S. Muraro
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1
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.
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.
Conference paper
(2025)
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A. Rosati, D. Gaudio, H. Falepin, W. Ovalle-Villamil, S. Muraro, M.A. Cabrera
Monopiles, commonly adopted as substructures in wind farms, are typically installed via impact driving. The heavy selfweight of the monopile and the impact hammer required for installation increase the risk of pile runs. Pile runs have been reported in cases of stronger soils overlying weaker layers, as well as in heterogeneous soil deposits (e.g., chalk). However, recent experience from the field showed that none of the reasons above could satisfactorily explain the observed pile run in the presence of silty or fine sandy soils, typically referred to as transitional soils. Conversely, back analysis of the driving data revealed a high dependency of the Soil Resistance to Driving (SRD) on the pile penetration rate. This behaviour is believed to be linked to the drainage response of the transitional soils and pile driving parameters, including impact energy and blow rate. The latter may combine so that Excess Pore Water Pressures (EPWP), without dissipating sufficiently, accumulate to the extent a pile run can be triggered, due to the reduction of the available shear strength of the soil. To investigate this hypothesis, an experimental testing program was conducted using the geotechnical centrifuge. The tests, involving a model monopile driven in a natural silt sample, aimed at demonstrating that the soil conditions believed to contribute to a pile run can be replicated in the centrifuge. Preliminary results of a testing sequence of single blows suggest that the EPWP accumulated around the pile between consecutive blows is responsible for a reduction of the unit shaft resistance.
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Monopiles, commonly adopted as substructures in wind farms, are typically installed via impact driving. The heavy selfweight of the monopile and the impact hammer required for installation increase the risk of pile runs. Pile runs have been reported in cases of stronger soils overlying weaker layers, as well as in heterogeneous soil deposits (e.g., chalk). However, recent experience from the field showed that none of the reasons above could satisfactorily explain the observed pile run in the presence of silty or fine sandy soils, typically referred to as transitional soils. Conversely, back analysis of the driving data revealed a high dependency of the Soil Resistance to Driving (SRD) on the pile penetration rate. This behaviour is believed to be linked to the drainage response of the transitional soils and pile driving parameters, including impact energy and blow rate. The latter may combine so that Excess Pore Water Pressures (EPWP), without dissipating sufficiently, accumulate to the extent a pile run can be triggered, due to the reduction of the available shear strength of the soil. To investigate this hypothesis, an experimental testing program was conducted using the geotechnical centrifuge. The tests, involving a model monopile driven in a natural silt sample, aimed at demonstrating that the soil conditions believed to contribute to a pile run can be replicated in the centrifuge. Preliminary results of a testing sequence of single blows suggest that the EPWP accumulated around the pile between consecutive blows is responsible for a reduction of the unit shaft resistance.
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.
Widely graded coarse soils are widely distributed in the mountainous regions of China and are extensively utilised in engineering construction. The volume change of these materials upon intense seasonal wetting and drying cycles presents challenges for the long-term performance of the engineering structures. In this study, the water migration and deformation behaviour of widely graded coarse soils are examined over multiple cycles of drying and wetting. Laboratory tests are integrated with nuclear magnetic resonance and scanning electron microscopy. The results demonstrate that capillary and adsorption forces drive the gradual expulsion and absorption of inter-aggregate and intra-aggregate pore water during the drying–wetting cycles. These two mechanisms differentially influence the microstructure and soil water characteristic curve, soil shrinkage curve, and their hysteretic response. The water content corresponding to the transition between these two regimes is influenced by the coarse content of the soil. In the capillary regime, capillary forces generate normal stresses on the soil skeleton, inducing significant volume changes and resulting in a denser soil structure. In the adsorption regime, on the other hand, adsorption forces control the drainage of pore water within the aggregates. This causes the clay aggregates to shrink without significant deformation of the soil skeleton, the latter being influenced by the coarse content. The hysteresis effect is notably more pronounced in the capillary regime.
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Widely graded coarse soils are widely distributed in the mountainous regions of China and are extensively utilised in engineering construction. The volume change of these materials upon intense seasonal wetting and drying cycles presents challenges for the long-term performance of the engineering structures. In this study, the water migration and deformation behaviour of widely graded coarse soils are examined over multiple cycles of drying and wetting. Laboratory tests are integrated with nuclear magnetic resonance and scanning electron microscopy. The results demonstrate that capillary and adsorption forces drive the gradual expulsion and absorption of inter-aggregate and intra-aggregate pore water during the drying–wetting cycles. These two mechanisms differentially influence the microstructure and soil water characteristic curve, soil shrinkage curve, and their hysteretic response. The water content corresponding to the transition between these two regimes is influenced by the coarse content of the soil. In the capillary regime, capillary forces generate normal stresses on the soil skeleton, inducing significant volume changes and resulting in a denser soil structure. In the adsorption regime, on the other hand, adsorption forces control the drainage of pore water within the aggregates. This causes the clay aggregates to shrink without significant deformation of the soil skeleton, the latter being influenced by the coarse content. The hysteresis effect is notably more pronounced in the capillary regime.
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.
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.
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.
Drying of silty soil treated with superabsorbent hydrogels
Retention behaviour and cracking
Desiccation cracks in soils pose risks to the serviceability and safety of geotechnical infrastructure worldwide. This paper aims to investigate the potential of superabsorbent hydrogels (SAH) as innovative soil amendment to mitigate soil drying effects and cracking. Laboratory tests were conducted on an initially saturated silty soil treated with different types and dosages of SAH. Desiccation cracking tests, shrinkage tests, and water retention tests were performed to analyse the cracking process, evaporation rate, and retention properties. The tests were integrated with micro-CT scan analyses to observe changes in soil fabric due to the SAH addition. The results indicate that SAH particles serve as internal water reservoirs, extending the normal shrinkage stage and maintaining higher suctions without significant desaturation, in comparison to untreated soil. The addition of SAH reduces the evaporation rate, particularly at a dosage of 0.1%. The progression of cracking occurs at suctions below the air entry value, and the inclusion of SAH reduces the rate of crack development. These findings highlight the need for additional research on SAH as a promising soil treatment for geotechnical applications.
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Desiccation cracks in soils pose risks to the serviceability and safety of geotechnical infrastructure worldwide. This paper aims to investigate the potential of superabsorbent hydrogels (SAH) as innovative soil amendment to mitigate soil drying effects and cracking. Laboratory tests were conducted on an initially saturated silty soil treated with different types and dosages of SAH. Desiccation cracking tests, shrinkage tests, and water retention tests were performed to analyse the cracking process, evaporation rate, and retention properties. The tests were integrated with micro-CT scan analyses to observe changes in soil fabric due to the SAH addition. The results indicate that SAH particles serve as internal water reservoirs, extending the normal shrinkage stage and maintaining higher suctions without significant desaturation, in comparison to untreated soil. The addition of SAH reduces the evaporation rate, particularly at a dosage of 0.1%. The progression of cracking occurs at suctions below the air entry value, and the inclusion of SAH reduces the rate of crack development. These findings highlight the need for additional research on SAH as a promising soil treatment for geotechnical applications.
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.
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.
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.
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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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.
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.
Peatlands have been recognised to provide a natural carbon sink thanks to waterlogged conditions, which keep summertime temperatures relatively low, increase their water holding capacity, decrease the organic soil decomposition rate by creating anoxic conditions and eventually keeping high water table. However, unfavourable environmental conditions due to increasing temperatures and more frequent droughts will reduce water retention of peats and the summertime insulation, in turn increasing their temperature sensitivity and their decomposition rate. As a result, peatlands may start inverting their positive cycle and emitting greenhouse gases, including CO2 and CH4, which suggests better investigating how increasing climate stresses will affect the efficiency of peats in the greenhouse gases cycle and CO2 sequestration. Some evidence of gas production from increasing decomposition rate in the Netherlands is coming from continuous pore pressure measurements in saturated layers below the water table, which are monitored to assess the safety of the water defence and the transportation infrastructures. Increasing water pressure in closed piezometers compared to vented ones seem to suggest that gas is produced and capped in the ground, until the breakthrough pressure is reached and the gas vents from cracks opened in the soil matrix. Besides the environmental issues, increasing gas production from decomposition is becoming of concern for the stability of embankments made of organic soils, where the effective stress may be lowered to such an extent to endanger their stability. As a matter of fact, in the last ten years, gas overpressure has been claimed to be a triggering or a contributing factor in few small failures experienced by regional dykes in the Netherlands. In spite of the evidence and the risk increasing with heat waves and drought events, the role of gas on the coupled hydromechanical response of organic soils has been seldom investigated nor properly understood yet. In the section of Geoengineering at TU Delft, a research effort has been undertaken in the last years to investigate in depth the role of gas formation and venting on the coupled hydro-mechanical response of organic layers in the subsoil of water defence embankments. Preliminary laboratory tests performed on peats to fill this gap showed the role of increasing gas content on their compressibility and on the mobilised shear strength at given strains. The volumetric response of peats including gas was tentatively interpreted with a simple non-linear elastic model, which proved able to model the experimental results. A similar model was used to numerically investigate the relevance of gas production and venting on the response of a regional dyke in the Netherlands, where gas bubbles from venting were observed after excavating - unloading - the toe of the dyke during a stress test. Fully coupled three-phases hydromechanical numerical analyses were performed with CODE_Bright to include gas overpressure. A gas content of 6% in volume was artificially generated in the peat layer, capped by a clay layer on top, and let reaching an equilibrium distribution, which depends on the stress-strain response of the different layers and their volumetric compressibility. Gas venting is triggered by simulating excavation at the toe of the dyke, which allows gas escaping after the capping clay is removed. The variation in the operative stress, on which stiffness and strength are assumed to depend, is shown in the figure over gas generation and venting. In spite of the small amount of gas generated, the predicted overpressure is enough to bring the operative stress to zero in the upper meter of soil at the toe of the embankment due to the light weight of peat and cover soil, which temporarily reduces the factor of safety of the water defence against global stability. As soon as the gas overpressure is released, the operative stress increases above the effective stress which would characterise saturated conditions, bringing the system back to safer conditions. These preliminary analyses are supporting an undergoing experimental and numerical thorough effort to better quantify the dynamics of gas generation and venting in organic soils to reduce the hazard associated with increasing climatic stresses.
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Peatlands have been recognised to provide a natural carbon sink thanks to waterlogged conditions, which keep summertime temperatures relatively low, increase their water holding capacity, decrease the organic soil decomposition rate by creating anoxic conditions and eventually keeping high water table. However, unfavourable environmental conditions due to increasing temperatures and more frequent droughts will reduce water retention of peats and the summertime insulation, in turn increasing their temperature sensitivity and their decomposition rate. As a result, peatlands may start inverting their positive cycle and emitting greenhouse gases, including CO2 and CH4, which suggests better investigating how increasing climate stresses will affect the efficiency of peats in the greenhouse gases cycle and CO2 sequestration. Some evidence of gas production from increasing decomposition rate in the Netherlands is coming from continuous pore pressure measurements in saturated layers below the water table, which are monitored to assess the safety of the water defence and the transportation infrastructures. Increasing water pressure in closed piezometers compared to vented ones seem to suggest that gas is produced and capped in the ground, until the breakthrough pressure is reached and the gas vents from cracks opened in the soil matrix. Besides the environmental issues, increasing gas production from decomposition is becoming of concern for the stability of embankments made of organic soils, where the effective stress may be lowered to such an extent to endanger their stability. As a matter of fact, in the last ten years, gas overpressure has been claimed to be a triggering or a contributing factor in few small failures experienced by regional dykes in the Netherlands. In spite of the evidence and the risk increasing with heat waves and drought events, the role of gas on the coupled hydromechanical response of organic soils has been seldom investigated nor properly understood yet. In the section of Geoengineering at TU Delft, a research effort has been undertaken in the last years to investigate in depth the role of gas formation and venting on the coupled hydro-mechanical response of organic layers in the subsoil of water defence embankments. Preliminary laboratory tests performed on peats to fill this gap showed the role of increasing gas content on their compressibility and on the mobilised shear strength at given strains. The volumetric response of peats including gas was tentatively interpreted with a simple non-linear elastic model, which proved able to model the experimental results. A similar model was used to numerically investigate the relevance of gas production and venting on the response of a regional dyke in the Netherlands, where gas bubbles from venting were observed after excavating - unloading - the toe of the dyke during a stress test. Fully coupled three-phases hydromechanical numerical analyses were performed with CODE_Bright to include gas overpressure. A gas content of 6% in volume was artificially generated in the peat layer, capped by a clay layer on top, and let reaching an equilibrium distribution, which depends on the stress-strain response of the different layers and their volumetric compressibility. Gas venting is triggered by simulating excavation at the toe of the dyke, which allows gas escaping after the capping clay is removed. The variation in the operative stress, on which stiffness and strength are assumed to depend, is shown in the figure over gas generation and venting. In spite of the small amount of gas generated, the predicted overpressure is enough to bring the operative stress to zero in the upper meter of soil at the toe of the embankment due to the light weight of peat and cover soil, which temporarily reduces the factor of safety of the water defence against global stability. As soon as the gas overpressure is released, the operative stress increases above the effective stress which would characterise saturated conditions, bringing the system back to safer conditions. These preliminary analyses are supporting an undergoing experimental and numerical thorough effort to better quantify the dynamics of gas generation and venting in organic soils to reduce the hazard associated with increasing climatic stresses.
Experimental determination of the shear strength of peat from standard undrained triaxial tests
Correcting for the effects of end restraint
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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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.
Coupled Hydro-Mechanical Analysis of the Pre-failure and the Failure Behaviour of a Dyke on Soft Subsoil
Formulation and Synthesis of Results
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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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.
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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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.
Journal article
(2020)
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Stefano Muraro, Cristina Jommi
The paper assesses fully coupled hydro-mechanical numerical approaches developed for unsaturated soils to model the effect of free gas overpressure on the response of peat layers. A simple linear model is used for the soil skeleton, however, the global response is non-linear due to changes over time of the compressibility of the solid skeleton over the compressibility of the fluid, and solubility of gas in water. The overpressure generated in foundation peat layers by barometric pressure oscillations is modelled, and the results are compared to literature data. The development of pore overpressure upon unloading is analysed as a function of the soil skeleton compressibility, and the consequences on the average stress acting on the soil skeleton are discussed.
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The paper assesses fully coupled hydro-mechanical numerical approaches developed for unsaturated soils to model the effect of free gas overpressure on the response of peat layers. A simple linear model is used for the soil skeleton, however, the global response is non-linear due to changes over time of the compressibility of the solid skeleton over the compressibility of the fluid, and solubility of gas in water. The overpressure generated in foundation peat layers by barometric pressure oscillations is modelled, and the results are compared to literature data. The development of pore overpressure upon unloading is analysed as a function of the soil skeleton compressibility, and the consequences on the average stress acting on the soil skeleton are discussed.