A time-dependent anisotropic model for argillaceous rocks: application to an underground excavation in Callovo-Oxfordian claystone

The paper presents a constitutive model for argillaceous rocks, developed within the framework of elastoplasticity, that includes a number of features that are relevant for a satisfactory description of their hydromechanical behaviour: anisotropy of strength and stiffness, behaviour nonlinearity and occurrence of plastic strains prior to peak strength, significant softening after peak, time-dependent creep deformations and permeability increase due to damage. Both saturated and unsaturated conditions are envisaged. The constitutive model is then applied to the simulation of triaxial and creep tests on Callovo-Oxfordian (COx) claystone. Although the main objective has been the simulation of the COx claystone behaviour, the model can be readily used for other argillaceous materials. The constitutive model developed is then applied, via a suitable coupled hydromechanical formulation, to the analysis of the excavation of a drift in the Meuse/Haute-Marne Underground Research Laboratory. The pattern of observed pore water pressures and displacements, as well as the shape and extent of the damaged zone, are generally satisfactorily reproduced. The relevance and importance of rock anisotropy and of the development of a damaged zone around the excavations are clearly demonstrated.Peer ReviewedPostprint (author's final draft

Numerical simulation of the undrained stability of slopes in anisotropic fine-grained soils

This is an Accepted Manuscript of an article published by Taylor & Francis Group in Geomechanics and geoengineering on 2019, available online at: http://www.tandfonline.com/10.1080/17486025.2018.1490460The undrained stability of slopes in anisotropic fine-grained soils is studied in this paper using the finite element method (FEM). A constitutive model is presented, able to account for the observed variation of undrained strength with loading direction. The model is able to encompass the different strength distributions observed in normally, slightly overconsolidated and heavily overconsolidated soils. A series of stability analyses have been performed to explore the effect of the type of undrained strength anisotropy on the stability and failure mechanisms of slopes of different inclinations. In addition, a real case study of the failure of an underwater slope is analysed with the numerical approach presented. It suggests that, by considering undrained strength anisotropy, the failure can be satisfactorily explained.Peer ReviewedPostprint (author's final draft

Nonlocal plasticity modelling of strain localisation in stiff clays

The paper addresses the numerical simulation of strain localisation in stiff clays that exhibit softening behaviour. An elastoplastic constitutive model developed to incorporate key features of stiff clay behaviour is described first. A non-local formulation is then introduced for the regularisation of the analysis of localisation. A series of analyses were conducted to explore relevant aspects of the numerical simulation of localisation. A 3D analysis was also performed to assess the suitability of the approach presented for 3D applications. Finally, application to the simulation of a laboratory test on Beaucaire marl results in an excellent reproduction of experimental observations.Peer ReviewedPostprint (author's final draft

Analysis of strain localization with a non-local plasticity model

Strain localization can be defined as the accumulation of deformations in narrow zones, in the form of shear bands or cracks, commonly observed in several geomaterials like concrete, rocks, dense sands, or stiff fine-grained soils. The numerical simulation of such phenomenon under the framework of continuum mechanics involves a number of difficulties, as standard formulations tend to deliver nonobjective results due to the loss of ellipticity of the governing equation at the onset of localization. In the present paper, a non-local plasticity model is described, capable of simulating strain localization objectively, without resulting in mesh-dependent results. The model is intended for modelling stiff fine-grained soils, thus the yield surface and its evolution with deformation captures the main behaviour characteristics of these materials. A number of analyses have been performed to assess the developed formulation for the simulation of localized deformation patterns. Relevant aspects have been addressed, such as the thickness of the shear band, its orientation, and the onset of localization in a boundary value problem. In addition, a real plane strain experiment on Beaucaire marl [2] has been simulated, and the results of the analysis are compared not only with global measurements, but with the entire strain field, observed experimentally using the false relief stereophotogrammetry technique [1]. The behaviour of the test has been satisfactorily reproduced in terms of both, global measurements and strain fields.Postprint (published version

Numerical simulation of underground excavations in an indurated clay using non-local regularisation. Part 2: Sensitivity analysis

A sensitivity study is presented to evaluate the influence of different parameters on the simulation of an underground excavation in the Callovo-Oxfordian (COx) argillaceous formation performed in the Meuse/Haute-Marne underground research laboratory. An elasto-viscoplastic constitutive law representing the characteristic behaviour of indurated mudrocks and stiff clays has been employed. It incorporates anisotropy, strain-softening, creep deformations and dependence of permeability on damage. In addition, a non-local formulation, able to simulate localised deformations objectively, has been incorporated in the analyses. The following features affecting the excavation have been studied: initial stress, strength and stiffness anisotropy, strength parameters, hydraulic and hydromechanical parameters, and scale effects. A simulation reported in a companion paper provides the base case for benchmarking. The results are compared in terms of extent and configuration of the excavation fractured zone, vertical and horizontal tunnel convergences, and the development and evolution of pore pressures in the rock. From the comparisons, an enhanced understanding of the hydromechanical mechanisms associated with underground excavations in COx claystone, and other similar argillaceous materials, has been achieved.We are grateful for the financial and technical assistance of the French national radioactive waste management agency (Andra) to the work presented. The technical assistance of Plaxis is also gratefully acknowledged. The first author has been supported by a Conacyt scholarship (Reg. No. 270190).Peer ReviewedPostprint (author's final draft

Numerical simulation of underground excavations in an indurated clay using non-local regularisation. Part 1: formulation and base case

The paper presents the numerical simulation of an underground excavation in the Callovo-Oxfordian argillaceous formation, performed in the Meuse/Haute-Marne underground research laboratory, within the context of deep geological nuclear waste disposal. The constitutive model adopted incorporates a number of characteristic features of the hydromechanical behaviour of the Callovo-Oxfordian claystone, and other indurated clays, including anisotropy and time-dependent deformations. Model parameters have been largely determined from available experimental results. Particular attention has been given to the incorporation of a non-local formulation in order to simulate localised deformations objectively. It is shown that the numerical analysis is able to provide a very satisfactory reproduction of the extent and configuration of the excavation-induced fractured zone, the development of rock displacements and the evolution of water pressures in the rock. The analysis is also able to provide a deeper insight into the mechanisms underlying the generation of water pressures inside the rock mass.Peer ReviewedPostprint (author's final draft

Time domain deconvolution in nonlinear elastoplastic soil deposits

The paper presents an iterative procedure for the time domain deconvolution in nonlinear elastoplastic materials. The approach is intended for the generation of input motions for dynamic soil–structure interaction (DSSI) numerical analyses when the desired earthquake is specified at the surface of a nonlinear soil deposit. The main advantage is that the same constitutive model (or models) to be used in the DSSI simulation to characterise the soil deposit is also employed in the deconvolution procedure. Therefore, the desired surface motion is recovered from the free-field propagation of the resulting input motion at the base of the numerical model, accounting for the assumed constitutive behaviour of the ground. An application example is also presented, where the potential of the proposed approach is shown.Peer ReviewedPostprint (published version

Analysis of underground excavations in argillaceous hard soils : weak rocks

Materiales arcillosos rígidos, que se encuentran en la zona de transición entre suelos duros y rocas blandas, están siendo considerados actualmente en varios países como la roca huésped para el almacenamiento geológico profundo de residuos nucleares de alta actividad y larga vida. Esta posibilidad a derivado en la construcción de laboratorios de investigación subterráneos (LIS), excavados en estos materiales arcillosos rígidos, para estudiar su comportamiento bajo condiciones reales de trabajo. Entre los diferentes aspectos estudiados en los LIS, el comportamiento hidromecánico de la roca huésped es el más relevante para la presente investigación. Observaciones in situ han revelado que las excavaciones inducen daño alrededor de los túneles, en la forma de redes de fracturas, contenidas dentro de una zona llamada la zona de daño de la excavación (ZDE). La ZDE se ha identificado como una de las principales causas afectando el comportamiento de las excavaciones. En este contexto, el principal objetivo de la presente investigación es la simulación numérica del comportamiento hidromecánico de excavaciones experimentales llevadas a cabo en el LIS Meuse/Haute-Marne (Francia). Para lograrlo, se desarrolló un modelo constitutivo para caracterizar la roca huésped. El modelado de estos materiales es una tarea desafiante. Estos materiales exhiben características más propias de los suelos como considerables deformaciones plásticas, dependencia con la velocidad de carga, y creep, aunque también muestran características más típicas de las rocas como un considerable reblandecimiento y deformaciones plásticas localizadas. Además, debido a su origen sedimentario, también exhiben anisotropía en propiedades como su rigidez, resistencia, y permeabilidad. Se prestó especial atención a la reproducción de la ZDE y, por lo tanto, a la simulación objetiva de deformaciones localizadas; se empleó un enfoque no local para la regularización del continuo, el cual evita la dependencia con la malla empleada. Los resultados obtenidos proporcionan importantes conclusiones respecto al comportamiento hidromecánico de estos materiales arcillosos rígidos, e indican los principales aspectos que afectan la respuesta de las excavaciones subterráneas. En particular, se demuestra la importancia de la ZDE.Stiff clayey materials, lying in the transition between hard soils and weak rocks, are being currently considered in several countries as possible host medium for deep geological disposal of high active and long-lived nuclear waste. This possibility has led to the construction of underground research laboratories (URL), excavated in these indurated clayey materials, to study their behaviour under real working conditions. Among the very different topics addressed in the URLs, the hydromechanical behaviour of the host rock is the one that most concerns the present research. In situ observations have revealed that excavation operations induce damage around the galleries, in the form of fracture networks, contained within a zone called excavation damaged zone (EDZ). The EDZ has been identified as one of the main aspects affecting the behaviour of the excavations. In this context, the main objective of the present study is the numerical simulation of the hydromechanical behaviour of experimental excavations performed at the Meuse/Haute-Marne URL (France). For this purpose, a constitutive model has been developed to characterise the host formation. The modelling of these stiff argillaceous materials is a quite challenging task. They exhibit soil-like features like considerable plastic strains, rate-dependency, and creep, although they also show characteristics more typical of a rock such as significant softening and localised deformations. In addition, due to their sedimentary origin, they often exhibit anisotropy in properties like stiffness, strength, and permeability. Special attention has been paid to the reproduction of the EDZ and, therefore, to the objective simulation of localised deformations; a nonlocal approach has been employed for the regularisation of the continuum, avoiding the dependence on the employed mesh. The obtained results provide relevant insights into the hydromechanical behaviour of these stiff clayey materials, and they indicate the main aspects affecting the response of the underground excavations. In particular, the relevance of the EDZ has been demonstrated

Analysis of underground excavations in argillaceous hard soils : weak rocks

Materiales arcillosos rígidos, que se encuentran en la zona de transición entre suelos duros y rocas blandas, están siendo considerados actualmente en varios países como la roca huésped para el almacenamiento geológico profundo de residuos nucleares de alta actividad y larga vida. Esta posibilidad a derivado en la construcción de laboratorios de investigación subterráneos (LIS), excavados en estos materiales arcillosos rígidos, para estudiar su comportamiento bajo condiciones reales de trabajo. Entre los diferentes aspectos estudiados en los LIS, el comportamiento hidromecánico de la roca huésped es el más relevante para la presente investigación. Observaciones in situ han revelado que las excavaciones inducen daño alrededor de los túneles, en la forma de redes de fracturas, contenidas dentro de una zona llamada la zona de daño de la excavación (ZDE). La ZDE se ha identificado como una de las principales causas afectando el comportamiento de las excavaciones. En este contexto, el principal objetivo de la presente investigación es la simulación numérica del comportamiento hidromecánico de excavaciones experimentales llevadas a cabo en el LIS Meuse/Haute-Marne (Francia). Para lograrlo, se desarrolló un modelo constitutivo para caracterizar la roca huésped. El modelado de estos materiales es una tarea desafiante. Estos materiales exhiben características más propias de los suelos como considerables deformaciones plásticas, dependencia con la velocidad de carga, y creep, aunque también muestran características más típicas de las rocas como un considerable reblandecimiento y deformaciones plásticas localizadas. Además, debido a su origen sedimentario, también exhiben anisotropía en propiedades como su rigidez, resistencia, y permeabilidad. Se prestó especial atención a la reproducción de la ZDE y, por lo tanto, a la simulación objetiva de deformaciones localizadas; se empleó un enfoque no local para la regularización del continuo, el cual evita la dependencia con la malla empleada. Los resultados obtenidos proporcionan importantes conclusiones respecto al comportamiento hidromecánico de estos materiales arcillosos rígidos, e indican los principales aspectos que afectan la respuesta de las excavaciones subterráneas. En particular, se demuestra la importancia de la ZDE.Stiff clayey materials, lying in the transition between hard soils and weak rocks, are being currently considered in several countries as possible host medium for deep geological disposal of high active and long-lived nuclear waste. This possibility has led to the construction of underground research laboratories (URL), excavated in these indurated clayey materials, to study their behaviour under real working conditions. Among the very different topics addressed in the URLs, the hydromechanical behaviour of the host rock is the one that most concerns the present research. In situ observations have revealed that excavation operations induce damage around the galleries, in the form of fracture networks, contained within a zone called excavation damaged zone (EDZ). The EDZ has been identified as one of the main aspects affecting the behaviour of the excavations. In this context, the main objective of the present study is the numerical simulation of the hydromechanical behaviour of experimental excavations performed at the Meuse/Haute-Marne URL (France). For this purpose, a constitutive model has been developed to characterise the host formation. The modelling of these stiff argillaceous materials is a quite challenging task. They exhibit soil-like features like considerable plastic strains, rate-dependency, and creep, although they also show characteristics more typical of a rock such as significant softening and localised deformations. In addition, due to their sedimentary origin, they often exhibit anisotropy in properties like stiffness, strength, and permeability. Special attention has been paid to the reproduction of the EDZ and, therefore, to the objective simulation of localised deformations; a nonlocal approach has been employed for the regularisation of the continuum, avoiding the dependence on the employed mesh. The obtained results provide relevant insights into the hydromechanical behaviour of these stiff clayey materials, and they indicate the main aspects affecting the response of the underground excavations. In particular, the relevance of the EDZ has been demonstrated.Postprint (published version

Efecto de la excavación de túneles en una roca arcillosa

En el laboratorio subterráneo de Meuse-Haute Marne (Francia), se han desarrollado extensas investigaciones para examinar el efecto de la excavación de túneles en una roca arcillosa: la argilita del Callovian-Oxfordian. Se ha prestado especial atención a la generación de una zona dañada alrededor de la excavación. El hecho de que la roca tenga una resistencia relativa-mente limitada implica que esta zona dañada pueda alcanzar una extensión significativa. Se ha demostrado que el daño en la roca se manifiesta en una serie de fracturas de tensión y de corte en la zona cercana al túnel. Una observación importante es que la configuración de la zona dañada y las deformaciones que se producen como consecuencia de la excavación dependen considera-blemente del estado inicial de las tensiones in situ. La anisotropía en el comportamiento mecánico de la roca también ejerce una influencia notable. En el presente trabajo se presenta una modelización numérica de la generación de la zona dañada por la exca-vación del túnel. La modelización tiene características especiales que son clave para poder reproducir la localización de defor-maciones y la generación de fracturas: el uso de un modelo constitutivo que tiene en cuenta las características básicas de este ti-po de rocas y la incorporación de una formulación no local para poder abordar de forma consistente problemas que involucran materiales cuasi-frágiles y reblandecibles. Se demuestra que el modelo resultante es capaz de reproducir satisfactoriamente las observaciones de campo.Postprint (published version