A microstructurally based effective stress for unsaturated soils / E. E. Alonso in Géotechnique, Vol. 60 N° 12 (Décembre 2010)  

Public ISBD [article]  inGéotechnique > Vol. 60 N° 12 (Décembre 2010) . - pp. 913–925 Titre : A microstructurally based effective stress for unsaturated soils Type de document : texte imprimé Auteurs : E. E. Alonso, Auteur ; J. M. Pereira, Auteur ; J. Vaunat, Auteur Année de publication : 2011 Article en page(s) : pp. 913–925 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Pore pressures Constitutive relations Suction Stiffness Shear strength Partial saturation Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : Current alternative choices of stress state variables in unsaturated soils are described and compared, with a special focus on the use of an effective stress. Experimental data on stiffness and shear strength evolution with suction suggest that the proportion of suction contributing to the effective stress is often much smaller than predicted by the term ‘suction times degree of saturation' generally used in effective stress expressions of the Bishop type. It is suggested that effective stress in unsaturated soils should be related to soil microstructure. An effective degree of saturation is defined as describing the volume of water partially filling the soil macroporosity. This effective degree of saturation defines the proportion of the prevailing suction that actually contributes to the effective stress. Two alternative expressions (piecewise linear and non-linear) are proposed for the effective degree of saturation. They offer a similar performance. Available data on stiffness and shear strength variation with suction of a few different soils, ranging from a markedly granular material to high-plasticity clay, have been analysed. The analysis supports the proposed microstructural interpretation of the effective stress. Indeed, for granular soils the effective degree of saturation is almost equal to the total degree of saturation, and therefore the Bishop-type expression generally used as an effective stress is recovered. As the soil becomes more plastic, the proportion of free water reduces, and the contribution of suction to the effective stress reduces. At the limit, when the proportion of free water is negligible (this is the case of high-plasticity clays at high values of suction) the proposed effective stress reduces to the net stress (excess of total stress over the air pressure). The proposed effective stress equation may be identified if information on the amount of immobile water is available for a given soil. Water retention or porosimetry data provide this information. This has been shown by comparing the present proposal with independently obtained information about immobile water in high-plasticity clays. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.002 [article] A microstructurally based effective stress for unsaturated soils [texte imprimé] / E. E. Alonso, Auteur ; J. M. Pereira, Auteur ; J. Vaunat, Auteur . - 2011 . - pp. 913–925. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 60 N° 12 (Décembre 2010) . - pp. 913–925 Mots-clés : Pore pressures Constitutive relations Suction Stiffness Shear strength Partial saturation Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : Current alternative choices of stress state variables in unsaturated soils are described and compared, with a special focus on the use of an effective stress. Experimental data on stiffness and shear strength evolution with suction suggest that the proportion of suction contributing to the effective stress is often much smaller than predicted by the term ‘suction times degree of saturation' generally used in effective stress expressions of the Bishop type. It is suggested that effective stress in unsaturated soils should be related to soil microstructure. An effective degree of saturation is defined as describing the volume of water partially filling the soil macroporosity. This effective degree of saturation defines the proportion of the prevailing suction that actually contributes to the effective stress. Two alternative expressions (piecewise linear and non-linear) are proposed for the effective degree of saturation. They offer a similar performance. Available data on stiffness and shear strength variation with suction of a few different soils, ranging from a markedly granular material to high-plasticity clay, have been analysed. The analysis supports the proposed microstructural interpretation of the effective stress. Indeed, for granular soils the effective degree of saturation is almost equal to the total degree of saturation, and therefore the Bishop-type expression generally used as an effective stress is recovered. As the soil becomes more plastic, the proportion of free water reduces, and the contribution of suction to the effective stress reduces. At the limit, when the proportion of free water is negligible (this is the case of high-plasticity clays at high values of suction) the proposed effective stress reduces to the net stress (excess of total stress over the air pressure). The proposed effective stress equation may be identified if information on the amount of immobile water is available for a given soil. Water retention or porosimetry data provide this information. This has been shown by comparing the present proposal with independently obtained information about immobile water in high-plasticity clays. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.002

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On-sample water content measurement for a complete local monitoring in triaxial testing of unsaturated soils / J. A. Munoz-Castelblanco in Géotechnique, Vol. 62 N° 7 (Juillet 2012)  

Public ISBD [article]  inGéotechnique > Vol. 62 N° 7 (Juillet 2012) . - pp. 595 –604 Titre : On-sample water content measurement for a complete local monitoring in triaxial testing of unsaturated soils Type de document : texte imprimé Auteurs : J. A. Munoz-Castelblanco, Auteur ; P. Delage, Auteur ; J. M. Pereira, Auteur Année de publication : 2012 Article en page(s) : pp. 595 –604 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Suction Partial saturation Laboratory equipment Résumé : To provide a complete local monitoring of the state of an unsaturated soil sample during triaxial testing, a local water content measurement device was adapted to a triaxial device comprising the measurement of local displacements (Hall effect transducers) and suction (high-capacity transducer). Water content was locally monitored by means of a resistivity probe. The water content/resistivity calibration curves of an intact, natural, unsaturated loess from northern France extracted by block sampling at two depths (1 and 3·3 m) were carefully determined, showing good accuracy and repeatability. The validity of two models giving the resistivity of unsaturated soils with respect to their water content was examined. The first triaxial tests carried out with this device in the range of in situ stresses gave satisfactory results, but with some effects of the applied cell stress on the water content measurements. Some preliminary behaviour characteristics of the natural unsaturated loess, a material rarely tested up to now in the literature, were evidenced. Also, the yield stresses appeared significantly higher than the in situ stresses, confirming the combined effect of partial saturation and bonding. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.10.P.129 [article] On-sample water content measurement for a complete local monitoring in triaxial testing of unsaturated soils [texte imprimé] / J. A. Munoz-Castelblanco, Auteur ; P. Delage, Auteur ; J. M. Pereira, Auteur . - 2012 . - pp. 595 –604. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 62 N° 7 (Juillet 2012) . - pp. 595 –604 Mots-clés : Suction Partial saturation Laboratory equipment Résumé : To provide a complete local monitoring of the state of an unsaturated soil sample during triaxial testing, a local water content measurement device was adapted to a triaxial device comprising the measurement of local displacements (Hall effect transducers) and suction (high-capacity transducer). Water content was locally monitored by means of a resistivity probe. The water content/resistivity calibration curves of an intact, natural, unsaturated loess from northern France extracted by block sampling at two depths (1 and 3·3 m) were carefully determined, showing good accuracy and repeatability. The validity of two models giving the resistivity of unsaturated soils with respect to their water content was examined. The first triaxial tests carried out with this device in the range of in situ stresses gave satisfactory results, but with some effects of the applied cell stress on the water content measurements. Some preliminary behaviour characteristics of the natural unsaturated loess, a material rarely tested up to now in the literature, were evidenced. Also, the yield stresses appeared significantly higher than the in situ stresses, confirming the combined effect of partial saturation and bonding. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.10.P.129

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Preliminary study on the mechanical behaviour of heat exchanger pile in physical model / A. Kalantidou in Géotechnique, Vol. 62 N° 11 (Novembre 2012)  

Public ISBD [article]  inGéotechnique > Vol. 62 N° 11 (Novembre 2012) . - pp. 1047 –1051 Titre : Preliminary study on the mechanical behaviour of heat exchanger pile in physical model Type de document : texte imprimé Auteurs : A. Kalantidou, Auteur ; A. M. Tang, Auteur ; J. M. Pereira, Auteur Année de publication : 2012 Article en page(s) : pp. 1047 –1051 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Settlement Soil/structure interaction Temperature effects Model tests Résumé : The effects of temperature changes on the mechanical behaviour of heat exchanger piles need to be taken into account in geotechnical design. In this preliminary study, the behaviour of an axially loaded pile under thermal cycles was investigated using a physical model. After applying the axial load on the pile head, the pile was heated from 25°C to 50°C and subsequently cooled to 25°C. Four tests (corresponding to four values of axial load) were performed and two temperature cycles were undertaken in each test. The pile temperature and the pile head displacement were monitored. When low axial loads were applied, the heating induced heave and cooling induced settlement of the pile head. In this case, the displacement–temperature curve was found to be reversible and similar to the thermal expansion curve of the pile. However, in the case of higher axial loads, the heave of the pile head, obtained during heating, was lower than the thermal expansion of the pile, suggesting settlement of the pile toe. Irreversible settlement of the pile head was observed after these thermal cycles. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.11.T.013 [article] Preliminary study on the mechanical behaviour of heat exchanger pile in physical model [texte imprimé] / A. Kalantidou, Auteur ; A. M. Tang, Auteur ; J. M. Pereira, Auteur . - 2012 . - pp. 1047 –1051. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 62 N° 11 (Novembre 2012) . - pp. 1047 –1051 Mots-clés : Settlement Soil/structure interaction Temperature effects Model tests Résumé : The effects of temperature changes on the mechanical behaviour of heat exchanger piles need to be taken into account in geotechnical design. In this preliminary study, the behaviour of an axially loaded pile under thermal cycles was investigated using a physical model. After applying the axial load on the pile head, the pile was heated from 25°C to 50°C and subsequently cooled to 25°C. Four tests (corresponding to four values of axial load) were performed and two temperature cycles were undertaken in each test. The pile temperature and the pile head displacement were monitored. When low axial loads were applied, the heating induced heave and cooling induced settlement of the pile head. In this case, the displacement–temperature curve was found to be reversible and similar to the thermal expansion curve of the pile. However, in the case of higher axial loads, the heave of the pile head, obtained during heating, was lower than the thermal expansion of the pile, suggesting settlement of the pile toe. Irreversible settlement of the pile head was observed after these thermal cycles. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.11.T.013

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Strain-rate effects in deep marine clays from the Gulf of Guinea / S. S. Torisu in Géotechnique, Vol. 62 N° 9 (Septembre 2012)  

Public ISBD [article]  inGéotechnique > Vol. 62 N° 9 (Septembre 2012) . - pp. 767 –775 Titre : Strain-rate effects in deep marine clays from the Gulf of Guinea Type de document : texte imprimé Auteurs : S. S. Torisu, Auteur ; J. M. Pereira, Auteur ; V. De Gennaro, Auteur Année de publication : 2012 Article en page(s) : pp. 767 –775 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Creep Laboratory tests Stress path Fabric/structure of soils Time dependence Shear strength Clays Résumé : Current developments in deepwater offshore oil production in the Gulf of Guinea (GoG) make it necessary to characterise the mechanical behaviour of the GoG deepwater sediments better. This paper examines some aspects of the strain-rate-dependent behaviour of intact and reconstituted GoG clay specimens. Results from isotropic compression and undrained shearing laboratory tests with changing rates of stress and strain are presented. They show that the soil generally follows an isotach behaviour, with clear strain-rate dependence of the stress–strain behaviour and pore pressure response in the undrained shear tests. The effects of structure and its degradation on the time-dependent response of the sediments seem to confirm general trends observed in other structured clays. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.12.OG.015 [article] Strain-rate effects in deep marine clays from the Gulf of Guinea [texte imprimé] / S. S. Torisu, Auteur ; J. M. Pereira, Auteur ; V. De Gennaro, Auteur . - 2012 . - pp. 767 –775. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 62 N° 9 (Septembre 2012) . - pp. 767 –775 Mots-clés : Creep Laboratory tests Stress path Fabric/structure of soils Time dependence Shear strength Clays Résumé : Current developments in deepwater offshore oil production in the Gulf of Guinea (GoG) make it necessary to characterise the mechanical behaviour of the GoG deepwater sediments better. This paper examines some aspects of the strain-rate-dependent behaviour of intact and reconstituted GoG clay specimens. Results from isotropic compression and undrained shearing laboratory tests with changing rates of stress and strain are presented. They show that the soil generally follows an isotach behaviour, with clear strain-rate dependence of the stress–strain behaviour and pore pressure response in the undrained shear tests. The effects of structure and its degradation on the time-dependent response of the sediments seem to confirm general trends observed in other structured clays. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.12.OG.015

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The water retention properties of a natural unsaturated loess from northern France / J. A. Munoz-Castelblanco in Géotechnique, Vol. 62 N° 2 (Fevrier 2012)  

Public ISBD [article]  inGéotechnique > Vol. 62 N° 2 (Fevrier 2012) . - pp. 95 –106 Titre : The water retention properties of a natural unsaturated loess from northern France Type de document : texte imprimé Auteurs : J. A. Munoz-Castelblanco, Auteur ; J. M. Pereira, Auteur ; P. Delage, Auteur Année de publication : 2012 Article en page(s) : pp. 95 –106 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Suction Permeability Laboratory tests Résumé : The water retention properties of a natural loess from northern France were investigated on intact block samples that were excavated along a high-speed train line (TGV) that experienced stability problems during heavy rain episodes. Suction measurements were made by using the filter paper method and an in-house constructed high-capacity tensiometer (HCT) so as to determine the water retention curve of the loess. The results showed that an alternative approach using a previously wetted filter paper could fruitfully be used with suction values reasonably compatible with the HCT measurements. The water retention curve obtained exhibited a peculiar form, with no hysteresis observed around the natural water content (14·4%) and two hysteresis loops on both the wet and dry sides of the curve. It is hypothesised that this zone with no hysteresis corresponds to the natural variations of the water content under seasonal changes, providing a range of natural suction between 20 and 80 kPa, and it is suggested that this should be checked by in situ measurements. The loess microstructure was investigated by using scanning electron microscopy and mercury intrusion porosimetry. It appeared that the clay fraction (16%) was not uniformly distributed, with some areas composed of clean grains and a well-graded corresponding intergrain pore population. Conversely, the clay aggregations define a smaller-sized porosity. The calculation of a water retention curve derived from the pore size distribution curve showed that water retention is governed by capillarity in the largest pores between clean grains, whereas clay adsorption becomes dominant in the smallest pores, a zone where the microstructure is sensitive to change in water content. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.9.P.084 [article] The water retention properties of a natural unsaturated loess from northern France [texte imprimé] / J. A. Munoz-Castelblanco, Auteur ; J. M. Pereira, Auteur ; P. Delage, Auteur . - 2012 . - pp. 95 –106. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 62 N° 2 (Fevrier 2012) . - pp. 95 –106 Mots-clés : Suction Permeability Laboratory tests Résumé : The water retention properties of a natural loess from northern France were investigated on intact block samples that were excavated along a high-speed train line (TGV) that experienced stability problems during heavy rain episodes. Suction measurements were made by using the filter paper method and an in-house constructed high-capacity tensiometer (HCT) so as to determine the water retention curve of the loess. The results showed that an alternative approach using a previously wetted filter paper could fruitfully be used with suction values reasonably compatible with the HCT measurements. The water retention curve obtained exhibited a peculiar form, with no hysteresis observed around the natural water content (14·4%) and two hysteresis loops on both the wet and dry sides of the curve. It is hypothesised that this zone with no hysteresis corresponds to the natural variations of the water content under seasonal changes, providing a range of natural suction between 20 and 80 kPa, and it is suggested that this should be checked by in situ measurements. The loess microstructure was investigated by using scanning electron microscopy and mercury intrusion porosimetry. It appeared that the clay fraction (16%) was not uniformly distributed, with some areas composed of clean grains and a well-graded corresponding intergrain pore population. Conversely, the clay aggregations define a smaller-sized porosity. The calculation of a water retention curve derived from the pore size distribution curve showed that water retention is governed by capillarity in the largest pores between clean grains, whereas clay adsorption becomes dominant in the smallest pores, a zone where the microstructure is sensitive to change in water content. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.9.P.084

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