is addressed.

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Conventional triaxial tests on peats are strongly criticised due to the very high shear strength parameters obtained from standard data elaboration, leading to unrealistic factors of safety when used in geotechnical design and assessment. Various operational approaches have been proposed in the literature to overcome this difficulty; however, they seem to lack consistent mechanical background. Some of the issues related to the shear strength evaluation of peats from triaxial tests come from the non-uniform stress and strain states developing in the samples well before failure is attained, due to end restraint effects. Undrained triaxial compression tests were performed on reconstituted peat to examine the influence of end restraint on the deviatoric stress, excess pore pressure and deviatoric strain response. Samples were tested with standard rough end platens and with modified platens to reduce the friction between the sample and bottom and top caps. Four different initial height-to-diameter ratios were examined, to reduce the consequences of rough end platens on the sample response. The results indicate that end restraint contributes dramatically to overestimating the shear strength of peat, due to the increase in both the calculated deviatoric stress and the measured excess pore pressure at the bottom of the sample. Suggestions are given to quantify the influence of end restraint in the interpretation of standard data, in an attempt to suggest viable procedures to determine more reliable effective and undrained shear strength parameters from standard triaxial tests.

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In an attempt to evaluate current models for the safety assessment of dykes on soft soils, STOWA, the foundation for research on regional dykes in the Netherlands, launched and supported a full scale test on a regional historical dyke, which included observation of the pre-failure response and the design of its failure. The stress test on the dyke included progressive excavation at the toe and rapid drawdown in the ditch next to the toe of the embankment, until failure eventually occurred. The data and the observations collected on site during the test are a unique body of information on the coupled hydro-mechanical pre-failure behaviour and on the resistance of the earth construction. A selection of these data was included in the formulation of the Theme C of the 15th International Benchmark Workshop on Numerical Analysis of Dams, held in September 2019 in Milano, Italy. This contribution presents the main outcomes of the numerical benchmark, coming from the results of the different groups, which analysed the case with current geotechnical constitutive and numerical models.

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A simple numerical model was set up to investigate the hydraulic behaviour of a regional dyke to improve understanding of the response under variable atmospheric conditions. The unsaturated hydraulic properties of the dyke body and the relevant foundation layers were calibrated either on the results of laboratory tests or on a national database, namely the Staringreeks, compiled for typical Dutch soils. The boundary conditions were imposed according to the weather history at the top, and to the pore pressures measured in the field at the bottom of the representative soil column. The results indicate that a simple 1D model is able to accurately reproduce the suction time history in the dyke core, provided the hydraulic conductivity and soil water retention properties are properly calibrated. The optimised hydraulic conductivities are typically two orders of magnitude higher than the saturated hydraulic conductivity from the laboratory tests, but comparable to the ones suggested in the database developed on field data. The work highlights that cautious evaluation of laboratory data is needed for field applications, and that direct information from the field should be used to validate numerical models in the presence of organic soils.

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This paper discusses the results of an experimental programme designed to investigate the deviatoric behaviour of peats. The results are obtained from triaxial experiments carried out on reconstituted peat samples. The interpretation of the experimental results follows a hierarchical approach in an attempt to derive the ingredients that an elastic–plastic model for peats should contain, including the yield locus, the hardening mechanism and the flow rule. The results obtained from stress tests along different loading directions show that purely volumetric hardening is not adequate to describe the deviatoric response of peat and that a deviatoric strain-dependent component should be included. The plastic deformation mechanism also depends on the previous stress history experienced by the sample. Stress and strain path dependence of the interaction mechanisms between the peat matrix and the fibres is discussed as a possible physical reason for the observed behaviour. This work offers a relevant set of data and information to guide the rational development and the calibration of constitutive laws able to model the deviatoric behaviour of peats.

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Hindcasting of past levee failures enhances insights in the performance and vulnerability of levees. The scarcity of field evidence makes identifying the cause(s) of failure difficult. Under these circumstances, multiple scenarios and model choices are possible to characterise and to model the failure. This paper shows how probabilistic Bayesian techniques advance the procedure of hindcasting of levee failures. In the developed approach, a-priori levee information, and failure observations are systematically taken into account to determine the most likely scenario and the most representative model choices to characterise the failure most accurately. Observations, such as the slip surface, are taken into account in the probability estimates. The levee failure near Breitenhagen, Germany (2013) is used as a case study. The levee failed during river floods due the instability of the landside slope. The levee failure was most likely triggered by locally weak soil conditions and unexpected high water pressures due a connection between a pond on the riverside of the levee and the aquifer. These conditions were likely caused by the occurrence of a previous breach at this location. The approach developed in this paper is expected to support a more systematic and objective method of analysis of other levee failures.

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