Themed issue on selected papers SEG2015: part II

This themed issue of Geomechanics for Energy and the Environment ‘SEG-2015-Part II’, presents papers selected from extended abstracts submitted to the Symposium on Energy Geotechnics (SEG-2015) held in the Civil Engineering School of the Universitat Politècnica de Catalunya in Barcelona, Spain, between June 2nd and 4th, 2015. This symposium SEG-2015 was the first event organized by the Technical Committee TC308 of the International Society for Soil Mechanics and Geotechnical Engineering, which will have continuity in the next symposium SEG-2018 to be held in the École Polytechnique Fédérale de Lausanne (Lausanne, Switzerland). The first issue ‘SEG-2015-Part I’ appeared just before the 1st International Conference on Energy Geotechnics (ICEG-2016), which was held in Kiel (Germany) at the end of August 2016. The second volume ‘SEG-2015-Part II’ on December 2016 will thus close a first cycle of successful conferences and publications along this initial stage of TC308.Peer ReviewedPostprint (author's final draft

Yielding of rockfill in relative humidity-controlled triaxial experiments

The final publication is available at Springer via http://dx.doi.org/10.1007/s11440-016-0437-9The paper reports the results of suction controlled triaxial tests performed on compacted samples of two well graded granular materials in the range of coarse sand-medium gravel particle sizes: a quartzitic slate and a hard limestone. The evolution of grain size distributions is discussed. Dilatancy rules were investigated. Dilatancy could be described in terms of stress ratio, plastic work input and average confining stress. The shape of the yield locus in a triaxial plane was established by different experimental techniques. Yielding loci in both types of lithology is well represented by approximate elliptic shapes whose major axis follows approximately the Ko line. Relative humidity was found to affect in a significant way the evolution of grain size distribution, the deviatoric stress-strain response and the dilatancy rules.Peer ReviewedPostprint (author's final draft

Ice formation in unsaturated frozen soils

This paper presents a procedure for determining unfrozen water saturation in a partially saturated frozen soil (clayey silt) using bulk electrical conductivity (EC) measurements. A modification of Archie’s law is proposed to describe the relationship between soil bulk EC, temperature, porosity and degree of unfrozen water saturation. Compacted samples have been prepared at a dry density around 1.90 Mg/m3 and at dif-ferent degrees of saturation. Samples have been then subjected to freezing paths up to -15 °C. Measurements of bulk EC along the temperature decrease and freezing paths have been used to calibrate parameters associ-ated with the proposed model. These calibrated models allow determining the amount of ice content for a given state of the partially saturated soil (porosity, initial degree of water saturation and temperature). The soil freezing retention curve has been also estimated by combining the Clausius-Clapeyron equation with water retention data on drying. A good agreement has been observed between the estimation based on EC measurements and results from water retention data, which validates the proposed procedure.Postprint (published version

Pore size distribution effects on the hydro-chemo-mechanical behaviour of bentonite

Material and its preparation. Hydro – mechanical behaviour in oedometer conditions with 0.05 M , 0.10 M and 5.5 M NaCl solutions. Chemical induced swelling of samples saturated with a 5.5 M NaCl solutionPeer ReviewedPostprint (author’s final draft

Assessment of the volume change behaviour of clay aggregates by ESEM observations

Peer ReviewedPostprint (author’s final draft

Hygroscopicity issues in powder and grain technology

The effect of hygroscopicity on flowability of powders and bulk solids with applications in the packaging industry is experimentally and numerically investigated. Firstly, four granular materials are tested at different water contents to study the impact of relative humidity on some hydro-mechanical properties, namely the hydraulic diffusivity on wetting, as well as the shear strength and compressibility properties of the materials. Next, a capillary model covering a wider water content range –compared to the previous tests– is applied to discrete element simulations of a granular column collapse set-up. These simulations give further insight into important aspects of grain hygroscopicity in packaging and other industrial applications (such as the kinematics of flow and flowability issues), which are outside the scope of conventional experimental testing.Postprint (published version

Coupled thermo-hydro-mechanical behaviour of a deep clay

An experimental study on Ypresian clays –one of the potential deep formations in Belgium for the geological disposal of heat-emitting radioactive waste– was undertaken to systematically study its thermal properties and coupled hydro-mechanical response during fast heating pulse tests. An accurate characterisation of the thermal properties is required for assessing the near-field perturbations around disposal galleries that the sedimentary host rock formation will undergo. A new experimental cell adapted to apply the high in situ stresses and with thermal flux sensors was used to directly measure the thermal conductivity at different sample orientations (heat flux orthogonal and parallel to bedding planes). A clear influence of the degree of saturation – despite being close to saturation – and anisotropic features on thermal conductivity have been detected. The study was complemented by performing fast heating pulse tests under constant volume on a new and fully-instrumented axisymmetric cell. The cell allowed recording the pore pressure build-up and dissipation along a heating pulse and under water-undrained conditions.Postprint (published version

Correlation between chemical and mineralogical characteristics and permeability of phyllite clays using multivariate statistical analysis

Phyllite clays are applied as a layer on a surface to be waterproofed and subsequently compacted. For this purpose, phyllite clays deposits can be grouped by their chemical and mineralogical characteristics, and these characteristics can be connected with their properties, mainly permeability, in order to select those deposits with the lowest permeability values. Several deposits of phyllite clays in the provinces of Almería and Granada (SE Spain) have been studied. The results of applying a multivariate statistical analysis (MVA) to the chemical data analysed from 52 samples determined by XRF, mineralogical analysis by XRD and permeability are reported. Permeability, a characteristic physical property of phyllite clays, was calculated using the results for experimental nitrogen gas adsorption and nitrogen adsorption-desorption permeability dependence. According to the results, permeability values differentiated two groups, i.e. group 1 and group 2, with two subgroups in the latter. The influence of chemical as well as mineralogical characteristics on the permeability values of this set of phyllite clays was demonstrated using a multiple linear regression model. Two regression equations were deduced to describe the relationship between adsorption and desorption permeability values, which support this correlation. This was an indication of the statistical significance of each chemical and mineralogical variable, as it was added to the model. The statistical tests of the residuals suggested that there was no serious autocorrelation in the residuals.Peer ReviewedPostprint (author's final draft

A multi-scale insight into gas transport in a deep Cenozoic clay

The migration of gases is crucial to ensure the long-term feasibility of argillaceous formations for the deep disposal of radioactive waste. This paper presents an experimental investigation with a multi-scale perspective on the response to gas transport of initially saturated Boom Clay (Belgium). Gas injection tests have been performed under oedometer conditions at different controlled-volume rates, constant total vertical stress and different sample orientations (flow orthogonal or parallel to bedding planes). The results confirm soil expansion and consequent degradation during injection that has a significant impact on the aperture of localised gas pathways (fissures) and increases intrinsic permeability during the gas pressure dissipation stage. The analyses with complementary techniques (mercury intrusion porosimetry, field-emission scanning electron microscopy and X-ray micro-tomography) confirm the opening of fissures with different apertures and separations at the microstructural scale. Large-aperture fissures develop along the weaker bedding planes. These techniques allow the volume of fissures to be quantified, which does not significantly depend on gas flow direction, as also measured in the isotropic response of the gas effective permeability. A scalar damage variable derived from the fissured fraction has been used to assess the gas-entry pressure reduction and the intrinsic permeability increase after the gas tests in both directions.The authors are grateful to ONDRAF/NIRAS (Belgian National Agency for Radioactive Waste and Enriched Fissile Material) for funding this research programme under contract no. XSI/AV/2012-1952 (2012–2016) and no. CCHO 2018-0089/00/00 (2018–2020). This project has received funding from the European Union's Horizon 2020 research and innovation programme ‘European Joint Programme on Radioactive Waste Management’ EURAD (2019-2024) WP-Gas ‘Mechanistic understanding of gas transport in clay materials’ under the grant agreement No. 847593. The Spanish ‘Severo Ochoa Programme for Centres of Excellence in R&D’ (CEX2018-000797-S) is also acknowledged.Peer ReviewedPostprint (published version