Documents disponibles écrits par cet auteur

Centrifuge and numerical modeling of normal fault-rupture propagation in clay with and without a preexisting fracture / C. W.W. Ng in Journal of geotechnical and geoenvironmental engineering, Vol. 138 N° 12 (Décembre 2012) 

Public ISBD [article]  in Journal of geotechnical and geoenvironmental engineering > Vol. 138 N° 12 (Décembre 2012) . - pp. 1492–1502 Titre : Centrifuge and numerical modeling of normal fault-rupture propagation in clay with and without a preexisting fracture Type de document : texte imprimé Auteurs : C. W.W. Ng, Auteur ; Q. P. Cai, Auteur ; P. Hu, Auteur Année de publication : 2013 Article en page(s) : pp. 1492–1502 Note générale : Géotechnique Langues : Anglais (eng) Mots-clés : Centrifuge  model  Geological  faults  Soil  cement  Earthquakes Résumé : Over the last few decades, extensive research has been conducted to investigate fault-rupture propagation induced by faulting through uncemented soil. While it has long been realized that many natural soils are cemented, relatively few publications have been devoted to investigating fault-rupture propagation in cemented soil. Moreover, preexisting fracture in soil may be reactivated during earthquakes, and its influence on fault-rupture propagation is not fully understood. In this study, three centrifuge model tests are reported on investigating fault-rupture propagation induced by normal faulting in uncemented clay and cemented clay with and without a preexisting fracture. Preliminary numerical analyses of the centrifuge tests are also carried out to study the mechanism of fault-rupture propagation in these three ground conditions. Ground deformation was dominated by a shear mechanism in uncemented clay, and a shear zone along the projection of the bedrock-fault plane was observed. On the other hand, a bending deformation mechanism was identified at the ground surface in cemented clay with and without a preexisting fracture. In the presence of cementation, two main ruptures were observed, where one initiated at the cemented ground surface and propagated downward toward the center of the cemented clay, whereas the other initiated at the bedrock fault and propagated upward. The presence of a fracture in cemented clay was found to initiate a fault rupture at the tip of the preexisting fracture. ISSN : 1090-0241 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29GT.1943-5606.0000719 [article] Centrifuge and numerical modeling of normal fault-rupture propagation in clay with and without a preexisting fracture [texte imprimé] / C. W.W. Ng, Auteur ; Q. P. Cai, Auteur ; P. Hu, Auteur . - 2013 . - pp. 1492–1502. Géotechnique Langues : Anglais (eng) in Journal of geotechnical and geoenvironmental engineering > Vol. 138 N° 12 (Décembre 2012) . - pp. 1492–1502 Mots-clés : Centrifuge  model  Geological  faults  Soil  cement  Earthquakes Résumé : Over the last few decades, extensive research has been conducted to investigate fault-rupture propagation induced by faulting through uncemented soil. While it has long been realized that many natural soils are cemented, relatively few publications have been devoted to investigating fault-rupture propagation in cemented soil. Moreover, preexisting fracture in soil may be reactivated during earthquakes, and its influence on fault-rupture propagation is not fully understood. In this study, three centrifuge model tests are reported on investigating fault-rupture propagation induced by normal faulting in uncemented clay and cemented clay with and without a preexisting fracture. Preliminary numerical analyses of the centrifuge tests are also carried out to study the mechanism of fault-rupture propagation in these three ground conditions. Ground deformation was dominated by a shear mechanism in uncemented clay, and a shear zone along the projection of the bedrock-fault plane was observed. On the other hand, a bending deformation mechanism was identified at the ground surface in cemented clay with and without a preexisting fracture. In the presence of cementation, two main ruptures were observed, where one initiated at the cemented ground surface and propagated downward toward the center of the cemented clay, whereas the other initiated at the bedrock fault and propagated upward. The presence of a fracture in cemented clay was found to initiate a fault rupture at the tip of the preexisting fracture. ISSN : 1090-0241 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29GT.1943-5606.0000719

Effects of wetting–drying and stress ratio on anisotropic stiffness of an unsaturated soil at very small strains / C. W.W. Ng in Canadian geotechnical journal, Vol. 46 N° 9 (Septembre 2009) 

Public ISBD [article]  in Canadian geotechnical journal > Vol. 46 N° 9 (Septembre 2009) . - pp. 1062–1076 Titre : Effects of wetting–drying and stress ratio on anisotropic stiffness of an unsaturated soil at very small strains Type de document : texte imprimé Auteurs : C. W.W. Ng, Auteur ; J. Xu, Auteur ; S. Y. Yung, Auteur Article en page(s) : pp. 1062–1076 Note générale : Sciences de la Terre Langues : Anglais (eng) Mots-clés : Stiffness  anisotropy  Very  small  strain  shear  modulus  Unsaturated  Completely  decomposed  tuff  Wetting–drying  Stress  ratio  Rigidité  anisotrope  Module  de  cisaillement  à  très  faibles  déformations  Non  saturé  Tuff  complètement  décomposé  Mouillage–drainage  Ratio  des  contraintes Index. décimale : 550 Sciences auxiliaires de la géologie. Résumé : The very small strain shear modulus of soil, G0, is affected by many factors including soil properties, current stress state, stress history, and matric suction. Very little research has been conducted on anisotropic shear moduli of unsaturated soils. In this study, the effects of wetting–drying and stress ratio on anisotropic shear stiffness of an unsaturated completely decomposed tuff (CDT) at very small strains have been investigated using a modified triaxial testing system equipped with three pairs of bender elements. During drying and wetting tests, the measured very small strain shear moduli increased in a nonlinear fashion, but at a reduced rate as the matric suction increased. Similar to the stress-dependent soil-water characteristic curves (SDSWCCs), there was hysteresis between the drying and wetting curves showing the variations in shear moduli with matric suction. Variation in suction on the specimens under isotropic conditions produced changes in stiffness anisotropy (expressed as G0(hh)/G0(hv)) together with anisotropic strains. In shearing tests at constant suctions, significant stress-induced stiffness anisotropy was observed due to a change in the stress ratio. While shearing at a constant stress ratio, G0(hh)/G0(hv) appeared to be constant. Le module de cisaillement à très faibles déformations du sol, G0, est affecté par plusieurs facteurs, dont les propriétés du sol, l’état actuel des contraintes, l’historique des contraintes et la succion matricielle. Très peu de recherches ont été effectuées sur les modules de cisaillement anisotropes de sols non saturés. Dans cette étude, les effets du mouillage-drainage et du ratio des contraintes sur la rigidité en cisaillement anisotrope d’un sol non saturé de tuff complètement décomposé (TCD) à très faibles déformations ont été évalués à l’aide d’un système d’essai triaxial modifié qui était pourvu de trois paires d’éléments piézocéramiques. Durant les essais en drainage et en mouillage, le module de cisaillement à faibles déformations mesuré a augmenté de façon linéaire mais à un taux qui diminue selon l’augmentation de la succion matricielle. Il y avait des effets d’hystérésis entre les courbes en mouillage et en drainage montrant les variations des modules de cisaillement selon la succion matricielle, de façon semblable aux courbes caractéristiques sol-eau dépendantes des contraintes. Des variations de succion sur les échantillons soumis à des conditions isotropes ont causé des variations de rigidité anisotrope (exprimé G0(hh)/G0(hv)) combinées à des déformations anisotropes. Durant les essais en cisaillement à succion constante, de la rigidité anisotrope induite par les contraintes a été observée, due au changement du ratio des contraintes. Pendant le cisaillement à un ratio de contraintes constant, G0(hh)/G0(hv ) semble demeurer constant. DEWEY : 550 ISSN : 0008-3674 En ligne : http://rparticle.web-p.cisti.nrc.ca/rparticle/AbstractTemplateServlet?calyLang=f [...] [article] Effects of wetting–drying and stress ratio on anisotropic stiffness of an unsaturated soil at very small strains [texte imprimé] / C. W.W. Ng, Auteur ; J. Xu, Auteur ; S. Y. Yung, Auteur . - pp. 1062–1076. Sciences de la Terre Langues : Anglais (eng) in Canadian geotechnical journal > Vol. 46 N° 9 (Septembre 2009) . - pp. 1062–1076 Mots-clés : Stiffness  anisotropy  Very  small  strain  shear  modulus  Unsaturated  Completely  decomposed  tuff  Wetting–drying  Stress  ratio  Rigidité  anisotrope  Module  de  cisaillement  à  très  faibles  déformations  Non  saturé  Tuff  complètement  décomposé  Mouillage–drainage  Ratio  des  contraintes Index. décimale : 550 Sciences auxiliaires de la géologie. Résumé : The very small strain shear modulus of soil, G0, is affected by many factors including soil properties, current stress state, stress history, and matric suction. Very little research has been conducted on anisotropic shear moduli of unsaturated soils. In this study, the effects of wetting–drying and stress ratio on anisotropic shear stiffness of an unsaturated completely decomposed tuff (CDT) at very small strains have been investigated using a modified triaxial testing system equipped with three pairs of bender elements. During drying and wetting tests, the measured very small strain shear moduli increased in a nonlinear fashion, but at a reduced rate as the matric suction increased. Similar to the stress-dependent soil-water characteristic curves (SDSWCCs), there was hysteresis between the drying and wetting curves showing the variations in shear moduli with matric suction. Variation in suction on the specimens under isotropic conditions produced changes in stiffness anisotropy (expressed as G0(hh)/G0(hv)) together with anisotropic strains. In shearing tests at constant suctions, significant stress-induced stiffness anisotropy was observed due to a change in the stress ratio. While shearing at a constant stress ratio, G0(hh)/G0(hv) appeared to be constant. Le module de cisaillement à très faibles déformations du sol, G0, est affecté par plusieurs facteurs, dont les propriétés du sol, l’état actuel des contraintes, l’historique des contraintes et la succion matricielle. Très peu de recherches ont été effectuées sur les modules de cisaillement anisotropes de sols non saturés. Dans cette étude, les effets du mouillage-drainage et du ratio des contraintes sur la rigidité en cisaillement anisotrope d’un sol non saturé de tuff complètement décomposé (TCD) à très faibles déformations ont été évalués à l’aide d’un système d’essai triaxial modifié qui était pourvu de trois paires d’éléments piézocéramiques. Durant les essais en drainage et en mouillage, le module de cisaillement à faibles déformations mesuré a augmenté de façon linéaire mais à un taux qui diminue selon l’augmentation de la succion matricielle. Il y avait des effets d’hystérésis entre les courbes en mouillage et en drainage montrant les variations des modules de cisaillement selon la succion matricielle, de façon semblable aux courbes caractéristiques sol-eau dépendantes des contraintes. Des variations de succion sur les échantillons soumis à des conditions isotropes ont causé des variations de rigidité anisotrope (exprimé G0(hh)/G0(hv)) combinées à des déformations anisotropes. Durant les essais en cisaillement à succion constante, de la rigidité anisotrope induite par les contraintes a été observée, due au changement du ratio des contraintes. Pendant le cisaillement à un ratio de contraintes constant, G0(hh)/G0(hv ) semble demeurer constant. DEWEY : 550 ISSN : 0008-3674 En ligne : http://rparticle.web-p.cisti.nrc.ca/rparticle/AbstractTemplateServlet?calyLang=f [...]

A field study of stress-dependent soil–water characteristic curves and permeability of a saprolitic slope in Hong Kong / C. W.W. Ng in Géotechnique, Vol. 61 N° 6 (Juin 2011) 

Public ISBD [article]  in Géotechnique > Vol. 61 N° 6 (Juin 2011) . - pp. 511–521 Titre : A field study of stress-dependent soil–water characteristic curves and permeability of a saprolitic slope in Hong Kong Type de document : texte imprimé Auteurs : C. W.W. Ng, Auteur ; H. N. Wong, Auteur ; Y. M. Tse, Auteur Année de publication : 2011 Article en page(s) : pp. 511–521 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Field  instrumentation  Permeability  Pore  pressures  Suction  In  Situ  testing  Partial  saturation  Slopes Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : It is well recognised that field measurements of stress-dependent soil–water characteristic curves (SDSWCCs) and the permeability function (coefficient of permeability with respect to the water phase) are vital for assessing transient seepage, pore water pressure changes and the stability of unsaturated soil slopes. In this study, SDSWCCs and the permeability function were measured by using the instantaneous profile method on a saprolitic hillside in Tung Chung, Hong Kong. The ground profile comprises colluvium and completely decomposed tuff (CDT). A 3 m diameter circular plot was instrumented and subjected to two wetting–drying cycles. Throughout a 48-day monitoring period, variations of volumetric water content and matrix suction in the uppermost 3 m of soil were measured continuously using time-domain reflectometry moisture probes and jet-fill tensiometers, respectively. The field observations reveal that there is a clear trend of decreasing sizes of hysteretic loops of SDSWCCs with depth. In a comparison of the field- and laboratory-measured SDSWCCs, general agreement can be seen in the first wetting–drying cycle but not in the second cycle. The field-observed unsaturated permeability function is highly dependent on suction history (wetting–drying cycles). The field-measured permeability ranges from 4 × 10−7 m/s to 3 × 10−4 m/s in the colluvium, whereas the corresponding measured values vary from 4 × 10−7 m/s to 1 × 10−4 m/s in the CDT. Predictions of SWCC and permeability function using existing empirical approaches based on particle size distributions (PSDs) should be treated with caution. There is a clear qualitative and quantitative inconsistency between predictions of permeability function from PSD and field measurements. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.157 [article] A field study of stress-dependent soil–water characteristic curves and permeability of a saprolitic slope in Hong Kong [texte imprimé] / C. W.W. Ng, Auteur ; H. N. Wong, Auteur ; Y. M. Tse, Auteur . - 2011 . - pp. 511–521. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 61 N° 6 (Juin 2011) . - pp. 511–521 Mots-clés : Field  instrumentation  Permeability  Pore  pressures  Suction  In  Situ  testing  Partial  saturation  Slopes Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : It is well recognised that field measurements of stress-dependent soil–water characteristic curves (SDSWCCs) and the permeability function (coefficient of permeability with respect to the water phase) are vital for assessing transient seepage, pore water pressure changes and the stability of unsaturated soil slopes. In this study, SDSWCCs and the permeability function were measured by using the instantaneous profile method on a saprolitic hillside in Tung Chung, Hong Kong. The ground profile comprises colluvium and completely decomposed tuff (CDT). A 3 m diameter circular plot was instrumented and subjected to two wetting–drying cycles. Throughout a 48-day monitoring period, variations of volumetric water content and matrix suction in the uppermost 3 m of soil were measured continuously using time-domain reflectometry moisture probes and jet-fill tensiometers, respectively. The field observations reveal that there is a clear trend of decreasing sizes of hysteretic loops of SDSWCCs with depth. In a comparison of the field- and laboratory-measured SDSWCCs, general agreement can be seen in the first wetting–drying cycle but not in the second cycle. The field-observed unsaturated permeability function is highly dependent on suction history (wetting–drying cycles). The field-measured permeability ranges from 4 × 10−7 m/s to 3 × 10−4 m/s in the colluvium, whereas the corresponding measured values vary from 4 × 10−7 m/s to 1 × 10−4 m/s in the CDT. Predictions of SWCC and permeability function using existing empirical approaches based on particle size distributions (PSDs) should be treated with caution. There is a clear qualitative and quantitative inconsistency between predictions of permeability function from PSD and field measurements. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.157

Ground deformations and soil–structure interaction of a multi-propped excavation in Shanghai soft clays / C. W.W. Ng in Géotechnique, Vol. 62 N° 10 (Octobre 2012) 

Public ISBD [article]  in Géotechnique > Vol. 62 N° 10 (Octobre 2012) . - pp. 907 –921 Titre : Ground deformations and soil–structure interaction of a multi-propped excavation in Shanghai soft clays Type de document : texte imprimé Auteurs : C. W.W. Ng, Auteur ; Y. Hong, Auteur ; G. B. Liu, Auteur Année de publication : 2012 Article en page(s) : pp. 907 –921 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Ground  movements  Case  history  Soil/structure  interaction  Retaining  walls  Earth  pressure  Pore  pressures  Stress  path  Clays  Field  instrumentation Résumé : Ground deformations and soil–structure interaction were monitored at a comprehensively instrumented diaphragm wall panel constructed at a greenfield site. The diaphragm wall retains a 14·5 m deep multi-propped excavation in soft clays in Shanghai, P. R. China. At this greenfield site, the maximum ground surface settlement ( δ v-max) at the end of excavation ranged from 0·25% to 0·32% of the final excavation depth (H e), and the influence zone of ground surface settlement was more than 2·5H e behind the wall. Both the measured δ v-max and the influence zone of ground settlements at this site are generally larger than for excavations in other, more densely built areas of Shanghai. Force and moment equilibrium analyses of the instrumented panel validate the reliability of the measured earth pressures and prop forces. The measured prop loads from this greenfield site are generally larger than those reported from similar excavations in Shanghai. Earth pressure measurements reveal that concreting of the diaphragm wall panel increased the lateral earth pressure coefficients at the soil/wall interface by a factor of more than 1·4. The stress paths for the retained soil within the propped region generally moved towards the passive state during the last few stages of main excavation, and soil near the formation level approached the passive failure state prior to the completion of excavation. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.10.P.072 [article] Ground deformations and soil–structure interaction of a multi-propped excavation in Shanghai soft clays [texte imprimé] / C. W.W. Ng, Auteur ; Y. Hong, Auteur ; G. B. Liu, Auteur . - 2012 . - pp. 907 –921. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 62 N° 10 (Octobre 2012) . - pp. 907 –921 Mots-clés : Ground  movements  Case  history  Soil/structure  interaction  Retaining  walls  Earth  pressure  Pore  pressures  Stress  path  Clays  Field  instrumentation Résumé : Ground deformations and soil–structure interaction were monitored at a comprehensively instrumented diaphragm wall panel constructed at a greenfield site. The diaphragm wall retains a 14·5 m deep multi-propped excavation in soft clays in Shanghai, P. R. China. At this greenfield site, the maximum ground surface settlement ( δ v-max) at the end of excavation ranged from 0·25% to 0·32% of the final excavation depth (H e), and the influence zone of ground surface settlement was more than 2·5H e behind the wall. Both the measured δ v-max and the influence zone of ground settlements at this site are generally larger than for excavations in other, more densely built areas of Shanghai. Force and moment equilibrium analyses of the instrumented panel validate the reliability of the measured earth pressures and prop forces. The measured prop loads from this greenfield site are generally larger than those reported from similar excavations in Shanghai. Earth pressure measurements reveal that concreting of the diaphragm wall panel increased the lateral earth pressure coefficients at the soil/wall interface by a factor of more than 1·4. The stress paths for the retained soil within the propped region generally moved towards the passive state during the last few stages of main excavation, and soil near the formation level approached the passive failure state prior to the completion of excavation. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.10.P.072

Measurements of drying and wetting permeability functions using a new stress-controllable soil column / C. W.W. Ng in Journal of geotechnical and geoenvironmental engineering, Vol. 138 N° 1 (Janvier 2012) 

Public ISBD [article]  in Journal of geotechnical and geoenvironmental engineering > Vol. 138 N° 1 (Janvier 2012) . - pp. 58-68 Titre : Measurements of drying and wetting permeability functions using a new stress-controllable soil column Type de document : texte imprimé Auteurs : C. W.W. Ng, Auteur ; A. K. Leung, Auteur Année de publication : 2012 Article en page(s) : pp. 58-68 Note générale : Géotechnique Langues : Anglais (eng) Mots-clés : Permeability  function  Soil-water  characteristic  curve  Matric  suction  Net  normal  stress  Stress-controllable  soil  column  Instantaneous  profile  method Résumé : Direct measurement of a permeability function, k(ψ) (where ψ = suction), of an unsaturated soil is often tedious and time-consuming. Thus, researchers have proposed various semiempirical predictive methods to determine k(ψ) indirectly from a soil-water characteristic curve (SWCC) or a particle-size distribution. Also, stress effects on k(ψ) are generally not considered. To explore any stress effect on k(ψ), a new 1-m-high stress-controllable soil column is developed to measure stress-dependent SWCC (SDSWCC) and k(ψ) of soils directly. The objectives of this paper are to investigate the effects of (1) two stress-state variables (matric suction and net normal stress) and (2) a drying-wetting cycle on both SDSWCC and k(ψ) of a compacted decomposed silty clay. Each compacted soil column is subjected to an evaporation-ponding cycle under various vertical net normal stresses. The variations of pore-water pressure and volumetric water content (VWC, θw) profiles with time are measured instantaneously. The k(ψ) is hence determined by using the instantaneous profile method. At a given average vertical net normal stress, the measured k(ψ) exhibits a remarkable hysteresis loop, whereas the k(θw) seems to be less hysteretic. The effects of the vertical net normal stress appear to be more pronounced than that of the drying-wetting cycle on k(θw). As average vertical net normal stress is increased from 4 to 78 kPa, the measured permeability decreases by up to two orders of magnitude at a given VWC. DEWEY : 624.1 ISSN : 1090-0241 En ligne : http://ascelibrary.org/gto/resource/1/jggefk/v138/i1/p58_s1?isAuthorized=no [article] Measurements of drying and wetting permeability functions using a new stress-controllable soil column [texte imprimé] / C. W.W. Ng, Auteur ; A. K. Leung, Auteur . - 2012 . - pp. 58-68. Géotechnique Langues : Anglais (eng) in Journal of geotechnical and geoenvironmental engineering > Vol. 138 N° 1 (Janvier 2012) . - pp. 58-68 Mots-clés : Permeability  function  Soil-water  characteristic  curve  Matric  suction  Net  normal  stress  Stress-controllable  soil  column  Instantaneous  profile  method Résumé : Direct measurement of a permeability function, k(ψ) (where ψ = suction), of an unsaturated soil is often tedious and time-consuming. Thus, researchers have proposed various semiempirical predictive methods to determine k(ψ) indirectly from a soil-water characteristic curve (SWCC) or a particle-size distribution. Also, stress effects on k(ψ) are generally not considered. To explore any stress effect on k(ψ), a new 1-m-high stress-controllable soil column is developed to measure stress-dependent SWCC (SDSWCC) and k(ψ) of soils directly. The objectives of this paper are to investigate the effects of (1) two stress-state variables (matric suction and net normal stress) and (2) a drying-wetting cycle on both SDSWCC and k(ψ) of a compacted decomposed silty clay. Each compacted soil column is subjected to an evaporation-ponding cycle under various vertical net normal stresses. The variations of pore-water pressure and volumetric water content (VWC, θw) profiles with time are measured instantaneously. The k(ψ) is hence determined by using the instantaneous profile method. At a given average vertical net normal stress, the measured k(ψ) exhibits a remarkable hysteresis loop, whereas the k(θw) seems to be less hysteretic. The effects of the vertical net normal stress appear to be more pronounced than that of the drying-wetting cycle on k(θw). As average vertical net normal stress is increased from 4 to 78 kPa, the measured permeability decreases by up to two orders of magnitude at a given VWC. DEWEY : 624.1 ISSN : 1090-0241 En ligne : http://ascelibrary.org/gto/resource/1/jggefk/v138/i1/p58_s1?isAuthorized=no