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C. Chao
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1
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.
...
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.
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.
...
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.
A comprehensive journey on dutch organic clays
From element testing through constitutive modelling towards a novel cyclic multidirectional shear device
In the Netherlands, natural gas extraction from the Groningen field has induced seismic activities, notably recording the highest magnitude of ML 3.6 at Huizinge. Despite the lower magnitudes of these induced earthquakes compared to their natural counterparts, their impact is significantly magnified due to the shallow depths at which they occur and the high site amplification over areas with soft soils. In response, the Dutch Research Council (NWO) launched the DeepNL research programme to conduct thorough scientific research into these phenomena. The SOFTTOP project, part of this initiative, aims to fill the knowledge gaps regarding the effects of soft soil deposits on earthquake dynamics and subsequent soil responses. This thesis, nested within the SOFTTOP project, focuses on the cyclic and dynamic behaviours of soft organic clays, which are prevalent in the Netherlands' deltaic subsurface. It starts with a detailed investigation into the monotonic behaviours of these clays, laying the groundwork for further exploration into their cyclic and rate-dependent responses.
Through an extensive experimental programme, a characteristic organic diatomaceous clay from the Netherlands was tested under various stress paths, uncovering limitations in existing models. To address these, an advanced elastoplastic model, JMC-clay, was developed. Starting from a previous formulation for peats, modifications were made to the yield surface shape and the rotational hardening rule, enhancing the models' ability to predict pre-failure behaviours accurately. This foundational work paves the way for studying cyclic behaviour under ‘slow’ testing conditions, deliberately chosen to minimise the effects of non-uniform pore pressure distribution. The study's exploration into the slow cyclic response of Dutch organic clay has unearthed critical insights into the effects of loading frequency, soil composition, initial stress state, and cyclic stress amplitude on the soil's mechanical properties. Among the findings, the soil's cyclic response significantly depends on the cyclic strain amplitude.
A pivotal aspect of this thesis is extending the JMC-clay model to include cyclic behaviour, incorporating bounding surface plasticity for a more accurate and predictive modelling framework of soil behaviour under cyclic loading conditions. This development, however, brought to light challenges in model validation. Simulations indicated that a larger bounding surface, indicative of a higher apparent over-consolidation ratio (OCR), aligns more closely with experimental observations than anticipated. This suggests that contrary to the expected purely elastoplastic response, creep behaviour plays a significant role during consolidation, necessitating adjustments to the model to capture these observations accurately. The JMC-clay model is extended to include an elastoplastic-viscoplastic bounding surface formulation to capture time-dependent soil response. The strain-rate saturation feature of the coupled elastoplastic-viscoplastic framework requires further investigation with experimental data at a higher loading rate.
However, the existing equipment falls short when conducting ‘fast’ testing, designing and constructing a cyclic-dynamic multi-directional shear apparatus for organic soft soils (CYC-DoSS) in-house aimed to fully cover the earthquake frequency spectrum. The development and application of the device, a state-of-the-art earthquake simulator, marks a significant leap in element testing. Designed to overcome the limitations of traditional testing apparatuses, the CYC-DoSS features digitally controlled servo-hydraulic actuators and in-house developed local response sensors, offering a comprehensive suite of measurement capabilities. The inclusion of advanced measurement techniques, such as P-wave and S-wave bender element measurements, accelerometers, and fibre optic pore pressure transducers, alongside custom-developed sensors for capturing detailed deformation patterns, pore pressure responses, and acceleration data, significantly enhances the ability to measure cyclic responses. This is crucial for accurate seismic risk assessment and mitigation strategies and enables a deeper insight into the dynamic behaviours of soft soils under seismic load.
By weaving together experimental research, advanced constitutive modelling, and the deployment of an innovative testing apparatus, this thesis presents a comprehensive approach to unravelling the behaviour of Dutch soft organic clays under seismic conditions. The contributions of this research extend beyond theoretical advancements, offering practical insights and methodologies to enhance the resilience of infrastructure in seismic-prone areas. ...
Through an extensive experimental programme, a characteristic organic diatomaceous clay from the Netherlands was tested under various stress paths, uncovering limitations in existing models. To address these, an advanced elastoplastic model, JMC-clay, was developed. Starting from a previous formulation for peats, modifications were made to the yield surface shape and the rotational hardening rule, enhancing the models' ability to predict pre-failure behaviours accurately. This foundational work paves the way for studying cyclic behaviour under ‘slow’ testing conditions, deliberately chosen to minimise the effects of non-uniform pore pressure distribution. The study's exploration into the slow cyclic response of Dutch organic clay has unearthed critical insights into the effects of loading frequency, soil composition, initial stress state, and cyclic stress amplitude on the soil's mechanical properties. Among the findings, the soil's cyclic response significantly depends on the cyclic strain amplitude.
A pivotal aspect of this thesis is extending the JMC-clay model to include cyclic behaviour, incorporating bounding surface plasticity for a more accurate and predictive modelling framework of soil behaviour under cyclic loading conditions. This development, however, brought to light challenges in model validation. Simulations indicated that a larger bounding surface, indicative of a higher apparent over-consolidation ratio (OCR), aligns more closely with experimental observations than anticipated. This suggests that contrary to the expected purely elastoplastic response, creep behaviour plays a significant role during consolidation, necessitating adjustments to the model to capture these observations accurately. The JMC-clay model is extended to include an elastoplastic-viscoplastic bounding surface formulation to capture time-dependent soil response. The strain-rate saturation feature of the coupled elastoplastic-viscoplastic framework requires further investigation with experimental data at a higher loading rate.
However, the existing equipment falls short when conducting ‘fast’ testing, designing and constructing a cyclic-dynamic multi-directional shear apparatus for organic soft soils (CYC-DoSS) in-house aimed to fully cover the earthquake frequency spectrum. The development and application of the device, a state-of-the-art earthquake simulator, marks a significant leap in element testing. Designed to overcome the limitations of traditional testing apparatuses, the CYC-DoSS features digitally controlled servo-hydraulic actuators and in-house developed local response sensors, offering a comprehensive suite of measurement capabilities. The inclusion of advanced measurement techniques, such as P-wave and S-wave bender element measurements, accelerometers, and fibre optic pore pressure transducers, alongside custom-developed sensors for capturing detailed deformation patterns, pore pressure responses, and acceleration data, significantly enhances the ability to measure cyclic responses. This is crucial for accurate seismic risk assessment and mitigation strategies and enables a deeper insight into the dynamic behaviours of soft soils under seismic load.
By weaving together experimental research, advanced constitutive modelling, and the deployment of an innovative testing apparatus, this thesis presents a comprehensive approach to unravelling the behaviour of Dutch soft organic clays under seismic conditions. The contributions of this research extend beyond theoretical advancements, offering practical insights and methodologies to enhance the resilience of infrastructure in seismic-prone areas. ...
In the Netherlands, natural gas extraction from the Groningen field has induced seismic activities, notably recording the highest magnitude of ML 3.6 at Huizinge. Despite the lower magnitudes of these induced earthquakes compared to their natural counterparts, their impact is significantly magnified due to the shallow depths at which they occur and the high site amplification over areas with soft soils. In response, the Dutch Research Council (NWO) launched the DeepNL research programme to conduct thorough scientific research into these phenomena. The SOFTTOP project, part of this initiative, aims to fill the knowledge gaps regarding the effects of soft soil deposits on earthquake dynamics and subsequent soil responses. This thesis, nested within the SOFTTOP project, focuses on the cyclic and dynamic behaviours of soft organic clays, which are prevalent in the Netherlands' deltaic subsurface. It starts with a detailed investigation into the monotonic behaviours of these clays, laying the groundwork for further exploration into their cyclic and rate-dependent responses.
Through an extensive experimental programme, a characteristic organic diatomaceous clay from the Netherlands was tested under various stress paths, uncovering limitations in existing models. To address these, an advanced elastoplastic model, JMC-clay, was developed. Starting from a previous formulation for peats, modifications were made to the yield surface shape and the rotational hardening rule, enhancing the models' ability to predict pre-failure behaviours accurately. This foundational work paves the way for studying cyclic behaviour under ‘slow’ testing conditions, deliberately chosen to minimise the effects of non-uniform pore pressure distribution. The study's exploration into the slow cyclic response of Dutch organic clay has unearthed critical insights into the effects of loading frequency, soil composition, initial stress state, and cyclic stress amplitude on the soil's mechanical properties. Among the findings, the soil's cyclic response significantly depends on the cyclic strain amplitude.
A pivotal aspect of this thesis is extending the JMC-clay model to include cyclic behaviour, incorporating bounding surface plasticity for a more accurate and predictive modelling framework of soil behaviour under cyclic loading conditions. This development, however, brought to light challenges in model validation. Simulations indicated that a larger bounding surface, indicative of a higher apparent over-consolidation ratio (OCR), aligns more closely with experimental observations than anticipated. This suggests that contrary to the expected purely elastoplastic response, creep behaviour plays a significant role during consolidation, necessitating adjustments to the model to capture these observations accurately. The JMC-clay model is extended to include an elastoplastic-viscoplastic bounding surface formulation to capture time-dependent soil response. The strain-rate saturation feature of the coupled elastoplastic-viscoplastic framework requires further investigation with experimental data at a higher loading rate.
However, the existing equipment falls short when conducting ‘fast’ testing, designing and constructing a cyclic-dynamic multi-directional shear apparatus for organic soft soils (CYC-DoSS) in-house aimed to fully cover the earthquake frequency spectrum. The development and application of the device, a state-of-the-art earthquake simulator, marks a significant leap in element testing. Designed to overcome the limitations of traditional testing apparatuses, the CYC-DoSS features digitally controlled servo-hydraulic actuators and in-house developed local response sensors, offering a comprehensive suite of measurement capabilities. The inclusion of advanced measurement techniques, such as P-wave and S-wave bender element measurements, accelerometers, and fibre optic pore pressure transducers, alongside custom-developed sensors for capturing detailed deformation patterns, pore pressure responses, and acceleration data, significantly enhances the ability to measure cyclic responses. This is crucial for accurate seismic risk assessment and mitigation strategies and enables a deeper insight into the dynamic behaviours of soft soils under seismic load.
By weaving together experimental research, advanced constitutive modelling, and the deployment of an innovative testing apparatus, this thesis presents a comprehensive approach to unravelling the behaviour of Dutch soft organic clays under seismic conditions. The contributions of this research extend beyond theoretical advancements, offering practical insights and methodologies to enhance the resilience of infrastructure in seismic-prone areas.
Through an extensive experimental programme, a characteristic organic diatomaceous clay from the Netherlands was tested under various stress paths, uncovering limitations in existing models. To address these, an advanced elastoplastic model, JMC-clay, was developed. Starting from a previous formulation for peats, modifications were made to the yield surface shape and the rotational hardening rule, enhancing the models' ability to predict pre-failure behaviours accurately. This foundational work paves the way for studying cyclic behaviour under ‘slow’ testing conditions, deliberately chosen to minimise the effects of non-uniform pore pressure distribution. The study's exploration into the slow cyclic response of Dutch organic clay has unearthed critical insights into the effects of loading frequency, soil composition, initial stress state, and cyclic stress amplitude on the soil's mechanical properties. Among the findings, the soil's cyclic response significantly depends on the cyclic strain amplitude.
A pivotal aspect of this thesis is extending the JMC-clay model to include cyclic behaviour, incorporating bounding surface plasticity for a more accurate and predictive modelling framework of soil behaviour under cyclic loading conditions. This development, however, brought to light challenges in model validation. Simulations indicated that a larger bounding surface, indicative of a higher apparent over-consolidation ratio (OCR), aligns more closely with experimental observations than anticipated. This suggests that contrary to the expected purely elastoplastic response, creep behaviour plays a significant role during consolidation, necessitating adjustments to the model to capture these observations accurately. The JMC-clay model is extended to include an elastoplastic-viscoplastic bounding surface formulation to capture time-dependent soil response. The strain-rate saturation feature of the coupled elastoplastic-viscoplastic framework requires further investigation with experimental data at a higher loading rate.
However, the existing equipment falls short when conducting ‘fast’ testing, designing and constructing a cyclic-dynamic multi-directional shear apparatus for organic soft soils (CYC-DoSS) in-house aimed to fully cover the earthquake frequency spectrum. The development and application of the device, a state-of-the-art earthquake simulator, marks a significant leap in element testing. Designed to overcome the limitations of traditional testing apparatuses, the CYC-DoSS features digitally controlled servo-hydraulic actuators and in-house developed local response sensors, offering a comprehensive suite of measurement capabilities. The inclusion of advanced measurement techniques, such as P-wave and S-wave bender element measurements, accelerometers, and fibre optic pore pressure transducers, alongside custom-developed sensors for capturing detailed deformation patterns, pore pressure responses, and acceleration data, significantly enhances the ability to measure cyclic responses. This is crucial for accurate seismic risk assessment and mitigation strategies and enables a deeper insight into the dynamic behaviours of soft soils under seismic load.
By weaving together experimental research, advanced constitutive modelling, and the deployment of an innovative testing apparatus, this thesis presents a comprehensive approach to unravelling the behaviour of Dutch soft organic clays under seismic conditions. The contributions of this research extend beyond theoretical advancements, offering practical insights and methodologies to enhance the resilience of infrastructure in seismic-prone areas.
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.
...
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.
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.
...
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.
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.
...
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.
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.
...
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.
Energy technologies, which work by extracting or injecting fluids in the ground, such as geothermal energy systems or underground liquefied gas storage, may induce seismic events, see e.g., [1]. In the Netherlands, induced earthquakes are continuously recorded from the Groningen gas field, with the largest magnitude ever recorded of ML 3.6 at Huizinge. Even though the magnitude of these events is not high, compared to natural earthquakes, damage to the built environment is still caused because of the shallow depth of the events and site amplification, especially where soft soils are encountered [2]. Proper quantification of the induced seismic risk requires better understanding of the response of soft soils to these repeated short events, covering a range of frequencies from 1 to about 20 Hz. This motivated the development of a new advanced dynamic equipment to experimentally investigate the coupled response of soft organic clays and peats from the typical deltaic areas of the Netherlands. Direct simple shear (DSS) apparatuses are preferred usually to investigate the soil behaviour under cyclic and dynamic loading. Among them, a number of multi-directional DSS setups have been developed to investigate the soil behaviour under multidirectional loading [3, 4, 5, 6, 7, 8]. Applying multi-directional loading to soil specimens in the laboratory is a keystone for elucidating the cyclic and dynamic soil response, as several studies have shown that the cyclic and post-cyclic response of soils is affected by multiple loading directions [6, 9, 10, 11, 12]. However, traditional DSS devices have a number of shortcomings, which are inherited by multi-directional DSS devices. The main deficiency of the DSS device is that the shear stress acting on the lateral side of the specimen cannot be controlled, and hence, a homogeneous stress state cannot be achieved, in spite of the common assumptions. Lateral stresses cannot be measured either in traditional setups, which leaves a knowledge gap on the stress state and the stress path of the sample. In addition, the majority of laboratory element tests are performed by imposing “slow” undrained cyclic loads, to try to guarantee uniform water pressure distribution within the sample, for the sake of interpretation and modelling. However, seismic events encompass much higher loading frequencies than typically available, with loading rate effects playing a key role in the response of soft soils such as organic clays and peats. In order to fully understand the cyclic behaviour of soft soils, “fast” cyclic tests are crucial. The innovative multidirectional shear device, developed in the section of Geoengineering at TU Delft (Cyclic-Dynamic shear simulator for Organic Soft Soils, CYC-DOSS), was designed to overcome some limitations of previous equipment. The underlying idea is to abandon the homogenous stress-strain state assumption and monitor the response with local sensors, which allows conditioning a numerical back-analysis of the test data. The new device shown in Figure 1 is characterised by (1) servo-hydraulic control; (2) multi-directional loading in 3 axes; (3) bender elements to measure both P-wave and S-wave velocity; (4) fully controlled cell pressure and back-pressure; and (5) possibility to reproduce the full acceleration time history of seismic events. The device is capable to apply loading frequencies up to 25Hz and a wide variety of multidirectional cyclic loading patterns. The apparatus is equipped with advanced sensors, also developed at TU Delft, including local pressure, displacements, and accelerations devices. The sensors are installed to reduce a priori assumptions on the soil response, better interpret the experimental results as a small-scale physical model and further investigate in depth the soil response under a variety of cyclic loading histories. The experimental information from the setup will be used to develop and calibrate an advanced bounding surface constitutive model for soft organic soils.
...
Energy technologies, which work by extracting or injecting fluids in the ground, such as geothermal energy systems or underground liquefied gas storage, may induce seismic events, see e.g., [1]. In the Netherlands, induced earthquakes are continuously recorded from the Groningen gas field, with the largest magnitude ever recorded of ML 3.6 at Huizinge. Even though the magnitude of these events is not high, compared to natural earthquakes, damage to the built environment is still caused because of the shallow depth of the events and site amplification, especially where soft soils are encountered [2]. Proper quantification of the induced seismic risk requires better understanding of the response of soft soils to these repeated short events, covering a range of frequencies from 1 to about 20 Hz. This motivated the development of a new advanced dynamic equipment to experimentally investigate the coupled response of soft organic clays and peats from the typical deltaic areas of the Netherlands. Direct simple shear (DSS) apparatuses are preferred usually to investigate the soil behaviour under cyclic and dynamic loading. Among them, a number of multi-directional DSS setups have been developed to investigate the soil behaviour under multidirectional loading [3, 4, 5, 6, 7, 8]. Applying multi-directional loading to soil specimens in the laboratory is a keystone for elucidating the cyclic and dynamic soil response, as several studies have shown that the cyclic and post-cyclic response of soils is affected by multiple loading directions [6, 9, 10, 11, 12]. However, traditional DSS devices have a number of shortcomings, which are inherited by multi-directional DSS devices. The main deficiency of the DSS device is that the shear stress acting on the lateral side of the specimen cannot be controlled, and hence, a homogeneous stress state cannot be achieved, in spite of the common assumptions. Lateral stresses cannot be measured either in traditional setups, which leaves a knowledge gap on the stress state and the stress path of the sample. In addition, the majority of laboratory element tests are performed by imposing “slow” undrained cyclic loads, to try to guarantee uniform water pressure distribution within the sample, for the sake of interpretation and modelling. However, seismic events encompass much higher loading frequencies than typically available, with loading rate effects playing a key role in the response of soft soils such as organic clays and peats. In order to fully understand the cyclic behaviour of soft soils, “fast” cyclic tests are crucial. The innovative multidirectional shear device, developed in the section of Geoengineering at TU Delft (Cyclic-Dynamic shear simulator for Organic Soft Soils, CYC-DOSS), was designed to overcome some limitations of previous equipment. The underlying idea is to abandon the homogenous stress-strain state assumption and monitor the response with local sensors, which allows conditioning a numerical back-analysis of the test data. The new device shown in Figure 1 is characterised by (1) servo-hydraulic control; (2) multi-directional loading in 3 axes; (3) bender elements to measure both P-wave and S-wave velocity; (4) fully controlled cell pressure and back-pressure; and (5) possibility to reproduce the full acceleration time history of seismic events. The device is capable to apply loading frequencies up to 25Hz and a wide variety of multidirectional cyclic loading patterns. The apparatus is equipped with advanced sensors, also developed at TU Delft, including local pressure, displacements, and accelerations devices. The sensors are installed to reduce a priori assumptions on the soil response, better interpret the experimental results as a small-scale physical model and further investigate in depth the soil response under a variety of cyclic loading histories. The experimental information from the setup will be used to develop and calibrate an advanced bounding surface constitutive model for soft organic soils.
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.
A simple numerical model was set up to investigate the hydraulic behaviour of a regional dyke to improve understanding of the response under variable atmospheric conditions. The unsaturated hydraulic properties of the dyke body and the relevant foundation layers were calibrated either on the results of laboratory tests or on a national database, namely the Staringreeks, compiled for typical Dutch soils. The boundary conditions were imposed according to the weather history at the top, and to the pore pressures measured in the field at the bottom of the representative soil column. The results indicate that a simple 1D model is able to accurately reproduce the suction time history in the dyke core, provided the hydraulic conductivity and soil water retention properties are properly calibrated. The optimised hydraulic conductivities are typically two orders of magnitude higher than the saturated hydraulic conductivity from the laboratory tests, but comparable to the ones suggested in the database developed on field data. The work highlights that cautious evaluation of laboratory data is needed for field applications, and that direct information from the field should be used to validate numerical models in the presence of organic soils.
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A simple numerical model was set up to investigate the hydraulic behaviour of a regional dyke to improve understanding of the response under variable atmospheric conditions. The unsaturated hydraulic properties of the dyke body and the relevant foundation layers were calibrated either on the results of laboratory tests or on a national database, namely the Staringreeks, compiled for typical Dutch soils. The boundary conditions were imposed according to the weather history at the top, and to the pore pressures measured in the field at the bottom of the representative soil column. The results indicate that a simple 1D model is able to accurately reproduce the suction time history in the dyke core, provided the hydraulic conductivity and soil water retention properties are properly calibrated. The optimised hydraulic conductivities are typically two orders of magnitude higher than the saturated hydraulic conductivity from the laboratory tests, but comparable to the ones suggested in the database developed on field data. The work highlights that cautious evaluation of laboratory data is needed for field applications, and that direct information from the field should be used to validate numerical models in the presence of organic soils.