A procedure for the direct determination of Bishop's chi parameter from changes in pore size distribution

Permission is granted by ICE Publishing to print one copy for personal use. Any other use of these PDF files is subject to reprint fees.Most of the recent works relating to the concept of effective stress in unsaturated soils focus on the proposal by Bishop, and, more particularly, on the search for suitable relationships between Bishop's ¿ parameter and the main controlling variables. These relationships are generally formulated by theoretical derivations and back-analyses of the dependency of mechanical parameters on hydraulic variables such as suction or saturation. In this note, a new procedure is proposed to evaluate directly, and without any a priori assumptions, values for Bishop's ¿ parameter. In the first part, a general derivation based on the definition of work conjugated variables allows the ¿ parameter to be defined as the ratio of the change of water volume over the change in pore volume during a process at constant suction. This definition is further exploited to evaluate Bishop's parameter from the changes suffered by material pore size distribution during loading. The method is applied to data obtained by mercury intrusion porosimetry tests on low-plasticity silt (Jossigny silt), low-plasticity sandy clay (lean clay) and highly plastic clay (Febex clay). Values obtained for these materials show that the ¿ parameter is close to the effective degree of saturation rather than the total degree of saturation.Peer ReviewedPostprint (published version

Analysis of strain localization with a nonlocal plasticity model

In the present paper a nonlocal plasticity model is described, intended to reproduce the mechanical behaviour of stiff fine-grained soils, including the objective simulation of strain localization; the phenomenon of accumulation of deformations in narrow zones in the form of shear bands or fractures. A number of analyses have been performed to assess the developed formulation. 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 (BVP). Results provide useful insigths into relevant aspects of the numerical simulation of strain localization

Thermo-hydro-mechanical simulation of a full-scale steel-lined micro-tunnel excavated in the callovooxfordian claystone

The paper presents an interpretation of the full-scale ALC1604 in situ heating test carried out in Callovo-Oxfordian claystone (COx) in the Meuse/Haute-Marne underground research laboratory (MHM URL). The MHM URL is a site-specific facility planned to study radioactive waste disposal in the COx. The thermo-hydro-mechanical (THM) behaviour of the host rock is significant for the design of the underground radioactive waste disposal facility and for its long-term safety. When subjected to thermal loading, the Callovo-Oxfordian claystone of low permeability (~10-20-10-21 m2) exhibits a strong pore pressure response that significantly affects the hydraulic and mechanical behaviour of the material. The observations gathered in the in situ test have provided an opportunity to examine the integrated thermo-hydromechanical (THM) response of this sedimentary clay. Coupled THM numerical analyses have been carried out to provide a structured framework for interpretation, and to enhance understanding of THM behaviour of COx. Numerical analyses have been based on a coupled theoretical formulation that incorporates a constitutive law specially developed for this type of material. The law includes a number of features that are relevant for a satisfactory description of the hydromechanical behaviour. By performing the numerical analysis, it has been possible to incorporate anisotropy of material parameters and of in situ stresses. The performance and analysis of the in situ tests have significantly enhanced the understanding of a complex THM problem and have proved the capability of the numerical formulation to provide adequate predictive capacity

Coupled THM analysis of long-term anisotropic convergence in the full-scale micro tunnel excavated in the Callovo-Oxfordian argillite

The main purpose of this paper is to analyse the convergence measurements of the ALC1604 in situ heating test carried out in the Callovo-Oxfordian claystone formation (COx) in the Meuse/Haute-Marne underground research laboratory (MHM URL). The concept of the test consists of horizontal micro-tunnel, equipped with a steel casing. The micro-tunnel is excavated in the direction of the horizontal principal major stress (sH). In situ observations showed anisotropic convergence with the maximum and minimum values in the horizontal and vertical directions, respectively. Coupled THM numerical analyses have been carried out to provide a structured framework for interpretation, and to enhance understanding of THM behaviour of Callovo-Oxfordian claystone. However, a special mechanical constitutive law is adopted for the description of the time-dependent anisotropic behaviour of the COx. The simulation of the test using this enhanced model provides a satisfactory reproduction of the THM long-term anisotropic convergence results. It also provides a better understanding of the observed test response.Postprint (published version

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

An interior - point algorithm for hyper-plastic models for soils

In numerical analysis of geotechnical problems it is often necessary to use sophisticated elastoplastic constitutive models. Particularly, the presence of friction as the fundamental mechanism to dissipate energy provides most of the models used in this field with a non-associated character. This characteristic introduces theoretical and numerical complexity at time of deriving the models from thermo-mechanical principles and implementing them. The paper presents the formulation and implementation for a family of non-associated models based on the hyper-plasticity approach. The paper starts with the hyper-plastic framework, that proves to be a powerful technique to derive evolution equations using standard thermo-mechanical procedures based on the differentiation of the energy expressions (free Helmholtz energy fs, Gibbs energy gs, and dissipation d), even for non-associated laws. The characterization of the variational structure behind the discrete equations of the Interior-Point projection of approximation is discussed in the second part. Thirdly is described the implementation of the models using convex programming theory. The integration of the incremental response is achieved following a classical primal-dual interior-point algorithm which presents good performance to resolve saddle-point problems. This algorithm has the particularity to solve the optimality conditions associated with a suitably penalized mathematical program taking Newton steps or damped Newton steps toward points on the central path or quasi-central path previously de ned. The global convergence characteristic of the algorithm is achieved introducing an appropriate line search scheme. The algorithm is finally tested on initial-boundary-value problem for an elastoplastic continuum of Cam clay type. The performance of the algorithm is discussed on the basis of the results obtained.Peer ReviewedPostprint (published version

Caracterización experimental de la respuesta al corte de aridos para Balasto

"Si el ferrocarril lograba sobreponerse a las adversidades del siglo XX pasaría a ser el medio de transporte del siglo XXI", se ha dicho en varias ocasiones. Esto puede verse reflejado en el crecimiento que han experimentado las líneas de alta velocidad en la primer década del siglo XXI alrededor del mundo. Esta necesidad de mantener la competitividad con otros medios de transporte, ha obligado al ferrocarril a profundizar en materia de investigación brindando soluciones que lo hagan más eficiente, principalmente reduciendo los costes relacionados a mantenimiento e infraestructura. En el caso de las vías en balasto, aproximadamente el 90% de las vías que existen en el mundo, el coste de mantenimiento puede ser reducido significativamente resolviendo una de las problemáticas que conlleva un elevado coste, la durabilidad de la capa de balasto. El deterioro de las partículas que componen la capa de balasto tiene origen en las vibraciones que se generan con el paso de los ejes de los vehículos, provocando distintos tipos de rotura en las partículas de balasto que disminuyen la capacidad del conjunto a resistir cargas y por tanto conservar una geometría de vía adecuada.De los tipos de rotura que se originan, los que más perjudican a la durabilidad del conjunto son la rotura por fragmentación y por abrasión. Este tipo de deterioro además de redondear los áridos y de quitar la trabazón del conjunto, generan finos que al acumularse modifican el comportamiento mecánico de la capa de balasto. En base a esta problemática se está llevando a cabo una investigación que consiste en recubrir los áridos con neumático reciclado con el objeto de disminuir el deterioro que soportan los áridos aumentando el área y disminuyendo la rigidez del contacto entre partículas. A éste árido recubierto se lo denomina NEOBALLAST. Otro beneficio asociado que se intenta obtener es el de disminuir las vibraciones y el ruido que se generan con el paso de trenes reutilizando caucho reciclado, brindando de esta manera una solución ambiental a la problemática de los neumáticos fuera de uso. En la presente Tesina se describe el comportamiento de los mencionados áridos ensayados en el Corte Directo Tradicional y el Corte Directo de Gran Tamaño del Laboratorio de Geotecnia de la UPC y en base a los parámetros obtenidos se realiza un análisis de la influencia que tiene el recubrimiento aplicado sobre el árido original. Este ensayo es el paso previo a la realización de ensayos dinámicos y ayuda a definir el tipo de recubrimiento más conveniente para continuar con las siguientes etapas de proyecto. De esta manera se contribuye a la búsqueda de alternativas que solucionen la problemática existente en la capa de balasto y se complementan áreas de estudio, empleando ensayos tradicionalmente de Mecánica de Suelos y Geotecnia para estudiar la infraestructura ferroviaria

Modelling of a deep excavation in a stiff clay

This extended abstract presents an interpretation of the response of a mudstone during the excavation of a shaft at high depths (500m). Instrument layout and numerical model are described. A very good agreement is observed between the field measurements and computed values.Peer ReviewedPostprint (published version

Consequences on water retention properties of double-porosity features in a compacted silt

The paper deals with an experimental investigation aimed at studying microstructural features and their consequences on water retention properties of statically compacted unsaturated silt. The evolution of the microstructure of the aggregate fabric induced by compaction is investigated by studying the pore size distribution changes under different initial conditions (void ratio and water content). The material used is low plasticity silt from Jossigny near Paris, France. A series of mercury intrusion porosimetry tests (MIP) were performed at different void ratios and water contents to provide microstructural information. The arrangement of aggregation/particles and pore network was also investigated with environmental scanning electron microscopy (ESEM). The MIP data were used to determine the water retention curve on drying for the specific pore network configuration induced on compaction. The MIP data were used to formulate and calibrate a multimodal water retention model for a specific pore network configuration, which is obtained by linear superposition of subcurves of a modified van Genuchten type. The study is then complemented with controlled suction oedometer tests on compacted samples to obtain the water retention properties of the material at two different void ratios. Finally, we compare the water retention properties obtained by the simulated progression of the different pore network configurations induced on the hydraulic path with the water retention properties under suction-controlled conditions. Good agreement between the two methods for the drying path is reache

Thermal conductivity of argillaceous rocks: determination methodology using in situ heating tests

This study focuses on the characterisation of thermal conductivity for three potential host rocks for radioactive waste disposal. First, the heat conduction process is reviewed on the basis of an analytical solution and key aspects related to anisotropic conduction are discussed. Then the existing information on the three rocks is summarised and a broad uncertainty range of thermal conductivity is estimated based on the mineralogical composition. Procedures to backanalyse the thermal conductivity on the basis of in situ heating tests are assessed and a methodology is put forward. Finally, this methodology is used to estimate the impact of experimental uncertainties and applied to the four in situ heating tests. In the three potential host rocks, a clear influence of the bedding planes was identified and anisotropic heat conduction was shown to be necessary to interpret the observed temperature field. Experimental uncertainties were also shown to induce a larger uncertainty on the anisotropy ratio than on the equivalent thermal conductivity defined as the geometric mean of the thermal conductivity in the three principal directions