CJ
C. Jommi
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59 records found
1
Active MASW for railway ballast characterization in a metro tunnel
A field study in the Milano Metro Line
Reliable estimates of the small-strain stiffness of railway ballast are essential for modeling train-induced vibration transmission and supporting condition assessment in underground railways. This paper presents a Multichannel Analysis of Surface Waves campaign performed inside a tunnel of Milan Metro Line M1, using a controlled impact source, a setting not yet discussed in the literature. A short, densely sampled receiver array was installed along the track, Rayleigh-wave dispersion was obtained in the frequency-phase velocity domain using a phase-shift approach, and a 1-D layered shear-wave velocity Vs profile was obtained by inversion. A key methodological insight derives from repeating the survey under two track boundary conditions: fastened and unfastened rails. Comparison of dispersion images and inverted profiles shows that the fastened configuration yields higher phase velocities and Vs values, consistent with stronger rail-sleeper coupling and rail-guided energy, which can bias interpretation of ballast properties. Conversely, unfastening the rails suppresses these effects and produces lower misfits and Vs profiles more representative of the ballast-invert-subgrade system. Three progressively constrained inversion parameterizations were tested to address non-uniqueness and robustness of the obtained profiles. The results confirm that Rayleigh-wave dispersion is primarily controlled by Vs, whereas other characteristics remain weakly correlated. Finally, in-situ stiffness trends of ballast are benchmarked against a laboratory dataset on comparable materials, supporting the plausibility of the velocity profiles obtained. Overall, the study demonstrates the feasibility of active MASW in a tunnel environment and delivers an operational workflow to obtain ballast-scale Vs profiles, while highlighting the importance of controlling track boundary conditions.
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Reliable estimates of the small-strain stiffness of railway ballast are essential for modeling train-induced vibration transmission and supporting condition assessment in underground railways. This paper presents a Multichannel Analysis of Surface Waves campaign performed inside a tunnel of Milan Metro Line M1, using a controlled impact source, a setting not yet discussed in the literature. A short, densely sampled receiver array was installed along the track, Rayleigh-wave dispersion was obtained in the frequency-phase velocity domain using a phase-shift approach, and a 1-D layered shear-wave velocity Vs profile was obtained by inversion. A key methodological insight derives from repeating the survey under two track boundary conditions: fastened and unfastened rails. Comparison of dispersion images and inverted profiles shows that the fastened configuration yields higher phase velocities and Vs values, consistent with stronger rail-sleeper coupling and rail-guided energy, which can bias interpretation of ballast properties. Conversely, unfastening the rails suppresses these effects and produces lower misfits and Vs profiles more representative of the ballast-invert-subgrade system. Three progressively constrained inversion parameterizations were tested to address non-uniqueness and robustness of the obtained profiles. The results confirm that Rayleigh-wave dispersion is primarily controlled by Vs, whereas other characteristics remain weakly correlated. Finally, in-situ stiffness trends of ballast are benchmarked against a laboratory dataset on comparable materials, supporting the plausibility of the velocity profiles obtained. Overall, the study demonstrates the feasibility of active MASW in a tunnel environment and delivers an operational workflow to obtain ballast-scale Vs profiles, while highlighting the importance of controlling track boundary conditions.
Corrosion-induced thickness loss in steel quay walls
Insights from long-term exposure measurements
Journal article
(2026)
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Federico Montali, Alfred Roubos, Marc Wormmeester, Kenneth Gavin, Cristina Jommi, Luca Flessati
Maritime ports are key components of global logistics networks, with steel quay walls providing berthing capacity and operational continuity. Their long-term structural performance is governed by corrosion driven by interactions between salinity, hydrodynamics, microbiological activity, and climatic conditions. Given that across Europe, many twentieth-century structures have exceeded their design life, reassessment of safety and residual capacity is essential. Conventional assessments typically use deterministic, uniform corrosion profiles based on simplified environmental classifications. In practice, however, field data show that corrosion is spatially variable, has short correlation lengths, and involves co-existing uniform and localised mechanisms. The scarcity of long-term, spatially detailed measurements has limited of site-specific deterioration models to be validated and included in design codes. This study analyses corrosion in steel quay walls at the Port of Rotterdam using ultrasonic thickness measurements and laboratory surface-morphology data. The database quantifies mean wall-thickness loss and spatial variability, enabling systematic comparison with design prescriptions. To interpret the observed variability, the study develops a stochastic corrosion representation based on random-fields, allowing explicit incorporation of spatial heterogeneity into structural assessments. The outcomes highlight the limitations of uniform corrosion assumptions and provide a basis for improved reliability evaluations and lifecycle-management strategies for ageing port infrastructure.
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Maritime ports are key components of global logistics networks, with steel quay walls providing berthing capacity and operational continuity. Their long-term structural performance is governed by corrosion driven by interactions between salinity, hydrodynamics, microbiological activity, and climatic conditions. Given that across Europe, many twentieth-century structures have exceeded their design life, reassessment of safety and residual capacity is essential. Conventional assessments typically use deterministic, uniform corrosion profiles based on simplified environmental classifications. In practice, however, field data show that corrosion is spatially variable, has short correlation lengths, and involves co-existing uniform and localised mechanisms. The scarcity of long-term, spatially detailed measurements has limited of site-specific deterioration models to be validated and included in design codes. This study analyses corrosion in steel quay walls at the Port of Rotterdam using ultrasonic thickness measurements and laboratory surface-morphology data. The database quantifies mean wall-thickness loss and spatial variability, enabling systematic comparison with design prescriptions. To interpret the observed variability, the study develops a stochastic corrosion representation based on random-fields, allowing explicit incorporation of spatial heterogeneity into structural assessments. The outcomes highlight the limitations of uniform corrosion assumptions and provide a basis for improved reliability evaluations and lifecycle-management strategies for ageing port infrastructure.
Peat is a highly organic and fibrous soil that often presents significant challenges in geotechnical engineering due to its unconventional high compressibility, shearing resistance and anisotropy. While there is empirical evidence about the role of fibres, a mechanistic model that systematically explains their contribution to the response of the material is lacking. This study presents an experimental and numerical methodology to investigate the reinforcing role of fibres on the mechanical response of peat. An experimental campaign characterised the geometric and mechanical properties of individual peat fibres, highlighting size-dependent variability in tensile strength and stiffness that was modelled with a stochastic approach developed for fracture mechanics. Dynamic image analysis provided a detailed understanding of fibre size distributions, and a novel function was proposed to flexibly model fibre orientations in three dimensions. These findings informed the development of a numerical framework which incorporates large-strain kinematics to examine the effects of fibre reorientation and volumetric changes during material deformation. The results highlight the importance of fibre kinematics in shaping the stress-strain behaviour of peat and offer a framework for further exploration of the role of fibres in soft organic soils. The numerical results compared with laboratory data highlight that fibre reinforcement during shearing depends strongly on the previous strain history and the alignment between fibre orientation and the loading direction.
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Peat is a highly organic and fibrous soil that often presents significant challenges in geotechnical engineering due to its unconventional high compressibility, shearing resistance and anisotropy. While there is empirical evidence about the role of fibres, a mechanistic model that systematically explains their contribution to the response of the material is lacking. This study presents an experimental and numerical methodology to investigate the reinforcing role of fibres on the mechanical response of peat. An experimental campaign characterised the geometric and mechanical properties of individual peat fibres, highlighting size-dependent variability in tensile strength and stiffness that was modelled with a stochastic approach developed for fracture mechanics. Dynamic image analysis provided a detailed understanding of fibre size distributions, and a novel function was proposed to flexibly model fibre orientations in three dimensions. These findings informed the development of a numerical framework which incorporates large-strain kinematics to examine the effects of fibre reorientation and volumetric changes during material deformation. The results highlight the importance of fibre kinematics in shaping the stress-strain behaviour of peat and offer a framework for further exploration of the role of fibres in soft organic soils. The numerical results compared with laboratory data highlight that fibre reinforcement during shearing depends strongly on the previous strain history and the alignment between fibre orientation and the loading direction.
The engineering response of soft organic clays is controlled by anisotropy, stress history and the nature of organic matter. The behaviour of these soils has been investigated extensively over compression triaxial paths, and models are available to successfully reproduce available experimental observations. However, open questions remain about the response over stress paths other than compression. In this study, an organic diatomaceous clay from the Netherlands was subjected to an extensive experimental programme, which included monotonic and non-monotonic axis-symmetric stress paths in both compression and extension. The comprehensive study introduces a new dataset to support the development and calibration of constitutive approaches. The collected experimental data revealed some limitations in current elastic–plastic models, which were addressed by introducing greater flexibility in the shape of the yield function and enhancing previous rotational hardening rules. The new model, named JMC-clay, is assessed and validated over a variety of stress paths. The comparison between experimental data and numerical simulations demonstrates the ability of the model to accurately describe the pre-failure behaviour. The findings emphasise that the model performance is particularly sensitive to elastic–plastic compressibility more than any other parameter. It is suggested that the true bottleneck in the practical implementation of this class of anisotropic formulations is their accurate initialisation, rather than calibration.
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The engineering response of soft organic clays is controlled by anisotropy, stress history and the nature of organic matter. The behaviour of these soils has been investigated extensively over compression triaxial paths, and models are available to successfully reproduce available experimental observations. However, open questions remain about the response over stress paths other than compression. In this study, an organic diatomaceous clay from the Netherlands was subjected to an extensive experimental programme, which included monotonic and non-monotonic axis-symmetric stress paths in both compression and extension. The comprehensive study introduces a new dataset to support the development and calibration of constitutive approaches. The collected experimental data revealed some limitations in current elastic–plastic models, which were addressed by introducing greater flexibility in the shape of the yield function and enhancing previous rotational hardening rules. The new model, named JMC-clay, is assessed and validated over a variety of stress paths. The comparison between experimental data and numerical simulations demonstrates the ability of the model to accurately describe the pre-failure behaviour. The findings emphasise that the model performance is particularly sensitive to elastic–plastic compressibility more than any other parameter. It is suggested that the true bottleneck in the practical implementation of this class of anisotropic formulations is their accurate initialisation, rather than calibration.
Interpretation of element testing in soil mechanics can be enhanced to a large extent with the use of local pressure measurements, helping in quantifying the consequences of non-uniform stress, strain, and pore pressure fields within the sample. Available diaphragm pressure transducers can be useful to this aim; however, they suffer from several limitations. A novel Fabry–Pérot fibre-optic sensor for local measurement of pore water pressure within the sample is presented and discussed. The sensor addresses several limitations of current mid-plane diaphragm transducers, including long-term drifting, temperature sensitivity, and maintenance difficulties. The new sensor offers significant advantages in terms of reduced sample disturbance, data acquisition frequency, and response time.
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Interpretation of element testing in soil mechanics can be enhanced to a large extent with the use of local pressure measurements, helping in quantifying the consequences of non-uniform stress, strain, and pore pressure fields within the sample. Available diaphragm pressure transducers can be useful to this aim; however, they suffer from several limitations. A novel Fabry–Pérot fibre-optic sensor for local measurement of pore water pressure within the sample is presented and discussed. The sensor addresses several limitations of current mid-plane diaphragm transducers, including long-term drifting, temperature sensitivity, and maintenance difficulties. The new sensor offers significant advantages in terms of reduced sample disturbance, data acquisition frequency, and response time.
A FEM-DEM model for immersed granular flows
From unresolved to semi-resolved scales
Journal article
(2025)
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Michel Henry, Laura Valentina Cote Martinez, Cristina Jommi, Michael Hicks, Jonathan Lambrechts, Vincent Legat, Miguel Cabrera
This work presents a coupling between the Finite Element Method (FEM) and the Discrete Element Method (DEM) to simulate immersed granular flows, transitioning from an unresolved to a semi-resolved representation. This refers to fluid discretisations ranging from a scale much larger than the grain size to one comparable to the grain size. The fluid phase is modelled using the Volume-Averaged Navier-Stokes (VANS) equations, which are solved with the FEM, while the granular phase is represented by the DEM with Non-Smooth Contact Dynamics (NSCD). An overlap-wise spatial coupling is proposed to bridge the gap between unresolved and semi-resolved scales. The method is validated by numerically reproducing experimental work on the fluidisation of a sand layer. Accuracy and stability are assessed by varying the mesh size down to half the grain diameter.
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This work presents a coupling between the Finite Element Method (FEM) and the Discrete Element Method (DEM) to simulate immersed granular flows, transitioning from an unresolved to a semi-resolved representation. This refers to fluid discretisations ranging from a scale much larger than the grain size to one comparable to the grain size. The fluid phase is modelled using the Volume-Averaged Navier-Stokes (VANS) equations, which are solved with the FEM, while the granular phase is represented by the DEM with Non-Smooth Contact Dynamics (NSCD). An overlap-wise spatial coupling is proposed to bridge the gap between unresolved and semi-resolved scales. The method is validated by numerically reproducing experimental work on the fluidisation of a sand layer. Accuracy and stability are assessed by varying the mesh size down to half the grain diameter.
The presence of entrapped gas, formed by the degradation of organic matter, complicates the pore pressure measurement in gassy soils. To address this challenge, fully coupled hydro-mechanical finite element simulations are presented to analyse the pore pressure response observed from triaxial tests of gassy peat samples. The experiments incorporated novel local pore pressure transducers, able to track distinct pore pressure measurements at separate locations. By replicating the experimental tests, the numerical simulations assessed the effects of non-uniform gas concentration and soil-porous disk interactions to refine gassy soil testing procedures and improve data interpretation.
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The presence of entrapped gas, formed by the degradation of organic matter, complicates the pore pressure measurement in gassy soils. To address this challenge, fully coupled hydro-mechanical finite element simulations are presented to analyse the pore pressure response observed from triaxial tests of gassy peat samples. The experiments incorporated novel local pore pressure transducers, able to track distinct pore pressure measurements at separate locations. By replicating the experimental tests, the numerical simulations assessed the effects of non-uniform gas concentration and soil-porous disk interactions to refine gassy soil testing procedures and improve data interpretation.
Peat is a highly organic material that poses significant environmental and geotechnical engineering challenges due to its hydrological relevance and atypical mechanical behaviour. Understanding its unsaturated response is essential for infrastructure built over organic soils, particularly under increasing seasonal variability associated with increased climate stresses. Modelling the water retention behaviour of peat remains complex due to its high compressibility and the fabric rearrangements induced by drying and wetting cycles. This study presents an experimental characterisation of the shrinkage and water retention behaviour of natural and reconstituted fibrous peat from the Netherlands. A combination of high-resolution laser scanning and suction measurements was employed to monitor volume change and water retention throughout drying. The results are interpreted through a framework that distinguishes between inter-and intra-ped porosities, allowing for the separation of their respective contributions to shrinkage and retention. Complementary mercury intrusion porosimetry (MIP) analyses provided insight into the evolution of pore size distribution during drying, supporting the interpretation of a sequential engagement of pore sizes. The findings underscore the importance of accounting for differential multiscale porosity evolution and fabric structure when evaluating the hydro-mechanical response of peat.
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Peat is a highly organic material that poses significant environmental and geotechnical engineering challenges due to its hydrological relevance and atypical mechanical behaviour. Understanding its unsaturated response is essential for infrastructure built over organic soils, particularly under increasing seasonal variability associated with increased climate stresses. Modelling the water retention behaviour of peat remains complex due to its high compressibility and the fabric rearrangements induced by drying and wetting cycles. This study presents an experimental characterisation of the shrinkage and water retention behaviour of natural and reconstituted fibrous peat from the Netherlands. A combination of high-resolution laser scanning and suction measurements was employed to monitor volume change and water retention throughout drying. The results are interpreted through a framework that distinguishes between inter-and intra-ped porosities, allowing for the separation of their respective contributions to shrinkage and retention. Complementary mercury intrusion porosimetry (MIP) analyses provided insight into the evolution of pore size distribution during drying, supporting the interpretation of a sequential engagement of pore sizes. The findings underscore the importance of accounting for differential multiscale porosity evolution and fabric structure when evaluating the hydro-mechanical response of peat.
Changing climatic conditions present an emerging threat to geo-structures. Climatic scenarios for the Netherlands indicate rising temperatures and larger variations in the atmospheric water balance. Consequently, geo-structures will be subjected to greater annual pore pressure variations and unprecedented stress levels. A particular concern is the impact of these changing conditions on the geotechnical performance of regional dykes, which are composed of and founded on organic soft soil layers susceptible to degradation. Given that changes in weather patterns are already observable, investigation of current in-situ soil state variations can provide valuable insight into the geotechnical response under future intensified environmental conditions. This study analyses in-situ monitoring data from a shallow-slope dyke system in the Netherlands to assess the persistence of atmospheric-driven pore pressure fluctuations in the dyke body and foundation layers. By correlating local weather conditions with soil response, the study identifies atmospheric events that trigger temporary or permanent variations in soil state, providing a guidance to address the consequences of possible future climatic events, which may compromise the geotechnical performance of soft soil dykes.
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Changing climatic conditions present an emerging threat to geo-structures. Climatic scenarios for the Netherlands indicate rising temperatures and larger variations in the atmospheric water balance. Consequently, geo-structures will be subjected to greater annual pore pressure variations and unprecedented stress levels. A particular concern is the impact of these changing conditions on the geotechnical performance of regional dykes, which are composed of and founded on organic soft soil layers susceptible to degradation. Given that changes in weather patterns are already observable, investigation of current in-situ soil state variations can provide valuable insight into the geotechnical response under future intensified environmental conditions. This study analyses in-situ monitoring data from a shallow-slope dyke system in the Netherlands to assess the persistence of atmospheric-driven pore pressure fluctuations in the dyke body and foundation layers. By correlating local weather conditions with soil response, the study identifies atmospheric events that trigger temporary or permanent variations in soil state, providing a guidance to address the consequences of possible future climatic events, which may compromise the geotechnical performance of soft soil dykes.
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.
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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.
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.
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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.
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.
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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.
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.
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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.
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.
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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.
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.
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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.
Failures that occurred in the last few decades highlighted the need to raise awareness about the emergent risk related to the impact localised degradation phenomena have on embankments. Common interventions aimed to improve embankments, such as the reconstruction of the damaged area or the injection of low-pressure grouts to fill fractures and burrows, may cause the weakening of the structure due to discontinuities between natural and treated zones. Moreover, since such repair techniques require huge volumes of materials, more sustainable solutions are encouraged. At the same time, the textile and fashion industries are looking for sustainable waste management and disposal strategies to face environmental problems concerned with the voluminous textile waste dispatched to landfills or incinerators. The use of soil mixed with textile waste in embankment improvement has been investigated to identify an effective engineering practice and to provide a strategy for the circular economy of textiles. Preliminary laboratory tests have been conducted on soil specimens collected from the Secchia River embankment, Northern Italy, to define the appropriate mixture proportions and to compare physical properties and hydro-mechanical behaviour of natural and treated soils. The results show that an appropriate fibre content offers manageable and relatively homogeneous mixtures. The indluence on soil consistency is mainly due to the textile fibre hydrophilic nature. The addition of fibres reduces the maximum dry density and increases the optimum water content. At low stress levels, the compressibility and hydraulic conductivity appear higher, however macro voids produced during sample preparation may alter the findings.
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Failures that occurred in the last few decades highlighted the need to raise awareness about the emergent risk related to the impact localised degradation phenomena have on embankments. Common interventions aimed to improve embankments, such as the reconstruction of the damaged area or the injection of low-pressure grouts to fill fractures and burrows, may cause the weakening of the structure due to discontinuities between natural and treated zones. Moreover, since such repair techniques require huge volumes of materials, more sustainable solutions are encouraged. At the same time, the textile and fashion industries are looking for sustainable waste management and disposal strategies to face environmental problems concerned with the voluminous textile waste dispatched to landfills or incinerators. The use of soil mixed with textile waste in embankment improvement has been investigated to identify an effective engineering practice and to provide a strategy for the circular economy of textiles. Preliminary laboratory tests have been conducted on soil specimens collected from the Secchia River embankment, Northern Italy, to define the appropriate mixture proportions and to compare physical properties and hydro-mechanical behaviour of natural and treated soils. The results show that an appropriate fibre content offers manageable and relatively homogeneous mixtures. The indluence on soil consistency is mainly due to the textile fibre hydrophilic nature. The addition of fibres reduces the maximum dry density and increases the optimum water content. At low stress levels, the compressibility and hydraulic conductivity appear higher, however macro voids produced during sample preparation may alter the findings.
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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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.
Conference paper
(2023)
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C. Chao, C. Jommi, S. Muraro
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.
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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.
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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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.