Dashnor Hoxha

Thermo-Mechanical Behaviour of an Innovant Straw Lime Composite for Thermal Insulation Applications

This paper presents an investigation on the use of new light-weight construction material, composed of lime, water and cereal straw fiber. Two types of fibers were used: wheat and barley straw. The influence of some parameters such as fiber types, binder types (lime and/ or lime-cement), fiber to binder ratio (F/B) and Water to Binder ratio (W/B) on the mechanical and thermal properties is studied. Compressive strength, thermal conductivity and density of the material were investigated. The results indicated that the thermal conductivity of the straw-lime composites decreased with increasing straw content. The result comparisons also revealed that the composite reinforced by wheat straw fibres has the highest compressive strength.

Thermo-hydro-mechanical behaviour of tuffeau stone masonry

This paper deals with thermo-hydromechanical behaviour of a stone masonry wall assembled with mortar. Macroscopic modelling is proposed that allows an evaluation of the impact of everyday thermal and hydraulic variations on the generation of variations in effective stress. The thermo-hydromechanic parameters of the unsaturated stone have been estimated by experimental tests for tuffeau, such as that found in the Chambord monument. An inverse identification has been used for the optimisation of these parameters based on in- situ measurements, before using the model for long-term analysis. The results of this numerical study show that the contrast between the properties alone is not sufficient to generate a stress that is superior to the stone strength. Otherwise, the variations in thermal and hydraulic conditions on the surface of the stone could lead to a fatigue-like degradation of historic monuments. Cet article s’intéresse au comportement thermo-hydromécanique d’un mur maçonné en pierre liée avec un mortier. Nous proposons une modélisation macroscopique qui permet d’évaluer l’impact de la variation quotidienne hydromécanique à la génération des variations des contraintes effectives au sein de la pierre. Les paramètres utilisés dans le modèle sont ceux identifiés au cours d’essais de laboratoire sur des échantillons de pierres similaires à ceux du château de Chambord. Une identification inverse a été utilisée pour les paramètres inconnus basée sur les mesures in situ. Les résultats de nos modélisations montrent que seul le contraste des propriétés ne conduit pas à des valeurs de contraintes suffisantes pour générer la rupture de la pierre. En revanche les variations cycliques d’hygrométrie et de température conduisent à des variations cycliques de la contrainte effective et la contrainte totale susceptibles d’endommager la pierre par fatigue.

On the numerical evaluation of historical stone sculpture artwork restoration

Traditionally, the restoration of historical artworks found during archaeological expeditions is based on the experience and artistic feeling of restaurateur as well as on a simplified analyse considering parts of artworks as rigid solids. Taking advantages from the progress of digitising techniques and virtually restoration of artworks, it is now possible to perform advanced (geo)mechanical analysis in order to evaluate the pertinence of alternative restoration solutions. In this paper, two alternative restoration hypotheses were studied for the statue of Neptune of Arles: in one hand, a metallic-insert-based restoration technique and on the other hand, a glue-based restoration. For the insert-based restoration scenario, the (geo)mechanical analysis aims at verifying of general stability and the local resistance of stone, especially at stress concentration zones as well as the optimisation of inserts. For the glue-based restoration hypothesis, firstly the linear fracture mechanic is used to evaluate the stress intensity factors at the tips of eventual glue-flaws. Then a parametric dynamic analysis is performed in order to identify the limits of glue-based restoration to resist typical transport vibration. The results of our analyses show that, unless special dumping devices are used for road transports, the glue-based restoration would fail against tensile stress induced in interfaces due to road vibrations.

Temperature and Pressure Dependence of the Effective Thermal Conductivity of Geomaterials: Numerical Investigation by the Immersed Interface Method

The present work aims to study the nonlinear effective thermal conductivity of heterogeneous composite-like geomaterials by using a numerical approach based on the immersed interface method (IIM). This method is particularly efficient at solving the diffusion problem in domains containing inner boundaries in the form of perfect or imperfect interfaces between constituents. In this paper, this numerical procedure is extended in the framework of non linear behavior of constituents and interfaces. The performance of the developed tool is then demonstrated through the studies of temperature- and pressure-dependent effective thermal conductivity of geomaterials with imperfect interfaces.

A closed-form hydro-mechanical solution for deep tunnels in elastic anisotropic rock

In this work, a closed-form hydro-mechanical solution for stresses and displacements around a deep tunnel excavated in an anisotropic poro-elastic medium is presented. The developed analytical solution is conducted by using the well-known complex potential approach and including the hydraulic effect. The validation of this analytical solution is done by comparing with the numerical results obtained from the finite element method Aster-Code. Through some numerical applications, the effect of the anisotropic poro-elastic behaviour of rock mass on the distribution of stresses and displacements around the tunnel will be detailed. The obtained results demonstrated the essential role of pore pressure and the hydro-mechanical coupling on the response of the tunnel and the analytical solution developed in this contribution would be useful for tunnel design thanks to its quick evaluation of the stress and displacement fields in the medium.

Modelling of two-phase flow in capillary porous medium by a microscopic discrete approach

The aim of this work is to study the fluid flow in porous media by a microscopic approach. For that, the microstructure of a porous material is represented by a network of capillary pipes with geometric and flow characteristics chosen in such a way as to fit both the pore volume and water saturated hydraulic conductivity of a studied porous material. In saturated state, a pipe is supposed to have a Kozney–Carman hydraulic conductivity. In partially saturated conditions, the “menisci model” as proposed by has been used. Such a representation is sufficient to satisfactory reproduce mostly of laboratory results on flow properties of a porous material in saturated and partially saturated conditions as demonstrated by our numerical results on a porous stone.

Hydromechanical Properties of some Mortars Used in some Ecologic Construction Techniques

This paper presents results of hydromechanical characterization tests performed on some mortars used in eco-construction practice. Typically, such mortars could be found in buildings constructed following so called GREB technique that uses straw bales as structural and insulating elements in addition to a wood frame. The full experimental program includes thermal, mechanical and hydraulic – hygroscopic tests. Mechanical tests, including uniaxial compression test and three point bending test and hydraulic tests including water and vapor water permeability, retention curve and unsaturated water permeability have been performed on three earth-cement mortars with sawdust additive. Tests were performed in age of 7, 14, 28 and 120 days. For retention curve and so called relative permeability a simple method has been used based on measurements of masse variations of samples on a controlled humidity environment and an inverse problem approach. Using of sawdust improves hydraulic properties of these mortars but the early age strength of these mortars has to be improved by cement additives.

Contribution to the Behavior Study and Collapse Risk of Underground Cavities in Highly Saline Geological Formations

The problem of saline soils reserved from occupation at Oran, North Algeria remained relatively unexplored or little known until recent years. Consequently, some studies were conducted, especially to characterize the real impact of an airport on these soils. The characterization of the real problems of saline soils, as well as the study of the behavior of their collapse under the coupled effect of thermal, mechanical and hydraulic, remains poorly known specially under an airport, where the instabilities and the risks of sudden collapses is an unknown problem for the authorities and citizens and the impact on the environment is not mastered. Under the action of water charged with carbon dioxide which dissolves the limestone, chalk or gypsum, many natural cavities are created. There are also pockets of dissolution filled with silt in the chalk, due to the irregularity of the contact chalk/silt. The flow of water can also enlarge the fractures at depth causing the silt that fills them and thus creating a surface subsidence due to infiltration. This phenomenon is found mainly in the dry valleys. To account for the effect of the hydro-thermomechanical coupling in predicting the collapse of saline soils, solutions were proposed for improvement of saline soil with a geosynthetic reinforcement, drainage, etc. These solutions are necessary for the proper design of airfield runways to avoid a disaster.

On the Modeling of Transition from a Diffuse to a Localized Damage

An approach to model long term hydromechanical behavior of rock masses around underground excavations is proposed. Implemented in a classical finite element numerical code this approach combine in one hand a continuum model to describe the progressive damage and strain localization and a fracture mechanics XFEM based numerical procedure to model the behavior of fractured mass once the macrofracturing takes places.

Fractured Reservoirs Modeling by Embedded Fracture Continuum Approach: Field-Scale Applications

In this paper, the recently developed Embedded Fracture Continuum (EFC) approach will be used to model the fractured reservoir at the large scale. This novel approach borrows the concept of continuum models and incorporates the effect of fractures explicitly by using the fracture cell concept which represents the grid mesh intersected by one or many fractures in the medium. Each fracture cell presents a porous medium that has its own properties calculated from the contributed properties of intact matrix and fractures. The considered problem consists of simulating the primary depletion of fractured reservoir in which the short and medium fractures will be accounted for implicitly in the homogenized porous medium through the upscaling procedure while the embedded long fracture networks are explicitly taken into account. Through this application, we demonstrate and highlight the performance of the EFC approach.