Browsing by Author "Hueckel, T"

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Open Access
A micro-scale inspired chemo-mechanical model of bonded geomaterials
(International Journal of Rock Mechanics and Mining Sciences, 2015-12-01) Gajo, A; Cecinato, F; Hueckel, T
© 2015 Elsevier Ltd.Chemical processes influence the mechanical properties of geomaterials, resulting in either strengthening or weakening effects, the latter being particularly critical for long-term safety assessment in civil and energy engineering. Coupling of chemical and mechanical processes in cemented soils and rocks is investigated starting form a micro-structural chemo-mechanical model. The model consists of an assembly of grains and bonds undergoing dissolution or precipitation of mineral mass, affecting geometrical characteristics of the assembly. The principal such characteristics are the evolution of specific surface area and of bond cross-sectional area at the micro-scale, and of porosity at the macro-scale, which become key variables linking the micro-scale and macro-scale mechanisms. This framework has the advantage of avoiding unphysical situations, such as the occurrence of mineral precipitation with no pore space available or the occurrence of dissolution with no cementing material left. The evolution of important micromechanical quantities, such as the number of active bonds and their cross section is tracked. At the macro-scale, a reactive chemo-plasticity model is combined with a model for bonded geomaterials. The resulting micro- to macro-scale transition, schematically applicable to both materials with reactive grains and bonds and materials with only reacting bonds, is validated against the available experimental evidence, concerning calcarenite with both reactive bonds and grains made of the same mineral. The model is thus shown to provide a flexible framework for a consistent interpretation of experimental loading paths, and can be readily extended to more complex circumstances.
Open Access
Efects of mineral suspension and dissolution on strength and compressibility of soft carbonate rocks
(Engineering Geology, 2015-01-04) Ciantia, MO; Castellanza, R; Crosta, GB; Hueckel, T
© 2014 Elsevier B.V.Calcarenites are highly porous soft rocks formed of mainly carbonate grains bonded together by calcite bridges. The above characteristics make them prone to water-induced weathering, frequently featuring large caverns and inland natural underground cavities. This study is aimed to determine the main physical processes at the base of the short- and long-term weakening experienced by these rocks when interacting with water. We present the results of microscale experimental investigations performed on calcarenites from four different sites in Southern Italy. SEM, thin sections, X-ray CT observations and related analyses are used for both the interpretation-definition of the structure changes, and the identification-quantification of the degradation mechanisms. Two distinct types of bonding have been identified within the rock: temporary bonding (TB) and persistent bonding (PB). The diverse mechanisms linked to these two types of bonding explain both the observed fast decrease in rock strength when water fills the pores (short-term effect of water), identified with a short-term debonding (STD), and a long-term weakening of the material, when the latter is persistently kept in water-saturated conditions (long-term effect of water), identified with a long-term debonding (LTD). To highlight the micro-hydro-chemo-mechanical processes of formation and annihilation of the TB bonds and their role in the evolution of the mechanical strength of the material, mechanical tests on samples prepared by drying partially saturated calcarenite powder, or a mix of glass ballotini and calcarenite powder were conducted. The long-term debonding processes have also been investigated, using acid solutions in order to accelerate the reaction rates. This paper attempts to identify and quantify differences between the two types of bonds and the relative micro-scale debonding processes leading to the macro-scale material weakening mechanisms.
Open Access
Evaporation-induced evolution of the capillary force between two grains
(Granular Matter, 2014-01-01) Mielniczuk, B; Hueckel, T; Youssoufi, MSE
© 2014, Springer-Verlag Berlin Heidelberg.The evolution of capillary forces during evaporation and the corresponding changes in the geometrical characteristics of liquid (water) bridges between two glass spheres with constant separation are examined experimentally. For comparison, the liquid bridges were also tested for mechanical extension (at constant volume). The obtained results reveal substantial differences between the evolution of capillary force due to evaporation and the evolution due to extension of the liquid bridges. During both evaporation and extension, the change of interparticle capillary forces consists in a force decrease to zero either gradually or via rupture of the bridge. At small separations between the grains (short & wide bridges) during evaporation and at large volumes during extension, there is a slight initial increase of force. During evaporation, the capillary force decreases slowly at the beginning of the process and quickly at the end of the process; during extension, the capillary force decreases quickly at the beginning and slowly at the end of the process. Rupture during evaporation of the bridges occurs most abruptly for bridges with wider separations (tall and thin), sometimes occurring after only 25% of the water volume was evaporated. The evolution (pinning/depinning) of two geometrical characteristics of the bridge, the diameter of the three-phase contact line and the “apparent” contact angle at the solid/liquid/gas interface, seem to control the capillary force evolution. The findings are of relevance to the mechanics of unsaturated granular media in the final phase of drying.
Open Access
Laplace pressure evolution and four instabilities in evaporating two-grain liquid bridges
(Powder Technology, 2015-10-01) Mielniczuk, B; Hueckel, T; El Youssoufi, MS
© 2015 Elsevier B.V.Dynamic variables characterizing evolution during evaporation of capillary bridge between two spheres are analyzed. The variables include: average Laplace pressure, pressure resulting force, surface tension force and total capillary force calculated based on the previously reported geometrical variables using Young-Laplace law [1,2]. This is the first time to our knowledge that Laplace pressure is calculated from the measured bridge curvatures along the process of evaporation and compared to experimental measurement data. A comparison with the experimental data from analogous capillary bridge extension tests is also shown and discussed.The behavior of evaporating liquid bridges is seen as strongly dependent on the grain separation. Initial negative Laplace pressure at small separations is seen to significantly augment during an advanced stage of evaporation, but to turn into positive pressure, after an instability toward the end of the process, and prior to rupture. At larger separations the pressure is positive all the time, changing a little, but rupturing early. Rupture in all cases occurs at positive pressure. However, because of the evolution of the surface area of contact, the resultant total capillary forces are always tensile, and decreasing toward zero in all cases. Comparison between measured total resultant capillary forces and those calculated from the Young-Laplace law is very good, except for some discrepancies at very small separations (below 50. μm). Up to four consecutive instabilities of capillary bridge are seen developing at some sphere separations. They are: re-pinning-induced suction (pressure) instability; Rayleigh nodoid/catenoid/unduloid unstable transition, associated with zero-pressure; Rayleigh unduloid/cylinder unstable transition, associated with the formation of a liquid-wire; and lastly, a pinching instability of the liquid-wire, associated with the bridge rupture. Rupture of the bridges is seen at large separations to occur quite early, at only 1/4-1/3 of the initial water volume evaporated. At smallest separations, rupture occurs in a seemingly unstable way when water evaporates from the bridge thinnest section of the neck.
Open Access
Microbially induced calcite precipitation effect on soil thermal conductivity
(Geotechnique Letters, 2016-01-06) Venuleo, S; Laloui, L; Terzis, D; Hueckel, T; Hassan, M
© 2016, Thomas Telford Services Ltd. All rights reserved.Efficiency of energy piles is strongly affected by soil saturation conditions: low water contents considerably decrease their performance thus limiting the possibility to extend their application to arid environments. This paper investigates the microbially induced calcite precipitation (MICP) technique as a potential means of enhancing the soil–pile heat exchange rates by improving the thermal properties of soil. The study puts the focus on measuring the thermal conductivity of untreated and treaded sand at various degrees of saturation. Experimental results clearly show a significant improvement of the thermal conductivity of soil especially for low degrees of saturation. This enhancement is attributed to the mineralised calcite crystals acting as ‘thermal bridges’ between the soil grains, offering a larger surface area for heat exchange compared with the untreated material in which exchanges occur through smaller contact points.
Open Access
Silica polymer bonding of stressed silica grains: An early growth of intergranular tensile strength
(Geomechanics for Energy and the Environment, 2015-01-01) Guo, R; Hueckel, T
© 2015 Elsevier Ltd. All rights reserved.Laboratory tests on microscale are reported in which millimeter-sized amorphous silica cubes were kept highly compressed in a liquid environment of de-ionized water solutions with different silica ion concentrations for up to four weeks. Such an arrangement simulates an early evolution of bonds between two sand grains stressed in situ. In-house designed Grain Indenter-Puller apparatus allowed measuring strength of such contacts after 3-4 weeks. Observations reported for the first time confirm a long-existing hypothesis that a stressed contact with microcracks generates silica polymers, forming a bonding structure between the grains on a timescale in the order of a few weeks. Such structure exhibits intergranular tensile force at failure of 1-1.5 mN when aged in solutions containing silica ion concentrations of 200-to 500-ppm. The magnitude of such intergranular force is 2-3 times greater than that of water capillary force between the same grains.