J. Liaudat
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1
Gas-induced fracturing in liquid-saturated clay-rich materials presents challenges in understanding and predicting fracture behaviour, due to the complex mechanical and transport properties of clays and the compressibility of gas. This paper introduces a novel experimental device for visualising fluid-driven cracks in clays. The device allows for the induction and observation of two-dimensional cracks in clay-rich, low-permeability materials through the injection of gas or water. The experimental setup comprises precision instrumentation for measuring compression forces, displacement, and fluid pressure, along with high-resolution imaging capabilities. Preliminary tests with Helium gas injection into Boom clay samples demonstrate the device's ability to track fracture evolution. This innovative experimental tool offers insights into the mechanisms governing fluid-driven fractures in clay-rich materials and provides a means to validate numerical models.
...
Gas-induced fracturing in liquid-saturated clay-rich materials presents challenges in understanding and predicting fracture behaviour, due to the complex mechanical and transport properties of clays and the compressibility of gas. This paper introduces a novel experimental device for visualising fluid-driven cracks in clays. The device allows for the induction and observation of two-dimensional cracks in clay-rich, low-permeability materials through the injection of gas or water. The experimental setup comprises precision instrumentation for measuring compression forces, displacement, and fluid pressure, along with high-resolution imaging capabilities. Preliminary tests with Helium gas injection into Boom clay samples demonstrate the device's ability to track fracture evolution. This innovative experimental tool offers insights into the mechanisms governing fluid-driven fractures in clay-rich materials and provides a means to validate numerical models.
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.
...
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.
The injection of gas in a liquid-saturated clay-rich material may lead to the formation of narrow channels or fractures created by the mechanical action of gas pressure. However, the complexity of the mechanical and transport properties of clays, combined with the high compressibility of gases, makes it challenging to understand and address this phenomenon. Gas fracturing can occur in a range of engineered processes, for instance the stimulation of sensitive hydro-carbon reservoirs, carbon dioxide injection and storage in subsurface reservoirs, pneumatic fracturing for enhanced remediation of contaminated soils, or gas transport through natural and engineered clay barriers in geological disposal facilities for radioactive waste. Despite the multiple environmental and economic implications associated with gas fracturing it remains difficult to predict and control due to the lack of fundamental scientific insight. One of the most challenging aspects of understanding gas-driven cracking in clays is the difficulty in visualising the crack formation in real time. In recent years, a new experimental setup has been proposed by the British Geological Survey, which makes it possible to induce and observe the formation of 'two dimensional' cracks in clay-rich low-permeability materials by the injection of gas or water. In order to aid the interpretation of the results obtained with this new device, the authors have carried out a series of numerical simulations performed with a finite element approach recently published, and compared them to experimental data. The model uses a fully coupled Pneumo-Hydro-Mechanical (PHM) formulation in 2D, and is implemented in the code LAGAMINE. These simulations show that the model proposed can reproduce the experimental results, providing valuable insights into the factors controlling the onset and propagation of the gas fractures. These insights have the potential to enhance the performance and safety of projects involving gas fracturing processes.
...
The injection of gas in a liquid-saturated clay-rich material may lead to the formation of narrow channels or fractures created by the mechanical action of gas pressure. However, the complexity of the mechanical and transport properties of clays, combined with the high compressibility of gases, makes it challenging to understand and address this phenomenon. Gas fracturing can occur in a range of engineered processes, for instance the stimulation of sensitive hydro-carbon reservoirs, carbon dioxide injection and storage in subsurface reservoirs, pneumatic fracturing for enhanced remediation of contaminated soils, or gas transport through natural and engineered clay barriers in geological disposal facilities for radioactive waste. Despite the multiple environmental and economic implications associated with gas fracturing it remains difficult to predict and control due to the lack of fundamental scientific insight. One of the most challenging aspects of understanding gas-driven cracking in clays is the difficulty in visualising the crack formation in real time. In recent years, a new experimental setup has been proposed by the British Geological Survey, which makes it possible to induce and observe the formation of 'two dimensional' cracks in clay-rich low-permeability materials by the injection of gas or water. In order to aid the interpretation of the results obtained with this new device, the authors have carried out a series of numerical simulations performed with a finite element approach recently published, and compared them to experimental data. The model uses a fully coupled Pneumo-Hydro-Mechanical (PHM) formulation in 2D, and is implemented in the code LAGAMINE. These simulations show that the model proposed can reproduce the experimental results, providing valuable insights into the factors controlling the onset and propagation of the gas fractures. These insights have the potential to enhance the performance and safety of projects involving gas fracturing processes.
Geological Disposal Facilities (GDF) for radioactive waste will generally rely on clay-rich materials as a host geological formation and/or engineered barrier. Gas will be produced within the GDF, which can build up significant gas pressure and will activate the migration of gas through the clay materials via different transport mechanisms. These transport mechanisms are usually investigated in laboratory tests on small clay samples of a few centimetres. In this paper, a new Pneumo-Hydro-Mechanical (PHM) Finite Element model to simulate gas migration in saturated clay samples of this scale is presented. In the proposed modelling approach, continuum elements are used to represent the mechanical and flow processes in the bulk clay material, while zero-thickness interface elements are used to represent existing or induced discontinuities (cracks). A new triple-node PHM interface element is presented to achieve this. The performance of model is illustrated with synthetic benchmark examples which show the ability of the model to reproduce observed PHM mechanisms leading to propagation of cracks due to the gas pressure (gas fracturing).
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Geological Disposal Facilities (GDF) for radioactive waste will generally rely on clay-rich materials as a host geological formation and/or engineered barrier. Gas will be produced within the GDF, which can build up significant gas pressure and will activate the migration of gas through the clay materials via different transport mechanisms. These transport mechanisms are usually investigated in laboratory tests on small clay samples of a few centimetres. In this paper, a new Pneumo-Hydro-Mechanical (PHM) Finite Element model to simulate gas migration in saturated clay samples of this scale is presented. In the proposed modelling approach, continuum elements are used to represent the mechanical and flow processes in the bulk clay material, while zero-thickness interface elements are used to represent existing or induced discontinuities (cracks). A new triple-node PHM interface element is presented to achieve this. The performance of model is illustrated with synthetic benchmark examples which show the ability of the model to reproduce observed PHM mechanisms leading to propagation of cracks due to the gas pressure (gas fracturing).
Conference paper
(2023)
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Wen Luo, Ouf Josselin, Philip J. Vardon, Anne-Catherine Dieudonné, Joaquín Liaudat, Kavan Khaledi, Reza Jalali, Florian Amann
Heat and electricity production from deep hot reservoirs through hydrothermal or petrothermal system requires to inject cold fluid in a naturally or artificially fractured medium. Cold water injection in a hot reservoirs causes thermo-hydro-mechanical (THM) coupled processes that may have several influences on operations and long term production. Cold water has a higher viscosity, which means injection can become difficult due to higher flow impedance, while thermal diffusion in the rock matrix causes shrinkage and thus an increasing fracture aperture over time that can lead to flow channeling and a reduction of operation time. A deep understanding of the contribution of coupled THM processes to injection into fractured media is thus important to predict the long-term performance of a geothermal power plant. In this study, coupled THM processes in a single fracture are numerically investigated with two different approaches to model the discontinuity. The first numerical approach proposes modelling an implicit fracture in a fracture zone using solid elements. The impact of the fracture is introduced by means of a stress dependent fracture aperture, which in turn determines the fracture transmissivity. This function allows recovery of the opening, and thus the model replicates opening and closure of the fracture. In the second approach, zero-thickness interface elements are used to explicitly model the opening of an existing fracture, following the work by Liaudat et al. A constitutive law that is capable of describing the fracture initiation, propagation, closing and opening is implemented in this approach, considering heat transfer in the discontinuity. In both approaches, the simulation of cold water injection requires to consider the heat and hydraulic flows in the fracture alongside mechanical behaviour, since changes in pore water pressure and temperature influence the fracture aperture, thus modifying the fracture transmissivity.
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Heat and electricity production from deep hot reservoirs through hydrothermal or petrothermal system requires to inject cold fluid in a naturally or artificially fractured medium. Cold water injection in a hot reservoirs causes thermo-hydro-mechanical (THM) coupled processes that may have several influences on operations and long term production. Cold water has a higher viscosity, which means injection can become difficult due to higher flow impedance, while thermal diffusion in the rock matrix causes shrinkage and thus an increasing fracture aperture over time that can lead to flow channeling and a reduction of operation time. A deep understanding of the contribution of coupled THM processes to injection into fractured media is thus important to predict the long-term performance of a geothermal power plant. In this study, coupled THM processes in a single fracture are numerically investigated with two different approaches to model the discontinuity. The first numerical approach proposes modelling an implicit fracture in a fracture zone using solid elements. The impact of the fracture is introduced by means of a stress dependent fracture aperture, which in turn determines the fracture transmissivity. This function allows recovery of the opening, and thus the model replicates opening and closure of the fracture. In the second approach, zero-thickness interface elements are used to explicitly model the opening of an existing fracture, following the work by Liaudat et al. A constitutive law that is capable of describing the fracture initiation, propagation, closing and opening is implemented in this approach, considering heat transfer in the discontinuity. In both approaches, the simulation of cold water injection requires to consider the heat and hydraulic flows in the fracture alongside mechanical behaviour, since changes in pore water pressure and temperature influence the fracture aperture, thus modifying the fracture transmissivity.
Rate-dependent behaviour of fracture propagation in salt rock
Proceedings of the 10th Conference on the Mechanical Behavior of Salt (SaltMech X), Utrecht, The Netherlands, 06-08 July 2022
Conference paper
(2022)
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Andreu Escanellas, Eduardo Cámara, J. Liaudat, Ignacio Carol
This paper describes an on-going experimental and numerical modelling
research project on salt rock specimens. The experimental part of the study consists of a number of Mode I fracture tests with the WST (Wedge-Splitting Test) configuration, which are performed at different loading rates and complemented by a series of standard uniaxial creep tests. The preliminary WST results show a greater mechanical fracture work accompanied with
lower force peaks, for the slower tests. As a first attempt to represent the experimental results, an in-house Finite Element model has been used, which combines an inviscid discrete fracture approach with a Maxwell chain model for the continuum material. The simulations show a decrease of the mechanical work needed for opening the fracture and higher peak force, as
foreseen by the ongoing experimental results, but not with the same intensity, which seems to indicate that work dissipation may not be caused exclusively by the bulk viscosity.
...
This paper describes an on-going experimental and numerical modelling
research project on salt rock specimens. The experimental part of the study consists of a number of Mode I fracture tests with the WST (Wedge-Splitting Test) configuration, which are performed at different loading rates and complemented by a series of standard uniaxial creep tests. The preliminary WST results show a greater mechanical fracture work accompanied with
lower force peaks, for the slower tests. As a first attempt to represent the experimental results, an in-house Finite Element model has been used, which combines an inviscid discrete fracture approach with a Maxwell chain model for the continuum material. The simulations show a decrease of the mechanical work needed for opening the fracture and higher peak force, as
foreseen by the ongoing experimental results, but not with the same intensity, which seems to indicate that work dissipation may not be caused exclusively by the bulk viscosity.
Conference paper
(2022)
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Lucía Barandiarán, J. Liaudat, Carlos Maria López, Ignacio Carol
In recent years, the authors and co-workers have developed a 3D finite element model for coupled thermo-hydro-mechanical (THM) problems in fractured rock masses. Zero-thickness interface elements are used for taking into account explicitly the effect of fractures and discontinuities in the fluid flow as well as the effect of fluid pressure in the crack propagation. Furthermore, the use of zero-thickness elements as a discrete modelling approach for fractures and discontinuities makes it possible to account for the heat transport taking place within these elements, even when advection dominates over diffusion (high Peclet number) [1]. The model has been implemented in the finite element code DRAC5, which is equipped with fracture-based interface elements and MPI parallel capabilities [2]. The code was originally developed considering water-saturated porous medium and fractures. The new developments described in the present paper, include the extension of the original formulation to the case of two-phase (liquid and gas) flow within the porous medium and discontinuities. The liquid includes only liquid water species, while the gas phase includes water vapour and gas species. The formulation includes the equilibrium equation, the mass balance of water and gas species and the energy balance equation. The parameters of the retention and relative permeability curves for the interface elements, such as the gas entry value and the residual water saturation, are updated with the variation of the normal aperture. The new capabilities of the model are illustrated with some academic verification examples.
...
In recent years, the authors and co-workers have developed a 3D finite element model for coupled thermo-hydro-mechanical (THM) problems in fractured rock masses. Zero-thickness interface elements are used for taking into account explicitly the effect of fractures and discontinuities in the fluid flow as well as the effect of fluid pressure in the crack propagation. Furthermore, the use of zero-thickness elements as a discrete modelling approach for fractures and discontinuities makes it possible to account for the heat transport taking place within these elements, even when advection dominates over diffusion (high Peclet number) [1]. The model has been implemented in the finite element code DRAC5, which is equipped with fracture-based interface elements and MPI parallel capabilities [2]. The code was originally developed considering water-saturated porous medium and fractures. The new developments described in the present paper, include the extension of the original formulation to the case of two-phase (liquid and gas) flow within the porous medium and discontinuities. The liquid includes only liquid water species, while the gas phase includes water vapour and gas species. The formulation includes the equilibrium equation, the mass balance of water and gas species and the energy balance equation. The parameters of the retention and relative permeability curves for the interface elements, such as the gas entry value and the residual water saturation, are updated with the variation of the normal aperture. The new capabilities of the model are illustrated with some academic verification examples.
Conference paper
(2022)
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Ariadna Martínez, J. Liaudat, Carlos Maria López, Ignacio Carol
In Carbon Capture Storage (CCS) sites, an important element of risk to be considered is the integrity of the cement seals of the abandoned wells in the reservoir [1]. The main goal of abandonment procedure once the life of a well is completed is to provide an effective isolation of the reservoir fluids in order to reduce environmental risk of contamination. In the case that the site has been reconverted to CCS, this is even more essential to prevent CO2 leaks from the storage site. It is important to note that the cracking conditions of the well cement seal can be affected by the long term changes in pore pressures that take place after the oil exploitation activities have stopped [2]. For example, slow pressure return around extraction wells (where the pore pressure had been subject to a sustained reduction during long extraction periods) may cause a progressive reduction of the effective stresses acting on the cement casing and plug, while the opposite can happen at injection wells. And these effects may be partially modified by the overall structural response due to the volume changes implied by the effective stress changes [3]. In this paper, a preliminary study of the effects of such stress changes on the potential integrity of a 2D cross-section of the sealed oil well system (caprock-external cement sheath-steel casingcement plug) during its service-life (injection/production activities and abandonment) has been performed by means of FE method including zero-thickness interface elements to represent potential cracks. In particular, these elements are pre-inserted in the analysis in between the contacts of caprock-external cement sheath, external cement sheath-casing and casing-cement plug. The results presented show that, depending on the initial state and range of pressure evolution, the different interfaces considered may open or close in a non-trivial manner during the pressure return process. This seems to indicate the importance of considering carefully the pressure return process and subsequent effective stresses evolution in abandoned reservoirs recycled to CCS, in order to avoid that new cracks in well cement seals may lead to potential CO2 leakage in the storage site.
...
In Carbon Capture Storage (CCS) sites, an important element of risk to be considered is the integrity of the cement seals of the abandoned wells in the reservoir [1]. The main goal of abandonment procedure once the life of a well is completed is to provide an effective isolation of the reservoir fluids in order to reduce environmental risk of contamination. In the case that the site has been reconverted to CCS, this is even more essential to prevent CO2 leaks from the storage site. It is important to note that the cracking conditions of the well cement seal can be affected by the long term changes in pore pressures that take place after the oil exploitation activities have stopped [2]. For example, slow pressure return around extraction wells (where the pore pressure had been subject to a sustained reduction during long extraction periods) may cause a progressive reduction of the effective stresses acting on the cement casing and plug, while the opposite can happen at injection wells. And these effects may be partially modified by the overall structural response due to the volume changes implied by the effective stress changes [3]. In this paper, a preliminary study of the effects of such stress changes on the potential integrity of a 2D cross-section of the sealed oil well system (caprock-external cement sheath-steel casingcement plug) during its service-life (injection/production activities and abandonment) has been performed by means of FE method including zero-thickness interface elements to represent potential cracks. In particular, these elements are pre-inserted in the analysis in between the contacts of caprock-external cement sheath, external cement sheath-casing and casing-cement plug. The results presented show that, depending on the initial state and range of pressure evolution, the different interfaces considered may open or close in a non-trivial manner during the pressure return process. This seems to indicate the importance of considering carefully the pressure return process and subsequent effective stresses evolution in abandoned reservoirs recycled to CCS, in order to avoid that new cracks in well cement seals may lead to potential CO2 leakage in the storage site.
Conference paper
(2022)
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J. Liaudat, A.A.M. Dieudonné, P.J. Vardon
Modelling gas flow along localized dilatant pathways
Challenges and perspective
Gas flow along localized dilatant pathways is considered as an important gas transfer mechanism in saturated plastic clays such as Boom clay. The process involves micro-fracturing of the pore space at rising gas pressures, gas flow and possibly sealing of the newly-created pathways.
Modelling gas flow along localized dilatant pathways presents a number of challenges associated with: (1) the very strong coupling between the hydraulic and mechanical aspects of the problem, (2) the microscopic scale of the process and (3) the important influence of material heterogeneities on the process dynamics.
This contribution presents a review of existing theoretical and numerical frameworks for damage, strain localisation and fracture of clay materials. In particular, it addresses the suitability of these approaches as a basis for the development of new modelling framework for gas flow along localized dilatant pathways.
The research was carried out within the WP GAS of EURAD (2019 - 2024). ...
Modelling gas flow along localized dilatant pathways presents a number of challenges associated with: (1) the very strong coupling between the hydraulic and mechanical aspects of the problem, (2) the microscopic scale of the process and (3) the important influence of material heterogeneities on the process dynamics.
This contribution presents a review of existing theoretical and numerical frameworks for damage, strain localisation and fracture of clay materials. In particular, it addresses the suitability of these approaches as a basis for the development of new modelling framework for gas flow along localized dilatant pathways.
The research was carried out within the WP GAS of EURAD (2019 - 2024). ...
Gas flow along localized dilatant pathways is considered as an important gas transfer mechanism in saturated plastic clays such as Boom clay. The process involves micro-fracturing of the pore space at rising gas pressures, gas flow and possibly sealing of the newly-created pathways.
Modelling gas flow along localized dilatant pathways presents a number of challenges associated with: (1) the very strong coupling between the hydraulic and mechanical aspects of the problem, (2) the microscopic scale of the process and (3) the important influence of material heterogeneities on the process dynamics.
This contribution presents a review of existing theoretical and numerical frameworks for damage, strain localisation and fracture of clay materials. In particular, it addresses the suitability of these approaches as a basis for the development of new modelling framework for gas flow along localized dilatant pathways.
The research was carried out within the WP GAS of EURAD (2019 - 2024).
Modelling gas flow along localized dilatant pathways presents a number of challenges associated with: (1) the very strong coupling between the hydraulic and mechanical aspects of the problem, (2) the microscopic scale of the process and (3) the important influence of material heterogeneities on the process dynamics.
This contribution presents a review of existing theoretical and numerical frameworks for damage, strain localisation and fracture of clay materials. In particular, it addresses the suitability of these approaches as a basis for the development of new modelling framework for gas flow along localized dilatant pathways.
The research was carried out within the WP GAS of EURAD (2019 - 2024).
In the framework of the Finite Element Method, zero-thickness interface elements have been widely used to model fracturing processes in quasi-brittle materials in a broad variety of problems. In particular, interface elements equipped with elastoplastic constitutive laws that account for the softening of the material strength parameters due to the fracturing mechanical work has been proved to accurately reproduce observed fracture propagation behaviour in concrete. Along this line, this paper presents the extension of an existing constitutive law of this kind to include the effect of chemical degradation of the material in the formation of fractures. The law is defined in terms of the normal and shear stresses on the average plane of the crack and the corresponding normal and shear relative displacements. A hyperbolic cracking (plastification) surface in the stress state determines the crack initiation. The softening of the cracking surface is governed by two history variables: an internal variable that accounts for the dissipated fracturing (plastic) work, and an external variable to be provided by a chemical degradation model that accounts for the effect of chemical degradation on the strength parameters. After a detailed discussion of the formulation, the main characteristics of the proposed law are illustrated with a number of academic examples for different combinations of mechanical loading and chemical degradation sequences. The model is finally validated against experimental results from the literature consisting of three-point bending tests performed on mortar samples previously exposed to an aggressive solution for different time periods.
...
In the framework of the Finite Element Method, zero-thickness interface elements have been widely used to model fracturing processes in quasi-brittle materials in a broad variety of problems. In particular, interface elements equipped with elastoplastic constitutive laws that account for the softening of the material strength parameters due to the fracturing mechanical work has been proved to accurately reproduce observed fracture propagation behaviour in concrete. Along this line, this paper presents the extension of an existing constitutive law of this kind to include the effect of chemical degradation of the material in the formation of fractures. The law is defined in terms of the normal and shear stresses on the average plane of the crack and the corresponding normal and shear relative displacements. A hyperbolic cracking (plastification) surface in the stress state determines the crack initiation. The softening of the cracking surface is governed by two history variables: an internal variable that accounts for the dissipated fracturing (plastic) work, and an external variable to be provided by a chemical degradation model that accounts for the effect of chemical degradation on the strength parameters. After a detailed discussion of the formulation, the main characteristics of the proposed law are illustrated with a number of academic examples for different combinations of mechanical loading and chemical degradation sequences. The model is finally validated against experimental results from the literature consisting of three-point bending tests performed on mortar samples previously exposed to an aggressive solution for different time periods.
Abstract
(2021)
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Andreu Escanellas, Eduardo Cámara, J. Liaudat, Ignacio Carol
In underground salt and potash mines, such as the ones in the region of Bages in central Catalonia, long-term deformations and stability of the mine tunnels are strongly influenced by the viscous (creep) behavior of saline rock. Tunnel excavation causes deviatoric stresses which in turn trigger creep strain in the salt rock. It has been often considered that the main consequences of creep are time-dependent convergences with potential consequences for the serviceability of the mine. However, creep deformations may be accompanied by stress redistribution, and potentially also by cracking and fracture, especially if rock exhibits preexisting discontinuities and/or layers separated by weaker contacts. In this way, tunnel crosssections that are perfectly stable after excavation, with time may not only deform, but also approach failure collapse. Traditionally, salt rock has been mainly characterized with regard to the creep behavior, while strength issues have been addressed independently in ways similar to other rock materials, via strength criteria such as Mohr-Coulomb, etc. However, there seem to be very few studies considering the fracture mechanics of salt rock. In this paper, the on-going experimental and numerical research along this line at ETSECCPBUPC is described. This includes standard creep tests as well as mode I fracture Wedge Splitting Tests (WST). WSTs have been performed at different loading rates with the purpose of assessing the time dependency of parameters such as the tensile strength and the specific apparent fracture energy. Preliminary test results show that as the loading rate is increased, the tensile strength seems higher but the apparent fracture decreases. Numerical calculations include finite element simulations of the WST, as it has been done before for other rock types by some of the authors1. Continuum elements with visco-elastic behavior were used to represent the salt rock material, while the fracture path was represented via preinserted zero-thickness interface elements. As a first attempt, the constitutive model used for the interface elements was a (time-independent) elasto-plasticity framework with fracture energybased evolution laws 2 . The preliminary numerical simulations qualitatively reproduce the experimental results, however, it seems that in order to quantitatively fit the experimental results, a new time-dependent constitutive model for the interface elements would be required in which fracture properties and evolution should interact with creep and time-dependent behavior.
...
In underground salt and potash mines, such as the ones in the region of Bages in central Catalonia, long-term deformations and stability of the mine tunnels are strongly influenced by the viscous (creep) behavior of saline rock. Tunnel excavation causes deviatoric stresses which in turn trigger creep strain in the salt rock. It has been often considered that the main consequences of creep are time-dependent convergences with potential consequences for the serviceability of the mine. However, creep deformations may be accompanied by stress redistribution, and potentially also by cracking and fracture, especially if rock exhibits preexisting discontinuities and/or layers separated by weaker contacts. In this way, tunnel crosssections that are perfectly stable after excavation, with time may not only deform, but also approach failure collapse. Traditionally, salt rock has been mainly characterized with regard to the creep behavior, while strength issues have been addressed independently in ways similar to other rock materials, via strength criteria such as Mohr-Coulomb, etc. However, there seem to be very few studies considering the fracture mechanics of salt rock. In this paper, the on-going experimental and numerical research along this line at ETSECCPBUPC is described. This includes standard creep tests as well as mode I fracture Wedge Splitting Tests (WST). WSTs have been performed at different loading rates with the purpose of assessing the time dependency of parameters such as the tensile strength and the specific apparent fracture energy. Preliminary test results show that as the loading rate is increased, the tensile strength seems higher but the apparent fracture decreases. Numerical calculations include finite element simulations of the WST, as it has been done before for other rock types by some of the authors1. Continuum elements with visco-elastic behavior were used to represent the salt rock material, while the fracture path was represented via preinserted zero-thickness interface elements. As a first attempt, the constitutive model used for the interface elements was a (time-independent) elasto-plasticity framework with fracture energybased evolution laws 2 . The preliminary numerical simulations qualitatively reproduce the experimental results, however, it seems that in order to quantitatively fit the experimental results, a new time-dependent constitutive model for the interface elements would be required in which fracture properties and evolution should interact with creep and time-dependent behavior.
Journal article
(2020)
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Joaquín Liaudat, Ignacio Carol, Carlos M. López
Alkali-Silica Reaction (ASR) is a particular type of chemical reaction in concrete, which produces cracking and overall expansion of the affected structural element due to the formation of expansive reaction products within the cracks. This paper develops the formulation of a coupled Chemo-Mechanical (C-M) Finite Element (FE) model for simulating ASR expansions in soda-lime glass concrete at the meso-scale. The model considers several C-M coupling mechanisms, including a reaction-expansion mechanism qualitatively proposed by the authors elsewhere on the basis of experimental results, which is introduced in order to reproduce the effect of compressive stresses on the development of ASR expansions. The model has the characteristic ingredient of using zero-thickness interface elements for modelling the C-M mechanisms leading to the propagation of cracks due to the formation of ASR products within them. This fact has required the development of: (i) a new FE formulation for diffusion-reaction processes occurring within discontinuities represented by interface elements, and (ii) a new mechanical constitutive law for interface elements, which is able to reproduce the propagation of a crack induced by the development of an internal pressure exerted by solid reaction products formed within it. In addition, the numerical implementation of the diffusion-reaction formulation has been advantageously performed with clear separation between the boundary-value or ‘structural’ governing equations (i.e. continuity and concentration gradient relations), and the ‘constitutive’ (i.e. chemical) equations. The model is illustrated with some application examples.
...
Alkali-Silica Reaction (ASR) is a particular type of chemical reaction in concrete, which produces cracking and overall expansion of the affected structural element due to the formation of expansive reaction products within the cracks. This paper develops the formulation of a coupled Chemo-Mechanical (C-M) Finite Element (FE) model for simulating ASR expansions in soda-lime glass concrete at the meso-scale. The model considers several C-M coupling mechanisms, including a reaction-expansion mechanism qualitatively proposed by the authors elsewhere on the basis of experimental results, which is introduced in order to reproduce the effect of compressive stresses on the development of ASR expansions. The model has the characteristic ingredient of using zero-thickness interface elements for modelling the C-M mechanisms leading to the propagation of cracks due to the formation of ASR products within them. This fact has required the development of: (i) a new FE formulation for diffusion-reaction processes occurring within discontinuities represented by interface elements, and (ii) a new mechanical constitutive law for interface elements, which is able to reproduce the propagation of a crack induced by the development of an internal pressure exerted by solid reaction products formed within it. In addition, the numerical implementation of the diffusion-reaction formulation has been advantageously performed with clear separation between the boundary-value or ‘structural’ governing equations (i.e. continuity and concentration gradient relations), and the ‘constitutive’ (i.e. chemical) equations. The model is illustrated with some application examples.