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Deep-penetrating spudcan foundations on layered clays / M. S. Hossain in Géotechnique, Vol. 60 N° 3 (Mars 2010) 

Public ISBD [article]  in Géotechnique > Vol. 60 N° 3 (Mars 2010) . - pp.157–170 Titre : Deep-penetrating spudcan foundations on layered clays : centrifuge tests Type de document : texte imprimé Auteurs : M. S. Hossain, Auteur ; M. F. Randolph, Auteur Année de publication : 2011 Article en page(s) : pp.157–170 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Bearing  capacity  Offshore  engineering  Footings/foundations  Centrifuge  modelling  Failure  Clays Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : Installation of independent-legged jack-up rigs in sea-bed sediments where a strong layer overlays weaker soil can lead to catastrophic ‘punch-through', with potential leg buckling or toppling of the unit. Although approximate methods of analysis exist for estimating the penetration resistance, these do not account for the distortion of the upper layer as it punches through into the lower layer and there has been only limited visual depiction of the failure mechanism that occurs for the case of strong clay overlying weaker soil. This paper reports results of centrifuge model tests undertaken to provide insight into spudcan foundation behaviour during undrained vertical penetration through a stronger clay layer into weaker material, varying the strength ratio between lower and upper soil layers, the thickness of the upper layer relative to the spudcan diameter and the strength gradient of the lower layer. The model tests included half-spudcan tests against a transparent window, allowing the soil flow to be captured continuously by a digital camera and subsequently quantified through particle image velocimetry (PIV) analysis, and separate full-spudcan tests to measure the penetration resistance. Four interesting aspects of the soil flow mechanisms were identified: (a) vertically downwards motion of the soil and consequent deformation of the layer interface; (b) trapping of the stronger material beneath the spudcan, with this material being carried down into the underlying soft layer; (c) delayed back-flow of soil around the spudcan into the cavity formed above the spudcan; (d) eventual localised flow around the embedded spudcan. The effect of normalised layer soil properties and geometry on the soil flow mechanisms and the form of the penetration resistance profile is discussed in the context of the likelihood and severity of punch-through failure. Typical critical failure modes involved punching shear, with clear shear planes in the shape of a truncated cone forming in the upper layer below the spudcan. Potential punch-through, with a peak in penetration resistance followed by some reduction, occurred for almost all cases investigated. The reduction in resistance became more severe as the strength ratio of the underlying layer to the upper layer reduced, and also as the upper layer thickness increased. The failure modes assumed by the currently available recommended practices are not consistent with those observed from the study, suggesting a more rational approach is needed. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.039 [article] Deep-penetrating spudcan foundations on layered clays : centrifuge tests [texte imprimé] / M. S. Hossain, Auteur ; M. F. Randolph, Auteur . - 2011 . - pp.157–170. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 60 N° 3 (Mars 2010) . - pp.157–170 Mots-clés : Bearing  capacity  Offshore  engineering  Footings/foundations  Centrifuge  modelling  Failure  Clays Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : Installation of independent-legged jack-up rigs in sea-bed sediments where a strong layer overlays weaker soil can lead to catastrophic ‘punch-through', with potential leg buckling or toppling of the unit. Although approximate methods of analysis exist for estimating the penetration resistance, these do not account for the distortion of the upper layer as it punches through into the lower layer and there has been only limited visual depiction of the failure mechanism that occurs for the case of strong clay overlying weaker soil. This paper reports results of centrifuge model tests undertaken to provide insight into spudcan foundation behaviour during undrained vertical penetration through a stronger clay layer into weaker material, varying the strength ratio between lower and upper soil layers, the thickness of the upper layer relative to the spudcan diameter and the strength gradient of the lower layer. The model tests included half-spudcan tests against a transparent window, allowing the soil flow to be captured continuously by a digital camera and subsequently quantified through particle image velocimetry (PIV) analysis, and separate full-spudcan tests to measure the penetration resistance. Four interesting aspects of the soil flow mechanisms were identified: (a) vertically downwards motion of the soil and consequent deformation of the layer interface; (b) trapping of the stronger material beneath the spudcan, with this material being carried down into the underlying soft layer; (c) delayed back-flow of soil around the spudcan into the cavity formed above the spudcan; (d) eventual localised flow around the embedded spudcan. The effect of normalised layer soil properties and geometry on the soil flow mechanisms and the form of the penetration resistance profile is discussed in the context of the likelihood and severity of punch-through failure. Typical critical failure modes involved punching shear, with clear shear planes in the shape of a truncated cone forming in the upper layer below the spudcan. Potential punch-through, with a peak in penetration resistance followed by some reduction, occurred for almost all cases investigated. The reduction in resistance became more severe as the strength ratio of the underlying layer to the upper layer reduced, and also as the upper layer thickness increased. The failure modes assumed by the currently available recommended practices are not consistent with those observed from the study, suggesting a more rational approach is needed. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.039

Deep-penetrating spudcan foundations on layered clays / M. S. Hossain in Géotechnique, Vol. 60 N° 3 (Mars 2010) 

Public ISBD [article]  in Géotechnique > Vol. 60 N° 3 (Mars 2010) . - pp. 171–184 Titre : Deep-penetrating spudcan foundations on layered clays : numerical analysis Type de document : texte imprimé Auteurs : M. S. Hossain, Auteur ; M. F. Randolph, Auteur Année de publication : 2011 Article en page(s) : pp. 171–184 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Offshore  engineering  Numerical  modelling  Bearing  capacity  Footings/foundations  Failure  Clays Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : This paper presents results of large deformation finite element (LDFE) analyses that investigate the penetration response of deep-penetrating spudcan foundations in stratified soil comprising a strong clay layer overlying weaker clay. Such strength profiles give rise to potential punch-through failure, with a local maximum penetration resistance followed by some reduction. The paper is a companion paper to one that presents results from centrifuge model tests of the same problem, with the overall aim being to investigate the potential for punch-through failure and its severity, as a basis for improving design methods for evaluating spudcan penetration. The LDFE analyses have simulated continuous penetration of smooth and rough spudcan foundations from the sea-bed surface. A detailed parametric study has been undertaken, exploring the relevant range of layer thickness (relative to the spudcan diameter), strength ratios and spudcan base roughness. The results have been validated against previously published results and the centrifuge test data presented in a companion paper. Excellent agreement was obtained between the results from LDFE analyses and centrifuge tests. As the ratio of the shear strength of the bottom layer to that of the top layer decreased, upwards soil flow around the spudcan shoulder became less pronounced and the failure mechanism was dominated by a steep-sided plug of soil from the top layer being forced deep into the lower layer. The potential for severe punch-through was demonstrated by a significant reduction in the resistance profile as the spudcan approached the interface between the two layers. A preliminary design chart is proposed to estimate the cavity depth above the penetrating spudcan, which was significantly greater than that for a single layer clay. The penetration responses have been presented in terms of profiles of normalised net bearing pressure for a range of layer geometries and soil properties. Potential punch-through occurred for all cases where the strength ratio was less than or equal to 0·6, with the reduction in resistance being greater the lower the strength ratio of the underlying layer to the upper layer, and the thicker the upper layer, but reducing as the strength gradient in the lower layer increased. The current approach suggested in offshore design guidelines is shown to underestimate the penetration resistance significantly, in most cases, and give poor estimates of the likelihood and severity of spudcan punch-through. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.040 [article] Deep-penetrating spudcan foundations on layered clays : numerical analysis [texte imprimé] / M. S. Hossain, Auteur ; M. F. Randolph, Auteur . - 2011 . - pp. 171–184. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 60 N° 3 (Mars 2010) . - pp. 171–184 Mots-clés : Offshore  engineering  Numerical  modelling  Bearing  capacity  Footings/foundations  Failure  Clays Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : This paper presents results of large deformation finite element (LDFE) analyses that investigate the penetration response of deep-penetrating spudcan foundations in stratified soil comprising a strong clay layer overlying weaker clay. Such strength profiles give rise to potential punch-through failure, with a local maximum penetration resistance followed by some reduction. The paper is a companion paper to one that presents results from centrifuge model tests of the same problem, with the overall aim being to investigate the potential for punch-through failure and its severity, as a basis for improving design methods for evaluating spudcan penetration. The LDFE analyses have simulated continuous penetration of smooth and rough spudcan foundations from the sea-bed surface. A detailed parametric study has been undertaken, exploring the relevant range of layer thickness (relative to the spudcan diameter), strength ratios and spudcan base roughness. The results have been validated against previously published results and the centrifuge test data presented in a companion paper. Excellent agreement was obtained between the results from LDFE analyses and centrifuge tests. As the ratio of the shear strength of the bottom layer to that of the top layer decreased, upwards soil flow around the spudcan shoulder became less pronounced and the failure mechanism was dominated by a steep-sided plug of soil from the top layer being forced deep into the lower layer. The potential for severe punch-through was demonstrated by a significant reduction in the resistance profile as the spudcan approached the interface between the two layers. A preliminary design chart is proposed to estimate the cavity depth above the penetrating spudcan, which was significantly greater than that for a single layer clay. The penetration responses have been presented in terms of profiles of normalised net bearing pressure for a range of layer geometries and soil properties. Potential punch-through occurred for all cases where the strength ratio was less than or equal to 0·6, with the reduction in resistance being greater the lower the strength ratio of the underlying layer to the upper layer, and the thicker the upper layer, but reducing as the strength gradient in the lower layer increased. The current approach suggested in offshore design guidelines is shown to underestimate the penetration resistance significantly, in most cases, and give poor estimates of the likelihood and severity of spudcan punch-through. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.040

Investigation of geotechnical parameters affecting electrical resistivity of compacted clays / G. Kibria 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. 1520–1529 Titre : Investigation of geotechnical parameters affecting electrical resistivity of compacted clays Type de document : texte imprimé Auteurs : G. Kibria, Auteur ; M. S. Hossain, Auteur Année de publication : 2013 Article en page(s) : pp. 1520–1529 Note générale : Géotechnique Langues : Anglais (eng) Mots-clés : Electrical  resistivity  Compacted  clays  Moisture  Unit  weight  Specific  surface  area  Ion  content Résumé : The use of resistivity imaging (RI) in subsurface investigation has increased in recent years. RI is a non-destructive method and provides a continuous image of the subsurface. However, only qualitative evaluation of the subsurface can be obtained from RI. The correlations between RI results and geotechnical engineering properties of soils have become important for site investigation using this method. The primary objective of the current study was to determine the geotechnical parameters affecting electrical resistivity of compacted clays. Understanding the influential factors will be helpful in determining the correlations between RI results and geotechnical properties of soil. The effects of moisture content, unit weight, degree of saturation, specific surface area, percentages of pores, and ion composition on soil resistivity were investigated. Soil samples used were classified as highly plastic clay (CH) according to the Unified Soil Classification System. High-energy X-ray fluorescence tests indicated the presence of high percentages of aluminum, silicon, and calcium ions in the samples. In addition, scanning electron microscope images were analyzed to identify clay structure and the distribution of pores. It was determined that the dominant clay mineral in the soil samples was montmorillonite. Soil resistivity tests were conducted in the laboratory at varying moisture contents and unit weights. Based on the experimental results, the average reduction in soil resistivity was 13.8 Ohm-m for an increase of moisture content from 10 to 20%. Test results indicated that soil resistivity decreased with an increase in moist unit weight. In addition, soil resistivity increased from 4.3 to 14.2 Ohm-m for an increase of surface area from 69.6 to 107.1 m2/g at 18% moisture content and 11.8 kN/m3 dry unit weight. Soil with high surface area required more water for the formation of water film and bridging between the particles. This might cause an increase in soil resistivity with an increase of surface area. Moreover, specific surface area also controlled resistivity when soil resistivity was plotted against calcium ions and pore spaces of the soil samples. Therefore, in addition to moisture content and unit weight, specific surface area of soils was identified as an important factor influencing soil resistivity. ISSN : 1090-0241 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29GT.1943-5606.0000722 [article] Investigation of geotechnical parameters affecting electrical resistivity of compacted clays [texte imprimé] / G. Kibria, Auteur ; M. S. Hossain, Auteur . - 2013 . - pp. 1520–1529. Géotechnique Langues : Anglais (eng) in Journal of geotechnical and geoenvironmental engineering > Vol. 138 N° 12 (Décembre 2012) . - pp. 1520–1529 Mots-clés : Electrical  resistivity  Compacted  clays  Moisture  Unit  weight  Specific  surface  area  Ion  content Résumé : The use of resistivity imaging (RI) in subsurface investigation has increased in recent years. RI is a non-destructive method and provides a continuous image of the subsurface. However, only qualitative evaluation of the subsurface can be obtained from RI. The correlations between RI results and geotechnical engineering properties of soils have become important for site investigation using this method. The primary objective of the current study was to determine the geotechnical parameters affecting electrical resistivity of compacted clays. Understanding the influential factors will be helpful in determining the correlations between RI results and geotechnical properties of soil. The effects of moisture content, unit weight, degree of saturation, specific surface area, percentages of pores, and ion composition on soil resistivity were investigated. Soil samples used were classified as highly plastic clay (CH) according to the Unified Soil Classification System. High-energy X-ray fluorescence tests indicated the presence of high percentages of aluminum, silicon, and calcium ions in the samples. In addition, scanning electron microscope images were analyzed to identify clay structure and the distribution of pores. It was determined that the dominant clay mineral in the soil samples was montmorillonite. Soil resistivity tests were conducted in the laboratory at varying moisture contents and unit weights. Based on the experimental results, the average reduction in soil resistivity was 13.8 Ohm-m for an increase of moisture content from 10 to 20%. Test results indicated that soil resistivity decreased with an increase in moist unit weight. In addition, soil resistivity increased from 4.3 to 14.2 Ohm-m for an increase of surface area from 69.6 to 107.1 m2/g at 18% moisture content and 11.8 kN/m3 dry unit weight. Soil with high surface area required more water for the formation of water film and bridging between the particles. This might cause an increase in soil resistivity with an increase of surface area. Moreover, specific surface area also controlled resistivity when soil resistivity was plotted against calcium ions and pore spaces of the soil samples. Therefore, in addition to moisture content and unit weight, specific surface area of soils was identified as an important factor influencing soil resistivity. ISSN : 1090-0241 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29GT.1943-5606.0000722